Anatomy of a mouse: structure and physiology

External Anatomy

Head Region

Eyes

The mouse eye is a compact organ adapted for nocturnal vision, consisting of a cornea, anterior chamber, lens, vitreous body, retina, and accessory structures that together form a functional visual system.

• Cornea: Transparent, avascular surface that refracts incoming light; thickness averages 120 µm.
• Lens: Biconvex, elastic tissue capable of accommodation; focal length adjusts via ciliary muscle activity.
• Retina: Multilayered neural tissue containing rods (≈ 95 % of photoreceptors) and cones (≈ 5 %); rods provide high sensitivity, cones support limited color discrimination.
• Optic nerve: Bundles retinal ganglion cell axons, transmitting visual signals to the lateral geniculate nucleus and visual cortex.
• Accessory structures: Iris regulates pupil diameter; ciliary body controls lens shape; sclera provides structural support.

Physiologically, mouse photoreceptors respond to wavelengths between 350 nm and 620 nm, with peak rod sensitivity near 500 nm. Signal transduction follows the canonical phototransduction cascade: photon absorption triggers rhodopsin activation, phosphodiesterase-mediated cGMP reduction, and closure of cGMP‑gated channels, resulting in hyperpolarization of photoreceptor membranes. Retinal ganglion cells encode spatial and temporal contrast, relaying processed information via the optic tract. Visual acuity averages 0.5 cycles/degree, sufficient for detecting predators and navigating confined environments. Pupillary reflexes modulate retinal illumination, while retinal pigment epithelium maintains photoreceptor health through phagocytosis of shed outer segments and recycling of visual pigments.

Ears

The mouse ear consists of three anatomically distinct regions: the outer ear, middle ear, and inner ear. The outer ear includes the pinna and external auditory canal, which channel sound waves toward the tympanic membrane. The tympanic membrane separates the outer and middle ear and transduces acoustic pressure into mechanical vibrations.

• Middle ear: houses the ossicular chain (malleus, incus, and stapes) that amplify vibrations and transmit them to the oval window of the inner ear.
• Inner ear: comprises the cochlea, vestibular apparatus, and associated nerves. The cochlea converts mechanical energy into neural signals via hair cells, while the vestibular system maintains balance and spatial orientation.

Physiological characteristics of mouse ears include a high-frequency hearing range, extending up to 100 kHz, and a rapid auditory brainstem response. The vascular supply derives from branches of the external carotid artery, and innervation is provided primarily by the facial and trigeminal nerves, facilitating both auditory perception and reflexive ear movements.

Nose

The mouse nose is a compact, forward‑projecting structure that houses the primary sensory organ for olfaction and the first stage of respiratory airflow. Externally, it comprises a pair of narial openings surrounded by vibrissae that aid in tactile exploration. The bony framework includes the premaxilla and maxilla, which support the nasal septum and lateral walls.

Internally, the nasal cavity contains a series of thin, scroll‑shaped bones called turbinates. These increase surface area for air conditioning and support the olfactory epithelium. The epithelium consists of:

• Olfactory receptor neurons with cilia that bind volatile molecules.
• Supporting sustentacular cells that maintain ionic balance.
• Basal cells that regenerate damaged receptors.

Mucus-producing goblet cells line the respiratory epithelium, providing a moist environment that traps particles and dissolves odorants. Blood supply derives from branches of the internal carotid artery, delivering oxygen and facilitating heat exchange. Sensory innervation is provided by the olfactory nerve (cranial nerve I) and trigeminal nerve fibers that convey irritation and temperature signals.

Physiologically, odor detection follows a sequence: odorant molecules dissolve in mucus, bind to specific receptor proteins on neuronal cilia, trigger a G‑protein cascade, and generate an electrical signal transmitted to the olfactory bulb. The mouse olfactory system can discriminate thousands of chemical cues, enabling behaviors such as foraging, predator avoidance, and social communication.

Key functional attributes of the mouse nose include:

1. Filtration of inhaled air through nasal hair and mucus.
2. Humidification and warming of airflow before it reaches the lungs.
3. Continuous regeneration of olfactory receptor cells, typically every 2–3 weeks.
4. Integration of olfactory and trigeminal inputs to produce reflexive respiratory adjustments.

Mouth and Vibrissae «Whiskers»

The mouse oral cavity is a compact structure adapted for gnawing, chewing, and taste perception. The upper and lower jaws are formed by the maxilla and mandible, each bearing a single row of continuously growing incisors that self‑sharpen through occlusal wear. Behind the incisors, the cheek teeth (molars and premolars) are limited in number and function primarily in grinding. The hard palate separates the nasal passages from the oral cavity and supports the attachment of the tongue, which contains intrinsic and extrinsic muscles for precise manipulation of food. Salivary glands—parotid, submandibular, and sublingual—secrete enzymes and lubricants that begin carbohydrate digestion and facilitate swallowing.

Vibrissae, commonly called whiskers, are specialized tactile hairs positioned primarily on the mystacial pads of the mouse’s snout. Each whisker is anchored in a follicle-sinus complex that includes a richly innervated blood sinus, providing a direct conduit for mechanoreceptive signals. The follicles contain:

• A deep dermal capsule surrounded by collagen fibers that transmit bending forces.
• A blood sinus that amplifies mechanical deflection.
• A dense array of Aβ and Aδ nerve fibers that convey high‑resolution spatial information to the brainstem.

The arrangement of whiskers follows a precise geometric pattern, allowing the mouse to construct a three‑dimensional map of its environment through active whisking movements. Motor control originates in the facial nucleus, which coordinates rhythmic protraction and retraction at frequencies up to 10 Hz. Sensory input from the follicles is relayed via the trigeminal ganglion to somatosensory cortices, where it is integrated with proprioceptive and visual data to guide navigation, prey capture, and obstacle avoidance.

Together, the oral apparatus and vibrissal system enable the mouse to acquire, process, and evaluate food while simultaneously detecting spatial cues essential for survival. Their integration exemplifies the efficiency of rodent morphology in supporting both feeding behavior and environmental exploration.

Torso

Fur and Skin

The mouse integument consists of a multilayered skin envelope covered by a dense pelage. The outermost layer, the epidermis, is composed of keratinized stratified squamous epithelium that provides a barrier against mechanical injury and microbial invasion. Beneath the epidermis lies the dermis, a connective tissue matrix rich in collagen and elastin fibers, blood vessels, lymphatics, and sensory nerve endings. The dermis supports hair follicles, sebaceous glands, and the vibrissal system.

Hair shafts emerge from follicular units distributed across the body. Each follicle undergoes a cyclic process of growth (anagen), regression (catagen), and rest (telogen), resulting in periodic shedding and replacement of fur. The pelage exhibits two primary coat types:

• Guard hairs: coarse, pigmented, and relatively long; protect underlying layers and contribute to visual camouflage.
• Undercoat hairs: fine, soft, and densely packed; provide insulation by trapping air close to the skin surface.

Melanin granules within the cortex determine fur color, while the distribution of melanocytes in the epidermis influences skin pigmentation. Specialized mechanoreceptive whiskers (vibrissae) are anchored in robust follicular sockets and contain a high concentration of tactile receptors, enabling precise spatial discrimination.

Sebaceous glands open onto the hair shaft, secreting lipid-rich sebum that maintains moisture, reduces friction, and imparts a modest antimicrobial effect. Sweat glands are sparsely distributed and function primarily in thermoregulation through evaporative cooling. The combination of fur density, hair length, and glandular activity allows the mouse to regulate body temperature across a wide range of ambient conditions.

Overall, the integumentary system integrates structural protection, sensory perception, and thermal balance, thereby supporting the animal’s survival in diverse environments.

Mammary Glands

Mammary glands in mice are paired exocrine organs situated ventrally along the milk line, typically numbering five pairs (thoracic, abdominal, and inguinal). Each gland comprises a branching ductal network that terminates in alveolar clusters responsible for milk synthesis. The epithelium is supported by a dense stromal matrix containing adipocytes, fibroblasts, and a vascular supply that delivers nutrients and hormones.

Key anatomical characteristics:

• Ductal hierarchy: primary ducts branch into secondary ducts, which further subdivide into terminal alveolar ducts.
• Alveoli: spherical secretory units lined by luminal epithelial cells and surrounded by myoepithelial cells that contract during milk ejection.
• Stroma: collagen-rich connective tissue provides structural integrity and houses immune cells.
• Vasculature: capillary loops closely associate with alveolar epithelium, facilitating nutrient exchange.
• Innervation: autonomic fibers regulate myoepithelial contraction and blood flow.

Physiologically, mammary development proceeds through three stages: embryonic bud formation, puberty‑induced ductal growth, and pregnancy‑driven alveolar differentiation. Estrogen and progesterone initiate ductal expansion, while prolactin and placental lactogen trigger alveolar proliferation and milk protein gene expression. During lactation, prolactin maintains synthesis of casein, whey proteins, lipids, and lactose; oxytocin induces myoepithelial contraction to expel milk through the nipple openings.

Functional outcomes include provision of nutrients, antibodies, and growth factors essential for neonatal growth. Milk composition shifts across lactation days, with early milk richer in immunoglobulins and later milk containing higher lipid concentrations. Male mice possess rudimentary mammary tissue that remains undeveloped due to the absence of the hormonal milieu required for full differentiation. Weaning marks involution, characterized by apoptosis of alveolar cells and remodeling of the glandular architecture back to a quiescent state.

Anus

The mouse anus forms the terminal segment of the gastrointestinal tract and lies at the posterior extremity of the body, opening to the external environment. It marks the transition from the rectum to the external environment and is surrounded by a complex arrangement of muscular and connective tissues that secure fecal continence and enable rapid expulsion when required.

Key structural components include:

• Internal anal sphincter – a ring of smooth muscle that maintains tonic contraction, providing baseline closure.
• External anal sphincter – striated muscle under voluntary control, allowing rapid increase in sphincter pressure.
• Mucosal lining – simple columnar epithelium with interspersed goblet cells, forming a protective barrier and secreting mucus.
• Anal crypts and glands – invaginations that house absorptive cells and contribute to fecal shaping.
• Connective tissue sheath – collagenous fibers that anchor the sphincters to the pelvic floor.

Histologically, the mucosa consists of a single layer of columnar cells, a thin lamina propria rich in fibroblasts, and a muscularis mucosa that supports peristaltic movements. Beneath the mucosa, the submucosa contains dense vascular networks, while the muscularis externa comprises the internal and external sphincter layers arranged in concentric circles.

Innervation derives from autonomic fibers (parasympathetic input via the pelvic nerve and sympathetic input via the hypogastric nerve) that modulate smooth‑muscle tone, and somatic fibers from the pudendal nerve that control the striated external sphincter. Arterial blood supply is provided by branches of the inferior rectal artery, with venous drainage through the rectal venous plexus into the portal system.

Physiologically, the anus regulates waste elimination by coordinating sphincter relaxation with rectal pressure, ensures continence through sustained sphincter tone, and contributes to immune defense by maintaining a mucus barrier that limits microbial invasion. The region also participates in signaling pathways that influence gut motility and systemic metabolic responses.

Genitalia

The murine reproductive system consists of distinct male and female structures adapted for rapid breeding cycles.

In males, the testes are positioned intra‑abdominally, attached to the caudal abdominal wall by the gubernaculum. The epididymis, coiled along the posterior surface of each testis, serves as the site of sperm maturation and storage. The vas deferens extends from the epididymis, traverses the inguinal canal, and joins the seminal vesicles to form the ejaculatory duct. Accessory glands include paired seminal vesicles, a single coagulating gland, and a prostate-like urethral gland, each contributing fluid components that support sperm viability. The penis is a short, keratinized organ with a preputial sheath; its glans lacks a baculum, reflecting the species’ reliance on copulatory thrusting rather than penile rigidity.

Female genitalia comprise paired ovaries located near the dorsal body wall, each containing follicles at various developmental stages. The oviducts (fallopian tubes) are slender, muscular tubes that transport ova to the uterine horns. The uterus is bicornuate, with two elongated horns converging into a short body leading to a single cervix. The vagina is a muscular canal terminating in a vulvar opening surrounded by a thin labial fold. The external genitalia include a modest clitoral projection and a perineal region lacking prominent scent glands.

Key physiological aspects:

• Sperm production peaks during the estrous cycle, with spermatogenesis completing in approximately 10 days.
• Ovarian estradiol and progesterone levels fluctuate cyclically, regulating ovulation and uterine receptivity.
• Seminal plasma constituents (e.g., fructose, prostaglandins) enhance sperm motility and facilitate uterine passage.
• Vaginal pH remains slightly acidic, providing a barrier to microbial invasion while allowing sperm passage during copulation.

These components collectively enable efficient gamete exchange and support the high reproductive output characteristic of laboratory and wild mouse populations.

Limbs

Forelimbs «Paws»

The mouse forelimb terminates in a compact paw that integrates skeletal, muscular, vascular, and sensory elements to support locomotion, manipulation, and tactile perception.

The skeletal framework consists of the humerus, radius, ulna, and a series of metacarpal bones that articulate with five phalanges per digit. The distal phalanges bear ungual keratinized claws, providing grip and digging capability. Joint capsules contain synovial fluid, enabling smooth flexion and extension during rapid strides.

Muscle groups originate on the scapula and humerus, inserting on the metacarpals and phalanges. Primary flexors (flexor digitorum superficialis and profundus) contract to curl the digits, while extensors (extensor digitorum) straighten them. Intrinsic muscles, including the lumbricals and interossei, fine‑tune digit positioning for object handling.

Blood supply is delivered by the brachial artery, which branches into the radial and ulnar arteries. These vessels form an extensive capillary network within the paw pads, maintaining tissue viability during prolonged contact with surfaces.

Sensory innervation derives from the median, ulnar, and radial nerves. Meissner’s corpuscles, Merkel cells, and free nerve endings populate the glabrous pad, delivering high‑resolution tactile feedback essential for navigation and foraging.

Key structural components can be summarized:

• Bones: humerus, radius, ulna, metacarpals, phalanges, ungual claws
• Muscles: flexor digitorum, extensor digitorum, lumbricals, interossei
• Vessels: brachial artery, radial and ulnar branches, capillary plexus
• Nerves: median, ulnar, radial nerves with specialized mechanoreceptors

Collectively, these systems confer the forelimb paw with the strength, dexterity, and sensory acuity required for the mouse’s ecological niche.

Hindlimbs «Paws»

The mouse hindlimb ends in a compact paw that supports rapid locomotion and precise manipulation of substrates. Each paw consists of five digits, a central pad, and lateral pads that distribute load and protect soft tissue during movement.

The skeletal framework includes the femur, tibia, fibula, and a fused tarsal–metatarsal complex. The metatarsals form a short, robust platform for digit attachment, while the calcaneus provides leverage for the ankle joint. The phalanges are short, with the distal phalanx bearing a keratinized claw.

Muscle groups are organized into flexors and extensors. The gastrocnemius–soleus complex extends the ankle, generating propulsion. The flexor digitorum brevis and flexor hallucis longus contract to curl the digits, enabling grasping and climbing. Antagonistic extensors, such as the extensor digitorum longus, straighten the toes during stance.

Sensory structures are concentrated in the pads and digit tips. Meissner’s corpuscles detect light touch, while Merkel cells respond to sustained pressure. Pacinian corpuscles register vibration, and nociceptors signal potentially damaging stimuli. The dense innervation provides high tactile acuity essential for environmental exploration.

Key anatomical features of the mouse paw:

• Five digits, each ending in a curved claw.
• Central plantar pad surrounded by two lateral pads.
• Thickened epidermis with keratinized layers for abrasion resistance.
• Rich vascular network supplying oxygenated blood to support muscular activity.
• Extensive nerve supply for rapid transmission of sensory information.

Tail

The mouse tail is a slender, cylindrical appendage extending posteriorly from the vertebral column, typically measuring 7–10 cm in adult specimens. Its surface is covered by fine pelage and a thin epidermis, beneath which lies a dense connective‑tissue sheath that encloses the underlying structures.

Internally, the tail consists of 20–30 caudal vertebrae lacking intervertebral discs, each bearing a pair of transverse processes that anchor longitudinal musculature. The musculature includes the extensor and flexor groups, which facilitate precise movements. A central medullary cavity contains a continuation of the spinal cord, providing neural pathways to distal tissues.

Arterial supply is delivered by the caudal artery, a branch of the abdominal aorta, while venous return occurs through the caudal vein. Sensory innervation arises from the dorsal root ganglia of the sacral spinal segments, supplying mechanoreceptors, thermoreceptors, and nociceptors distributed throughout the skin and subcutaneous layers.

The tail performs several physiological functions:

• Balance and locomotion: Muscular contraction adjusts tail position, counteracting body sway during rapid turns and vertical climbing.
• Thermoregulation: Vascular smooth muscle regulates blood flow, allowing heat dissipation or retention according to ambient temperature.
• Communication: Rapid flicking or positioning conveys social cues such as threat, submission, or exploratory intent.

These characteristics integrate structural specialization with functional demands, reflecting the tail’s adaptation to the mouse’s ecological niche.

Internal Anatomy: Organ Systems

Skeletal System

Skull

The mouse skull is a compact, fused cranium composed of eight major bones: the frontal, parietal, occipital, temporal, sphenoid, ethmoid, nasal, and maxillary. These elements interlock through sutures that permit limited growth during post‑natal development while maintaining rigidity for protection of the brain.

Key anatomical features include:

• Foramina: the optic canal, superior orbital fissure, and auditory meatus allow passage of cranial nerves and blood vessels.
• Dental arcade: incisors extend from the premaxilla and are continuously growing, while molars are situated in the maxilla and mandible.
• Muscle attachment sites: the temporal fossa and nuchal crest provide leverage for the temporalis and masseter muscles, enabling powerful gnawing motions.
• Sensory structures: the nasal cavity houses olfactory epithelium, and the orbit accommodates the visual apparatus.

Physiologically, the skull undergoes remodeling through osteoblastic and osteoclastic activity, regulated by mechanical loading from mastication and hormonal signals such as parathyroid hormone. Vascular supply is provided by branches of the internal carotid and vertebral arteries, delivering nutrients essential for bone turnover and repair.

Developmentally, the skull originates from neural crest cells that differentiate into mesenchymal tissue, forming the dermal bones, while the chondrocranium gives rise to the base. Fusion of the sutures occurs by four weeks of age, establishing the adult morphology required for efficient feeding, sensory processing, and protection of the central nervous system.

Vertebral Column

The vertebral column of the laboratory mouse consists of 26 individual vertebrae arranged into five distinct regions: seven cervical, thirteen thoracic, six lumbar, five sacral, and five caudal elements. Each vertebra features a centrum, a pair of neural arches, transverse processes, and a dorsal spinous process. Cervical vertebrae possess elongated transverse processes bearing foramen for the vertebral artery; thoracic vertebrae display rib facets for articulation with ribs; lumbar vertebrae have broad, robust bodies to support the abdominal cavity; sacral vertebrae are fused into a single sacrum that connects to the pelvis; caudal vertebrae are small, flexible, and terminate the spinal column.

The spinal cord runs within the vertebral canal, protected by the surrounding osseous structures and the meninges. Dorsal and ventral roots emerge from spinal nerve foramina at each segment, providing the conduit for sensory and motor signals throughout the body. Segmental innervation follows the typical mammalian pattern, with cervical nerves supplying forelimb musculature, thoracic nerves innervating intercostal muscles and skin, lumbar nerves controlling hindlimb muscles, and sacral nerves contributing to pelvic organ function.

Key physiological characteristics include:

• Intervertebral discs composed of fibrocartilage, allowing limited axial compression and rotational movement.
• Ligamentous complexes (ligamentum flavum, interspinous ligament) that stabilize adjacent vertebrae while permitting flexion and extension.
• Growth plates located at the ends of each vertebral body, responsible for longitudinal growth during post‑natal development.
• Muscular attachments (e.g., erector spinae, multifidus) that generate posture maintenance and locomotor forces.

Collectively, these structural and functional elements provide the mouse with a rigid yet adaptable axial framework essential for locomotion, protective housing of the central nervous system, and support of internal organs.

Rib Cage and Sternum

The mouse rib cage consists of twelve pairs of ribs that encircle the thoracic cavity, providing protection for the heart, lungs, and major blood vessels. Each rib attaches posteriorly to the vertebral column at the costovertebral joints and anteriorly to the sternum via costal cartilages, forming a semi‑rigid framework that permits limited expansion during respiration.

The sternum is a single, elongated bone situated in the midline of the ventral thorax. It comprises three regions: the manubrium, the body, and the xiphoid process. The manubrium articulates with the first pair of ribs, while the remaining ribs connect to the body through their respective cartilages. The xiphoid process, though small, serves as an attachment point for abdominal musculature.

Key physiological features of the mouse thoracic cage include:

• Flexibility: Overlapping costal cartilages allow the cage to expand and contract with each breath, supporting a tidal volume of approximately 0.5 ml.
• Structural support: The rib‑sternum junction distributes mechanical forces across the thorax, reducing stress on individual ribs.
• Muscle attachment: Intercostal muscles, the diaphragm, and portions of the pectoralis major originate on the ribs and sternum, facilitating ventilation and forelimb movement.

Vascular and nervous supply pass through intercostal spaces, where intercostal arteries, veins, and nerves run parallel to the ribs. Sensory innervation from the thoracic spinal nerves contributes to reflex control of breathing depth and rate.

Overall, the rib cage and sternum constitute a compact, resilient system that safeguards vital organs while accommodating the rapid respiratory cycles characteristic of small rodents.

Appendicular Skeleton «Limbs»

The mouse’s appendicular skeleton consists of paired forelimbs and hindlimbs that enable locomotion, manipulation of objects, and environmental interaction. Each limb is organized into three functional segments: the proximal girdle, the intermediate limb bones, and the distal elements that terminate in the digits.

The forelimb attaches to the scapula, which articulates with the humerus at the glenohumeral joint. The humerus extends distally to the radius and ulna, which are fused for most of their length, providing a rigid forearm. Distal to these bones, the carpal, metacarpal, and phalangeal series form the mouse’s hand, with an enlarged fifth digit that bears a claw used for digging and climbing.

The hindlimb connects to the pelvis via the ilium, ischium, and pubis. The femur projects posteriorly, articulating with the patella and then the tibia and fibula, which remain partially fused. The ankle region comprises the tarsal bones, followed by metatarsals and phalanges that constitute the foot. The hindlimb displays a more robust musculature and longer bones relative to the forelimb, reflecting its primary role in propulsion.

Key anatomical features of mouse limbs include:

• Scapular and pelvic girdles: Provide attachment points for major muscles and transmit forces to the limb bones.
• Fusion of radius‑ulna and tibia‑fibula: Increases structural stability during rapid movements.
• Elongated fifth digit (forelimb) and enlarged hindfoot: Adaptations for substrate grasping and burrowing.
• Highly mobile shoulder and hip joints: Allow a wide range of motion essential for climbing and sprinting.

Vascular and neural supply follows the major arterial trunks (subclavian → brachial in forelimbs; external iliac → femoral in hindlimbs) and the brachial and femoral nerves, ensuring precise motor control and sensory feedback throughout the appendicular skeleton.

Muscular System

Skeletal Muscles

The mouse’s skeletal musculature consists of over 400 distinct muscles that generate force for locomotion, posture, and manipulation of the environment. Each muscle originates on a bone or fascia and inserts on another skeletal element, forming a lever system that converts neural signals into mechanical work.

• Major muscle groups: forelimb (biceps brachii, triceps brachii, flexor digitorum), hindlimb (quadriceps femoris, gastrocnemius, tibialis anterior), axial muscles (erector spinae, intercostals), and masticatory muscles (masseter, temporalis).
• Fiber composition: a mix of fast‑twitch glycolytic fibers (type IIb) for rapid bursts, fast‑twitch oxidative fibers (type IIa) for sustained activity, and slow‑twitch oxidative fibers (type I) for postural maintenance. The proportion varies among muscles, reflecting functional demands.
• Innervation: motor neurons of the spinal ventral horns project via peripheral nerves (e.g., sciatic, median, facial) to neuromuscular junctions, where acetylcholine release triggers depolarization of the muscle fiber membrane.

Physiologically, muscle contraction follows the sliding‑filament mechanism: calcium released from the sarcoplasmic reticulum binds troponin, displacing tropomyosin and exposing actin sites for myosin cross‑bridge attachment. ATP hydrolysis drives cross‑bridge cycling, producing tension. Metabolic pathways differ by fiber type; glycolytic fibers rely on anaerobic glycolysis, whereas oxidative fibers depend on mitochondrial oxidative phosphorylation.

Regeneration capacity is high; satellite cells located between the basal lamina and sarcolemma activate after injury, proliferate, and differentiate into myoblasts that fuse with existing fibers. This process underlies the mouse’s utility in studies of muscle development, disease models (e.g., Duchenne muscular dystrophy), and therapeutic interventions.

Understanding the organization, fiber distribution, and contractile physiology of mouse skeletal muscles provides a foundation for interpreting experimental data and translating findings to broader mammalian biology.

Smooth Muscles

Smooth muscle tissue in the laboratory mouse forms the contractile component of many internal organs, including the gastrointestinal tract, urinary bladder, blood vessels, and the respiratory bronchioles. Unlike skeletal muscle, smooth muscle fibers are spindle‑shaped, lack sarcomeres, and contain dense bodies that anchor actin filaments. The cells are electrically coupled through gap junctions, allowing coordinated, slow waves of depolarization that generate tonic or phasic contractions.

Key structural features:

• Cellular morphology: elongated, uninucleated cells with a central nucleus; cytoplasm contains abundant mitochondria, endoplasmic reticulum, and myosin‑light chain kinase.
• Extracellular matrix: rich in collagen and elastin, providing tensile strength and elasticity.
• Innervation: primarily autonomic (sympathetic and parasympathetic) fibers; acetylcholine and norepinephrine modulate intracellular calcium levels.

Physiological mechanisms:

1. Calcium regulation: voltage‑dependent L‑type calcium channels and receptor‑operated channels permit calcium influx; intracellular stores release calcium via IP₃ receptors.
2. Signal transduction: calcium‑calmodulin complexes activate myosin‑light chain kinase, phosphorylating myosin heads and enabling cross‑bridge cycling; myosin‑light chain phosphatase mediates relaxation.
3. Contractile patterns: tonic contractions maintain baseline tension in vessels and sphincters; phasic contractions produce peristaltic waves in the gut.

In the mouse, smooth muscle responsiveness can be quantified by ex vivo organ bath assays, where dose‑response curves to agonists (e.g., acetylcholine) and antagonists (e.g., nifedipine) reveal receptor density and signaling efficiency. Histological analysis commonly employs α‑smooth muscle actin immunostaining to confirm tissue identity and assess hypertrophy or fibrosis in disease models.

Understanding smooth muscle structure and function is essential for interpreting experimental outcomes related to vascular tone, gastrointestinal motility, and urinary function in murine research.

Cardiac Muscle

The mouse heart consists of a compact layer of striated cardiac muscle that generates the contractile force required for circulation. Myocytes are elongated, cylindrical cells linked end‑to‑end by intercalated discs, which contain desmosomes, fascia adherens, and gap junctions. These structures provide mechanical continuity and rapid electrical coupling, enabling synchronous depolarization across the myocardium.

Cardiac muscle cells contain abundant mitochondria, reflecting a reliance on oxidative phosphorylation for ATP production. Each myocyte harbors a central nucleus, a well‑developed sarcoplasmic reticulum, and a dense network of transverse (T) tubules that align with the Z‑line of the sarcomere. The sarcomeric arrangement follows a 2‑to‑1 ratio of actin to myosin filaments, producing a characteristic striation pattern distinct from skeletal muscle.

Electrical activity originates in the sinoatrial (SA) node, propagates through the atrial myocardium, and reaches the atrioventricular (AV) node. From there, the impulse travels via the His‑Purkinje system to the ventricular walls, ensuring coordinated contraction. Action potentials in mouse cardiomyocytes display a rapid upstroke (phase 0), a brief plateau (phase 2), and a swift repolarization (phase 3), consistent with the high heart rate typical of rodents.

Key physiological properties include:

• Automaticity: SA‑node cells generate spontaneous depolarizations without neural input.
• Excitability: Voltage‑gated Na⁺ and Ca²⁺ channels mediate rapid depolarization and calcium influx.
• Conductivity: Gap junctions (connexin‑43) allow ion flow between adjacent myocytes.
• Contractility: Calcium release from the sarcoplasmic reticulum triggers actin–myosin cross‑bridge cycling; relaxation follows calcium reuptake by SERCA pumps.
• Responsiveness: β‑adrenergic stimulation increases intracellular cAMP, enhancing calcium handling and contractile force.

The mouse cardiac muscle exhibits a high heart rate (≈600 beats min⁻¹) and a short action‑potential duration, adaptations that support the metabolic demands of a small mammal. Comparative studies often use these characteristics to model human cardiac physiology and disease, leveraging the genetic tractability of the species.

Digestive System

Oral Cavity and Esophagus

The oral cavity of the laboratory mouse is a compact chamber optimized for gnawing and ingestion of solid food. Incisors dominate the dental arcade, continuously erupting to compensate for wear; they are supported by a thick enamel layer on the labial surface and a softer, dentin-rich lingual surface. The molars and premolars, situated posteriorly, possess complex occlusal ridges that facilitate grinding of seeds and pellets. The tongue is a muscular organ covered with papillae that aid in food manipulation and taste perception. Salivary glands—parotid, submandibular, and sublingual—secrete serous and mucous fluids rich in amylase, lysozyme, and electrolytes, initiating carbohydrate digestion and maintaining oral moisture. The hard palate forms a rigid barrier separating the nasal cavity, while the soft palate terminates the oral space and contributes to the closure of the nasopharynx during swallowing.

Swallowing transfers bolus material from the oral cavity into the esophagus through a coordinated series of neuromuscular events. The esophagus is a muscular tube approximately 3 cm in length, lined with a stratified squamous epithelium that resists abrasion. Its wall comprises:

• An inner circular muscle layer that contracts to generate peristaltic waves.
• An outer longitudinal muscle layer that shortens the tube and assists propulsion.
• A well‑developed myenteric plexus that regulates rhythmic contractions.
• A lower esophageal sphincter (LES) formed by thickened circular muscle, maintaining basal tone to prevent reflux.

Peristaltic activity proceeds at a velocity of 0.5–1 cm s⁻¹, moving ingested material toward the stomach. The LES relaxes transiently in response to swallow‑induced neural signals, allowing passage of the bolus before re‑establishing tonic closure. The esophageal mucosa secretes mucus containing bicarbonate, protecting the epithelium from acidic injury. Together, these structures ensure efficient transport of nutrients from the mouth to the gastric cavity in the mouse.

Stomach

The mouse stomach is a J‑shaped organ situated between the esophagus and the duodenum. Its ventral surface contacts the liver and pancreas, while the dorsal wall is supported by the diaphragm. The organ measures approximately 1.5 cm in length and 0.5 cm in diameter in adult laboratory strains, reflecting the animal’s small body size.

The gastric wall consists of four concentric layers:

• Mucosa: simple columnar epithelium with gastric pits leading to fundic glands that produce hydrochloric acid, pepsinogen, and intrinsic factor.
• Submucosa: loose connective tissue containing blood vessels, lymphatics, and a dense network of autonomic nerves.
• Muscularis externa: an inner circular and outer longitudinal smooth‑muscle layer that generates peristaltic contractions for mixing and propulsion of chyme.
• Serosa: thin connective tissue covered by mesothelium, providing structural support and facilitating movement within the abdominal cavity.

Acid secretion is regulated by three primary stimuli: vagal acetylcholine release, circulating gastrin, and local histamine from enterochromaffin‑like cells. Parietal cells respond to these signals by activating the H⁺/K⁺‑ATPase pump, maintaining gastric pH between 1.5 and 3.0 during active digestion. Chief cells secrete pepsinogen, which auto‑activates in the acidic environment to become pepsin, initiating protein hydrolysis.

Gastric motility follows a cyclical pattern of interdigestive migrating motor complexes and post‑prandial peristaltic waves. The interdigestive phase clears residual contents, while the post‑prandial phase mixes gastric secretions with ingested material, gradually releasing chyme into the duodenum through the pyloric sphincter. Neural control involves the dorsal motor nucleus of the vagus and intrinsic enteric circuits, ensuring coordinated contraction and relaxation.

Microbial colonization of the murine stomach is sparse due to low pH, yet a limited population of acid‑tolerant bacteria persists, influencing mucosal immunity and gastric hormone release. Experimental manipulation of gastric pH or secretion pathways in mice provides insight into digestive disorders, drug absorption, and the impact of diet on systemic metabolism.

Small Intestine

The mouse small intestine extends from the pyloric sphincter to the ileocecal valve, occupying roughly 30 % of total body length. Its average length of 25–35 cm provides a high surface‑to‑volume ratio essential for efficient nutrient extraction.

Anatomically, the tract divides into three contiguous regions. The duodenum, a short, C‑shaped segment, receives pancreatic secretions and bile. The jejunum, the central portion, exhibits pronounced villi and a dense capillary network. The ileum, the distal segment, contains numerous Peyer’s patches and a higher concentration of nutrient transporters.

The mucosal architecture follows the classic mammalian pattern. A single layer of columnar epithelium lines each villus, supported by a lamina propria rich in capillaries and lacteals. Crypts of Lieberkühn house proliferative stem cells and Paneth cells. The brush border presents enzymes such as sucrase‑isomaltase, aminopeptidase N, and intestinal alkaline phosphatase, each anchored to the microvillar membrane.

Physiological activity proceeds through coordinated steps:

• Carbohydrate digestion: disaccharidases convert oligosaccharides to monosaccharides, which enter enterocytes via SGLT1 and GLUT2 transporters.
• Protein breakdown: peptidases generate di‑ and tripeptides absorbed by PEPT1.
• Lipid assimilation: bile‑salt micelles deliver fatty acids and monoglycerides to the apical membrane; NPC1L1 mediates cholesterol uptake; chylomicron formation occurs within enterocytes before entry into lacteals.
• Electrolyte balance: Na⁺/K⁺‑ATPase maintains basolateral gradients, while NHE3 and CFTR regulate luminal pH.

Arterial inflow arrives through the superior mesenteric artery, branching into arcades that supply each villus. Venous drainage converges on the portal vein, delivering absorbed metabolites to the liver. Lacteals collect chylomicrons, feeding the systemic lymphatic circuit.

Neural control derives from the myenteric and submucosal plexuses of the enteric nervous system, modulated by sympathetic and parasympathetic fibers. Acetylcholine and vasoactive intestinal peptide stimulate peristalsis, whereas norepinephrine reduces motility.

The luminal environment hosts a dense microbial community, predominantly Firmicutes and Bacteroidetes. These organisms ferment indigestible carbohydrates, synthesize short‑chain fatty acids, and influence mucosal immunity through interaction with Toll‑like receptors on epithelial cells.

Large Intestine and Rectum

The large intestine of the laboratory mouse extends from the cecum to the rectum, measuring approximately 6–8 cm in length. Its wall consists of four layers: mucosa with a simple columnar epithelium bearing goblet cells, submucosa rich in connective tissue, a thin muscularis externa composed of an inner circular and outer longitudinal smooth‑muscle layer, and serosa. The mucosal surface forms numerous folds (plicae colica) that increase absorptive area, while the absence of villi distinguishes it from the small intestine. Lymphoid aggregates, known as colonic patches, are embedded in the submucosa and contribute to local immune surveillance.

Physiologically, the colon reabsorbs water and electrolytes, concentrating the luminal contents into feces. Sodium and chloride are absorbed via electrogenic transporters, while potassium secretion maintains ionic balance. The microbial community colonizes the distal colon, fermenting undigested carbohydrates to produce short‑chain fatty acids that serve as an energy source for colonocytes. Peristaltic contractions, regulated by the enteric nervous system and autonomic inputs, propel the fecal mass toward the rectum.

The rectum terminates the gastrointestinal tract and functions as a temporary storage site. Its wall shares the same four‑layer organization but exhibits a thicker muscularis externa, providing greater contractile force for evacuation. The internal anal sphincter comprises smooth muscle under involuntary control, whereas the external anal sphincter consists of skeletal muscle, allowing voluntary regulation of defecation. Sensory innervation within the rectal mucosa detects stretch, triggering the recto‑anal inhibitory reflex that coordinates sphincter relaxation and expulsion.

Accessory Organs «Liver, Pancreas, Gallbladder»

The mouse possesses three principal accessory organs that support digestive and metabolic processes: the liver, pancreas, and gallbladder. Each organ exhibits distinct anatomical features and physiological responsibilities that integrate with the gastrointestinal tract.

• Liver
  Location: occupies the right cranial abdominal cavity, extending beneath the diaphragm.
  Gross anatomy: lobulated parenchyma divided into right, left, and caudate lobes; surface covered by a thin connective tissue capsule.
  Histology: hepatocytes arranged in cords radiating from central veins; sinusoids lined with fenestrated endothelial cells; Kupffer cells scattered within sinusoidal lumen.
  Physiology: synthesizes plasma proteins, stores glycogen, detoxifies xenobiotics, and produces bile constituents. Bile flows into the biliary tree via intrahepatic ducts.

• Pancreas
  Location: retroperitoneal, situated between the duodenum and spleen, extending caudally along the left side of the abdominal cavity.
  Gross anatomy: composed of exocrine acini and endocrine islets of Langerhans; separated into head, body, and tail regions.
  Histology: acinar cells secrete digestive enzymes into a ductal network that merges with the main pancreatic duct; islets contain α‑, β‑, δ‑, and PP cells regulating glucose homeostasis.
  Physiology: releases amylase, lipase, proteases, and bicarbonate into the duodenum; secretes insulin, glucagon, somatostatin, and pancreatic polypeptide into the bloodstream.

• Gallbladder
  Location: resides on the visceral surface of the liver, adjacent to the cystic duct.
  Gross anatomy: thin‑walled, sac‑like organ capable of expanding to accommodate bile storage.
  Histology: mucosal epithelium of simple columnar cells, lamina propria with sparse smooth muscle, and a serosal covering.
  Physiology: concentrates and stores bile produced by the liver; releases bile into the cystic duct in response to cholecystokinin, facilitating lipid emulsification in the small intestine.

Collectively, these organs coordinate the processing, storage, and delivery of nutrients and metabolites, ensuring efficient digestion and systemic metabolic regulation in the mouse.

Respiratory System

Nasal Passages and Trachea

The mouse nasal cavity consists of paired passages lined with pseudostratified columnar epithelium that bears cilia and goblet cells. Respiratory turbinates increase surface area, supporting a dense capillary network that warms and humidifies inhaled air. The ventral region contains olfactory epithelium populated by sensory neurons, supporting cells, and basal cells, providing the primary site for odor detection. Mucus secreted by goblet cells traps particulate matter, while ciliary motion transports debris toward the nasopharynx for clearance.

Key structural and functional elements of the nasal passages include:

• Lateral and medial turbinates that create turbulent airflow.
• Olfactory epithelium with a high density of odorant receptors.
• Subepithelial venous plexus for thermoregulation of inhaled air.
• Rich innervation from the trigeminal and olfactory nerves facilitating sensory feedback.

The trachea extends caudally from the larynx, forming a rigid tube approximately 2 cm in length in adult mice. Its wall comprises C‑shaped hyaline cartilage rings that prevent collapse while allowing flexibility. The inner lining is a ciliated pseudostratified epithelium interspersed with mucus‑producing cells; coordinated ciliary beating and mucus flow constitute the mucociliary escalator, removing pathogens and particles. Longitudinal smooth muscle fibers encircle the trachea, modulating airway caliber through sympathetic and parasympathetic inputs.

Principal characteristics of the tracheal system are:

• 4–6 cartilage rings per millimeter, providing structural support.
• Ciliary beat frequency averaging 12–15 Hz, essential for mucus transport.
• Submucosal glands contributing to the aqueous component of airway secretions.
• Branching at the carina into primary bronchi, marking the transition to the lower respiratory tract.

Together, the nasal passages and trachea form a continuous conduit that conditions inhaled air, facilitates olfactory perception, and protects the pulmonary system from environmental hazards.

Lungs

Mouse lungs are paired organs situated within the thoracic cavity, extending from the thoracic inlet to the diaphragm. Each lung comprises three lobes: a larger right lung with four distinct lobules (cranial, middle, caudal, and accessory) and a left lung with a single lobe. The lobes are demarcated by connective tissue septa that support the pleural surface and maintain structural integrity during respiration.

The respiratory parenchyma consists of a branching bronchial tree that terminates in alveolar sacs. Bronchioles transition to terminal bronchioles, which give rise to respiratory bronchioles lined with cuboidal epithelium. Alveolar walls contain type I pneumocytes for gas diffusion and type II pneumocytes that secrete pulmonary surfactant, reducing surface tension and preventing alveolar collapse. Capillary networks envelop each alveolus, establishing a thin diffusion barrier for oxygen and carbon dioxide exchange.

Blood supply is delivered via the pulmonary artery, which follows the bronchial branches and branches into a dense capillary plexus. Venous return occurs through the pulmonary veins, which drain oxygen‑rich blood into the left atrium. Autonomic innervation originates from the vagus nerve, providing parasympathetic input that modulates bronchoconstriction, while sympathetic fibers influence bronchodilation and vascular tone.

Key physiological parameters in the mouse respiratory system include:

• Resting respiratory rate: 80–150 breaths per minute.
• Tidal volume: approximately 0.2 mL per gram of body weight.
• Minute ventilation: 12–20 mL g⁻¹ min⁻¹.
• Arterial oxygen tension (PaO₂): 80–100 mm Hg under normoxic conditions.

These values reflect the high metabolic demand and rapid gas exchange required for the mouse’s small body size and elevated activity levels.

Diaphragm

The diaphragm of the laboratory mouse is a dome‑shaped musculotendinous sheet separating the thoracic and abdominal cavities. Its central tendon is thin and fibrous, while the peripheral portions consist of skeletal muscle fibers arranged in three overlapping layers: a superficial costal layer, a middle sternal layer, and a deep lumbar layer. The costal fibers originate from the inner surfaces of the lower ribs, the sternal fibers attach to the xiphoid process, and the lumbar fibers arise from the upper lumbar vertebrae via the crura. This multilayered architecture provides both flexibility for rapid respiratory cycles and strength for generating intra‑abdominal pressure.

Innervation is supplied primarily by the phrenic nerve, a branch of the cervical spinal nerves C3–C5. Each hemidiaphragm receives a distinct phrenic branch that penetrates the muscle bundles, delivering motor fibers that trigger contraction and sensory fibers that convey proprioceptive feedback. The nerve supply allows synchronized activation with the intercostal muscles during inspiration.

Blood flow is delivered by the musculophrenic and pericardiacophrenic arteries, branches of the internal thoracic artery, and by the inferior phrenic arteries arising directly from the abdominal aorta. Venous drainage follows the corresponding veins into the brachiocephalic and hepatic veins, ensuring efficient exchange of gases and metabolites during high‑frequency breathing.

Functional contributions include:

• Lowering of the thoracic cavity during inspiratory contraction, creating negative pressure that draws air into the lungs.
• Elevation of intra‑abdominal pressure during activities such as defecation, urination, and forced expiration.
• Participation in non‑respiratory motor patterns, for example, vocalization and sneeze reflexes.

Developmentally, the diaphragm originates from the septum transversum, pleuroperitoneal membranes, and contributions of the body wall mesenchyme. In the mouse, the organ reaches its adult morphology by embryonic day 15.5, with the central tendon forming a continuous sheet that integrates the muscular crura. Histological sections reveal a predominance of type II (fast‑twitch) fibers, reflecting the species’ high metabolic rate and rapid respiratory rhythm.

Morphometric data indicate an average resting dome height of 2.8 mm and a surface area of approximately 45 mm² in adult C57BL/6 mice. These dimensions scale proportionally with body mass, allowing comparative studies across genetically modified strains.

Cardiovascular System

Heart

The mouse heart is a four‑chambered organ located centrally within the thoracic cavity, occupying approximately 0.1 % of total body mass. Its dimensions average 5–7 mm in length and 3–4 mm in width, with a wall thickness of 0.5–0.8 mm in the left ventricle. The myocardial wall consists of an outer epicardium, a middle myocardium rich in contractile fibers, and an inner endocardium lined with endothelial cells.

Blood enters the right atrium from the superior and inferior vena cava, passes through the tricuspid valve into the right ventricle, and is propelled into the pulmonary artery. After oxygenation in the lungs, blood returns via the pulmonary veins to the left atrium, traverses the mitral valve, and is expelled from the left ventricle through the aortic valve into systemic circulation. The four cardiac valves ensure unidirectional flow and prevent regurgitation.

Key physiological characteristics include:

• Resting heart rate: 300–700 beats min⁻¹, regulated by autonomic input.
• Stroke volume: 0.1–0.2 µL, adjusted by preload, afterload, and contractility.
• Cardiac output: 0.2–0.4 mL min⁻¹, sufficient to meet metabolic demands of a small mammal.
• Conduction system: sino‑atrial node initiates impulses, atrioventricular node delays transmission, and Purkinje fibers distribute the signal throughout the ventricles.

The mouse heart exhibits a higher basal metabolic rate than larger mammals, reflected in its rapid contractile cycle and pronounced sympathetic tone. Experimental manipulation of genetically engineered mice leverages these physiological traits to model human cardiac diseases, allowing precise assessment of molecular pathways, drug effects, and electrophysiological alterations.

Arteries and Veins

The murine circulatory network consists of a high‑pressure arterial tree that distributes oxygenated blood from the heart to peripheral tissues and a low‑pressure venous system that returns deoxygenated blood to the right atrium. Arteries possess thick tunica media layers of smooth muscle and elastic fibers, enabling them to sustain systolic pressure and regulate flow through vasoconstriction and vasodilation. Veins have thinner walls, larger lumens, and prominent adventitial collagen, facilitating capacitance and venous return assisted by skeletal muscle contraction and respiratory movements.

Key arterial trunks include:

• Aortic arch and descending thoracic aorta
• Carotid arteries supplying the brain
• Subclavian arteries serving forelimbs
• Renal arteries delivering blood to the kidneys
• Mesenteric arteries perfusing the gastrointestinal tract

Corresponding major veins comprise:

• Inferior vena cava collecting systemic blood
• Jugular veins draining cranial circulation
• Subclavian veins returning forelimb flow
• Renal veins carrying filtered blood from the kidneys
• Portal vein channeling nutrient‑rich blood from the intestines to the liver

Physiological differences are reflected in pressure gradients: systolic arterial pressure in adult mice averages 100–110 mm Hg, while central venous pressure remains near 2–5 mm Hg. Pulse wave velocity, measured along the aorta, reaches 2.5 m s⁻¹, indicating high arterial compliance. Venous return depends on the pressure‑gradient between peripheral veins and the right atrium and is enhanced by the presence of one‑way valves that prevent backflow.

Blood flow regulation involves autonomic innervation and local metabolic signaling. Sympathetic fibers increase arterial tone, reducing lumen diameter and elevating resistance. Endothelial release of nitric oxide induces smooth‑muscle relaxation, lowering resistance and promoting tissue perfusion. The coordinated action of these mechanisms sustains tissue oxygenation and removes metabolic waste throughout the mouse body.

Capillaries

Capillaries in Mus musculus form the terminal vascular network where plasma, gases, nutrients, and metabolic waste are exchanged between blood and tissue interstitium. The vessels consist of a single layer of endothelial cells supported by a thin basal lamina; pericytes embed the basement membrane and modulate diameter through contractile activity. Endothelial junctions determine permeability: continuous capillaries possess tight junctions, fenestrated capillaries contain transcellular pores, and sinusoidal capillaries exhibit large gaps, allowing passage of cells and macromolecules.

Capillary density varies markedly across organs, reflecting metabolic demand. Typical values include:

• Skeletal muscle: 400–800 mm⁻², supporting rapid oxygen delivery during locomotion.
• Lung alveolar septa: dense continuous network enabling efficient O₂/CO₂ diffusion.
• Cerebral cortex: 600–900 mm⁻², with a predominance of tight‑junctioned vessels that maintain the blood‑brain barrier.
• Liver: sinusoidal capillaries with a surface area exceeding 10 m², facilitating plasma protein synthesis and detoxification.

Blood flow through capillaries is regulated by pericyte tone and endothelial release of vasoactive substances such as nitric oxide and endothelin‑1. In response to increased tissue activity, arteriolar precapillary sphincters relax, recruiting previously dormant capillaries and raising total exchange surface. This recruitment mechanism contributes to the mouse’s high basal metabolic rate and its ability to sustain rapid temperature fluctuations.

Capillary permeability is modulated by transcellular pathways (caveolae-mediated transport) and paracellular routes (junctional remodeling). In inflammatory conditions, endothelial expression of adhesion molecules promotes leukocyte extravasation, while cytokine‑induced widening of intercellular gaps accelerates plasma protein leakage. The precise structural organization and dynamic control of mouse capillaries underpin the organism’s capacity for efficient nutrient delivery, waste removal, and rapid physiological adaptation.

Blood Composition

Blood in the laboratory mouse consists of a liquid phase (plasma) and a cellular fraction. Plasma occupies roughly 55 % of total volume and contains water, electrolytes, nutrients, hormones, waste products, and plasma proteins such as albumin, globulins, and fibrinogen. The cellular fraction comprises erythrocytes, leukocytes, and platelets, each contributing to distinct physiological functions.

• Erythrocytes: Biconcave cells that transport oxygen bound to hemoglobin; typical mouse hematocrit ranges from 40 % to 50 %.
• Leukocytes: Diverse white‑blood cells classified into neutrophils, lymphocytes, monocytes, eosinophils, and basophils; total white‑cell count averages 5–10 × 10⁹ cells L⁻¹.
• Platelets: Small, anucleate fragments that mediate clot formation; platelet count in mice is approximately 800–1,200 × 10⁹ cells L⁻¹.

Plasma protein concentration averages 60–70 g L⁻¹, with albumin representing about 60 % of total protein mass. Electrolyte concentrations follow patterns similar to other mammals: sodium 140 mmol L⁻¹, potassium 4.5 mmol L⁻¹, chloride 100 mmol L⁻¹, and calcium 2.5 mmol L⁻¹. Glucose levels in fasting mice typically range from 5 to 8 mmol L⁻¹, while blood urea nitrogen averages 5–10 mmol L⁻¹.

The composition of mouse blood provides a reliable baseline for comparative studies, toxicology assessments, and genetic investigations. Deviations from these reference values indicate physiological stress, disease, or experimental manipulation.

Nervous System

Central Nervous System «Brain, Spinal Cord»

The central nervous system of the laboratory mouse consists of a compact brain and a slender spinal cord that together coordinate sensory input, motor output, and autonomic regulation.

The mouse brain occupies approximately 0.4 % of total body mass and contains an estimated 71 million neurons. Its organization mirrors that of other mammals and can be divided into three primary zones:

• Forebrain – cerebral cortex, olfactory bulb, hippocampus, basal ganglia, and thalamus; responsible for higher‑order processing, memory formation, and olfactory perception.
• Midbrain – tectum and tegmentum; integrates visual and auditory reflexes.
• Hindbrain – cerebellum, pons, and medulla oblongata; controls balance, coordination, and basic life‑support functions.

The spinal cord extends from the medulla to the lumbar region, measuring roughly 7 cm in a typical adult mouse. It is organized into 31 paired spinal segments, each containing:

• Dorsal (posterior) horn – receives afferent sensory fibers from dorsal root ganglia.
• Ventral (anterior) horn – houses motor neurons that project to skeletal muscles.
• Intermediate zone – integrates interneuronal circuits for reflex arcs.

White matter surrounds the gray core, forming dorsal, lateral, and ventral funiculi that transmit ascending sensory tracts and descending motor pathways. Myelination, primarily by oligodendrocytes, accelerates conduction velocities to 30–40 m s⁻¹, enabling rapid reflexes and coordinated locomotion.

Physiologically, neuronal communication relies on action potentials generated at the axon hillock, propagated along myelinated fibers, and terminated by synaptic release of neurotransmitters such as glutamate, GABA, and acetylcholine. Synaptic plasticity within the hippocampus and cortex underlies learning and memory, while cerebellar circuitry refines motor timing through long‑term depression of parallel‑fiber synapses.

Collectively, the mouse central nervous system provides a streamlined yet fully functional platform for investigating neurobiological mechanisms that translate to broader mammalian physiology.

Peripheral Nervous System

The peripheral nervous system (PNS) of the laboratory mouse comprises all neural elements located outside the central nervous system, linking the brain and spinal cord with peripheral tissues. It consists of cranial and spinal nerves, associated ganglia, and autonomic pathways that regulate somatic and visceral functions.

Cranial nerves (12 pairs) emerge from the brainstem and innervate head structures, providing sensory input from the olfactory epithelium, retina, and oral cavity, and motor output to facial muscles and tongue. Spinal nerves (31 pairs) arise from each spinal segment, each dividing into dorsal (sensory) and ventral (motor) roots that join to form mixed peripheral nerves. These nerves distribute to limbs, trunk, and tail, transmitting tactile, proprioceptive, and nociceptive signals while delivering motor commands to skeletal muscles.

Key components of the mouse PNS include:

• Dorsal root ganglia (DRG): clusters of sensory neuron cell bodies that relay peripheral afferent signals to the spinal cord.
• Autonomic ganglia: sympathetic chain ganglia and parasympathetic nuclei that control heart rate, respiration, gastrointestinal motility, and glandular secretion.
• Neuromuscular junctions: specialized synapses where motor axons terminate on skeletal muscle fibers, enabling rapid contraction.
• Myelinated and unmyelinated fibers: large-diameter, myelinated axons conduct impulses at high velocity; small-diameter, unmyelinated C-fibers transmit slower, diffuse pain and temperature information.

Physiologically, the mouse PNS maintains homeostasis through reflex arcs that bypass central processing, such as the withdrawal reflex mediated by spinal interneurons. Autonomic circuits modulate organ function via sympathetic activation (release of norepinephrine) and parasympathetic signaling (acetylcholine release), producing complementary effects on cardiovascular and digestive systems.

Developmentally, peripheral neurons arise from neural crest cells, migrate to target sites, and extend growth cones guided by molecular cues (e.g., neurotrophins). Adult mice exhibit continuous remodeling of peripheral terminals, allowing adaptation to injury and environmental changes.

Overall, the peripheral nervous system integrates sensory detection, motor execution, and autonomic regulation, forming the external interface through which the mouse interacts with its environment and sustains internal physiological balance.

Sensory Organs

The mouse possesses a compact yet highly specialized suite of sensory organs that enable rapid detection of environmental cues essential for survival.

Vision is mediated by large, laterally positioned eyes with a relatively high rod density, providing acute scotopic sensitivity. The retina contains a multilayered architecture of photoreceptors, bipolar cells, and ganglion cells that converge onto the optic nerve, transmitting signals to the superior colliculus and visual cortex. Pupil dilation is controlled by autonomic innervation, allowing rapid adjustment to light intensity.

Auditory perception relies on a streamlined outer ear leading to the tympanic membrane, which vibrates the ossicular chain (malleus, incus, stapes). These vibrations amplify sound pressure and transmit it to the cochlear fluid. Hair cells within the organ of Corti convert mechanical displacement into electrical impulses that travel via the auditory nerve to the brainstem and auditory cortex. Frequency discrimination extends up to 100 kHz, surpassing many mammals.

Olfaction is facilitated by an extensive nasal epithelium populated with millions of olfactory receptor neurons. Each neuron expresses a single G‑protein‑coupled receptor, binding volatile molecules and generating depolarizing currents. Signals are routed through the olfactory bulb to limbic structures, influencing feeding and predator avoidance behaviors.

Tactile sensing is dominated by the vibrissal system. Whiskers are anchored in follicular capsules rich in mechanoreceptors, including Merkel cells and lanceolate endings. Deflection of a whisker produces graded receptor potentials that travel via the trigeminal nerve to the somatosensory cortex, enabling precise spatial mapping of objects.

Taste buds, primarily located on the circumvallate and foliate papillae of the tongue, contain type I, II, and III cells that detect sweet, bitter, umami, salty, and sour stimuli. Transduction mechanisms involve G‑protein signaling and ion channel modulation, with gustatory fibers projecting to the nucleus of the solitary tract.

Collectively, these organs integrate multimodal inputs, allowing the mouse to navigate complex habitats, locate food, and evade threats with remarkable efficiency.

Endocrine System

Pituitary Gland

The pituitary gland in the laboratory mouse is a compact endocrine organ situated at the base of the brain, nestled within the sella turcica of the sphenoid bone and connected to the hypothalamus by the pituitary stalk (infundibulum). It consists of three distinct lobes—anterior (adenohypophysis), intermediate (pars intermedia), and posterior (neurohypophysis)—each composed of specialized cell populations that secrete defined hormone groups.

The anterior lobe contains somatotrophs, lactotrophs, corticotrophs, thyrotrophs, and gonadotrophs. These cells synthesize and release, respectively, growth hormone (GH), prolactin (PRL), adrenocorticotropic hormone (ACTH), thyroid‑stimulating hormone (TSH), and the gonadotropins luteinizing hormone (LH) and follicle‑stimulating hormone (FSH). Hormone secretion is regulated primarily by hypothalamic releasing and inhibiting factors delivered through the hypophyseal portal circulation.

The intermediate lobe is relatively small in adult mice but prominent during embryogenesis. It produces melanocyte‑stimulating hormone (MSH), contributing to pigment regulation and energy balance.

The posterior lobe stores neurohormones synthesized in hypothalamic nuclei. Oxytocin and vasopressin (antidiuretic hormone, ADH) are released directly into the systemic circulation in response to neuronal firing.

Key physiological functions of the mouse pituitary include:

• Growth regulation via GH, influencing skeletal development and tissue remodeling.
• Reproductive cycle control through LH and FSH, modulating gonadal steroidogenesis and gametogenesis.
• Metabolic homeostasis via ACTH‑driven cortisol production, TSH‑induced thyroid activity, and PRL’s effect on lactation.
• Fluid balance and uterine contractility mediated by ADH and oxytocin.

Vascular supply to the gland is dense; arterial branches from the internal carotid arteries form a portal capillary network that transports hypothalamic hormones to the anterior lobe, while the posterior lobe receives direct arterial input. The gland’s feedback loops involve peripheral hormone levels that inhibit or stimulate hypothalamic releasing factors, maintaining endocrine equilibrium.

Developmentally, pituitary organogenesis in mice proceeds from Rathke’s pouch, an ectodermal invagination that differentiates into the adenohypophysis, while the neurohypophysis originates from the neuroectoderm of the diencephalon. Genetic models, such as Pit1‑deficient mice, have clarified lineage specification and hormone synthesis pathways, reinforcing the gland’s utility in experimental physiology.

Overall, the mouse pituitary integrates central neural signals with peripheral endocrine responses, serving as a pivotal hub in the organism’s structural and functional regulation.

Thyroid Gland

The thyroid gland of the laboratory mouse is a paired, bilobed organ situated anterior to the trachea, extending from the laryngeal cartilage to the thoracic inlet. Each lobe consists of a capsule of dense connective tissue that encloses multiple follicles, the fundamental functional units. Follicles contain a lumen filled with colloid, primarily composed of thyroglobulin, the precursor of thyroid hormones.

Follicular epithelial cells line the colloid and synthesize thyroxine (T4) and triiodothyronine (T3) through iodination of tyrosine residues on thyroglobulin. Upon stimulation, these cells endocytose colloid, cleave thyroglobulin, and release T3 and T4 into the bloodstream. Interspersed among the follicular cells are parafollicular (C) cells that secrete calcitonin, a peptide regulator of calcium homeostasis.

Key physiological attributes include:

• Hormone production: Continuous synthesis of T4 and T3; conversion of T4 to the more active T3 occurs in peripheral tissues.
• Calcium regulation: Calcitonin release in response to elevated plasma calcium concentrations.
• Metabolic influence: Thyroid hormones modulate basal metabolic rate, thermogenesis, and protein synthesis.
• Developmental impact: Hormone levels affect growth, skeletal maturation, and neural development during the post‑natal period.

The gland receives arterial blood from the superior thyroid arteries, branches of the external carotid system, and drains via the thyroid veins into the brachiocephalic veins. Innervation is provided by sympathetic fibers originating from the cervical sympathetic trunk, influencing hormone secretion through adrenergic signaling.

Embryologically, the mouse thyroid arises from an endodermal thickening in the pharyngeal floor, descends along the midline, and differentiates into follicular and parafollicular cell lineages under the control of transcription factors such as Nkx2‑1 and Pax8. Genetic manipulation of these pathways produces predictable alterations in gland size, hormone output, and systemic metabolic parameters, making the mouse thyroid a valuable model for endocrine research.

Adrenal Glands

The adrenal glands of the laboratory mouse are paired, retroperitoneal organs situated cranial to the kidneys and attached to the caudal pole of each kidney by a connective tissue capsule. Each gland measures approximately 2–3 mm in length and is encapsulated by a dense fibrous layer that separates it from surrounding adipose tissue.

Structurally, the gland consists of two distinct regions. The outer cortex is subdivided into three concentric zones: the zona glomerulosa, which synthesizes mineralocorticoids; the zona fasciculata, which produces glucocorticoids; and the zona reticularis, responsible for androgen precursors. Beneath the cortex lies the medulla, composed of chromaffin cells that store and secrete catecholamines. Histologically, cortical cells display lipid-rich cytoplasm, whereas medullary cells exhibit dense-core secretory granules visible under electron microscopy.

Hormonal output includes:

• Mineralocorticoids (predominantly aldosterone) – regulate sodium‑potassium balance and blood pressure.
• Glucocorticoids (corticosterone) – modulate glucose metabolism, immune function, and stress adaptation.
• Androgen precursors (dehydroepiandrosterone sulfate) – serve as substrates for peripheral sex‑steroid synthesis.
• Catecholamines (epinephrine and norepinephrine) – mediate acute stress responses and cardiovascular regulation.

Physiologically, adrenal secretion integrates with the hypothalamic‑pituitary‑adrenal axis. Corticosterone release follows pulsatile hypothalamic corticotropin‑releasing hormone stimulation, while catecholamine discharge is triggered by sympathetic innervation. In experimental mouse models, adrenal function is frequently assessed through plasma hormone quantification, adrenalectomy, or pharmacological manipulation to elucidate disease mechanisms such as metabolic syndrome, hypertension, and stress‑related disorders.

Pancreas «Endocrine Function»

The endocrine pancreas of the laboratory mouse is confined to the core of the organ and consists of discrete clusters of hormone‑producing cells known as the islets of Langerhans. These microstructures are interspersed among the exocrine acini and receive a rich vascular supply that enables rapid delivery of secreted hormones into the circulation.

Islets contain four principal cell types arranged in a characteristic pattern: β‑cells dominate the interior, α‑cells occupy the periphery, δ‑cells are interspersed, and pancreatic polypeptide (PP) cells are found in smaller peripheral clusters. Each cell type synthesizes a specific peptide hormone that regulates metabolic homeostasis.

• Insulin (β‑cells): lowers blood glucose by promoting cellular uptake and storage of glucose, glycogen synthesis, and lipogenesis.
• Glucagon (α‑cells): raises blood glucose through hepatic glycogenolysis and gluconeogenesis.
• Somatostatin (δ‑cells): inhibits both insulin and glucagon secretion, modulating the activity of neighboring islet cells and gastrointestinal hormone release.
• Pancreatic polypeptide (PP cells): influences gastric motility, pancreatic enzyme secretion, and energy expenditure.

Secretion of these hormones is tightly coupled to plasma nutrient levels. Rising glucose concentrations trigger rapid insulin release, while falling glucose stimulates glucagon output. Somatostatin provides a negative feedback loop that prevents excessive hormone fluctuations. The coordinated endocrine response maintains euglycemia, supports growth, and adapts to fasting or feeding cycles. In mouse models, alterations in islet architecture or hormone output serve as primary indicators of metabolic disease and are central to studies of diabetes, obesity, and endocrine regulation.

Gonads

The gonads of the laboratory mouse consist of paired testes in males and paired ovaries in females. Both organs are encapsulated by a thin fibrous tunic that separates them from surrounding adipose tissue and provides attachment points for the vascular and nervous supply.

In the testis, seminiferous tubules occupy the majority of the parenchyma. Each tubule is lined by Sertoli cells that support the development of germ cells through the stages of spermatogenesis. Leydig cells reside in the interstitial space, producing testosterone that regulates secondary sexual characteristics and spermatogenic activity. The blood‑testis barrier, formed by tight junctions between adjacent Sertoli cells, creates a specialized microenvironment essential for germ cell maturation.

The ovary contains a cortex packed with follicles at various developmental stages and a medulla rich in blood vessels, lymphatics, and stromal cells. Granulosa cells surround the oocyte within each follicle, providing nutrients and producing estrogen. Theca cells, positioned external to the granulosa layer, synthesize androgens that granulosa cells convert to estrogen. After ovulation, the ruptured follicle transforms into the corpus luteum, a transient endocrine structure that secretes progesterone to support potential pregnancy.

Key physiological functions of mouse gonads include:

• Production of gametes (spermatozoa in testes, oocytes in ovaries)
• Synthesis of sex steroids (testosterone, estradiol, progesterone)
• Regulation of the hypothalamic‑pituitary‑gonadal axis via feedback loops
• Provision of paracrine signals that influence local tissue remodeling

Developmentally, gonadal differentiation follows a genetically programmed pathway. The presence of the Sry gene on the Y chromosome initiates testis formation, while its absence allows ovarian development. Early embryonic stages feature a bipotential gonadal ridge that later diverges into either testicular cords or ovarian follicles.

In experimental contexts, mouse gonads serve as models for investigating endocrine disorders, reproductive toxicology, and gene function. Their small size, well‑characterized genome, and rapid reproductive cycle facilitate high‑throughput studies of hormonal regulation, meiotic processes, and stem‑cell dynamics.

Overall, the structural organization and endocrine output of mouse gonads provide a coherent framework for understanding mammalian reproductive biology and for translating findings to broader biomedical research.

Urinary System

Kidneys

The mouse kidney is a paired organ situated retroperitoneally on either side of the vertebral column, extending from the lumbar vertebrae L2 to L5. Each kidney measures approximately 1.5 cm in length, weighs 120–150 mg, and is enveloped by a thin fibrous capsule that protects the underlying parenchyma.

Renal architecture mirrors that of other mammals, comprising an outer cortex and an inner medulla. The cortex contains the bulk of glomeruli, proximal and distal tubules, and the initial segments of the loop of Henle. The medulla is organized into parallel, urine‑concentrating columns called papillary cones, each formed by descending and ascending limbs of the loop of Henle and collecting ducts. The renal pelvis converges into a single ureter that transports urine to the bladder.

Key functional components include:

• Glomeruli: Spherical capillary networks that filter plasma at a rate of ~0.5 µL/min per kidney, producing primary urine.
• Proximal tubule: Reabsorbs ~65 % of filtered water, electrolytes, glucose, and amino acids via active and passive transport.
• Loop of Henle: Generates an osmotic gradient that enables water reabsorption in the descending limb and sodium chloride reabsorption in the ascending limb.
• Distal tubule and collecting duct: Fine‑tune electrolyte balance and urine volume under hormonal control (e.g., aldosterone, antidiuretic hormone).
• Vasa recta: Counter‑current blood vessels that preserve the medullary osmotic gradient while supplying oxygen and nutrients.

Blood supply originates from the renal artery, which branches into segmental arteries, interlobar arteries, arcuate arteries, and finally interlobular arteries that feed the glomeruli. Venous drainage follows a parallel pathway through interlobular veins to the renal vein, returning deoxygenated blood to the inferior vena cava.

Physiologically, mouse kidneys maintain fluid homeostasis, regulate plasma electrolytes, and excrete metabolic waste such as urea and creatinine. Their high metabolic rate and rapid urine turnover make them valuable models for studying renal disease, drug nephrotoxicity, and genetic mutations affecting kidney function. Comparative analysis reveals a shorter nephron length and higher glomerular filtration fraction relative to larger mammals, reflecting adaptations to the mouse’s small body size and high basal metabolic demand.

Ureters

The ureters are paired muscular tubes that convey urine from each kidney to the urinary bladder in the laboratory mouse. Each organ measures approximately 3–4 cm in length and 0.2 mm in internal diameter, dimensions that reflect the animal’s small body size while maintaining sufficient lumen for continuous urine flow.

The wall of the mouse ureter consists of three concentric layers:

• An inner urothelium composed of transitional epithelium, providing a barrier against urine toxins and allowing stretch during peristalsis.
• A middle smooth‑muscle layer arranged in an inner circular and an outer longitudinal orientation, generating the peristaltic waves that propel urine.
• An outer adventitial connective tissue containing blood vessels, lymphatics, and nerves that supply the organ.

Arterial blood is delivered primarily by branches of the renal and internal iliac arteries, while venous drainage follows the corresponding veins. Autonomic innervation derives from sympathetic fibers of the lumbar spinal cord and parasympathetic fibers of the pelvic plexus, modulating the frequency and force of ureteral contractions.

Physiologically, the ureters maintain a unidirectional flow through coordinated peristaltic activity. Intramural pressure gradients are established by rhythmic smooth‑muscle contraction, preventing retrograde movement of urine and ensuring efficient emptying of the renal pelvis into the bladder. In mice, the peristaltic frequency averages 2–3 cycles per minute under basal conditions, with adjustments mediated by neural and hormonal signals such as adrenergic and antidiuretic hormones.

Developmentally, ureteral formation originates from the ureteric bud, an outgrowth of the metanephric mesenchyme that elongates and branches to form the collecting system. Mutations affecting bud branching or smooth‑muscle differentiation produce congenital anomalies, including hydronephrosis and obstructive uropathy, which are frequently used as models for human renal disease.

Research employing mouse ureters benefits from their accessibility for ex vivo perfusion studies, high‑resolution imaging, and genetic manipulation. The organ’s simple architecture and well‑characterized innervation make it a reliable system for investigating renal physiology, drug delivery, and the mechanisms underlying urinary tract disorders.

Bladder

The mouse urinary bladder is a thin‑walled, distensible organ situated in the ventral pelvis, posterior to the pubic symphysis. Its external surface is covered by serosa on the dorsal aspect and peritoneum on the ventral aspect, providing a smooth interface with adjacent structures.

The bladder wall consists of three concentric layers. The innermost mucosa includes a transitional epithelium (urothelium) that forms a barrier to urine constituents and a lamina propria rich in connective tissue and blood vessels. The middle muscular layer (detrusor) is composed of smooth muscle fibers arranged in interlacing bundles, allowing coordinated contraction during voiding. The outermost adventitia contains collagen and elastic fibers that tether the bladder to surrounding tissues and accommodate changes in volume.

Innervation is supplied by autonomic and sensory fibers. Parasympathetic input from the pelvic nerve releases acetylcholine, inducing detrusor contraction. Sympathetic fibers from the hypogastric nerve release norepinephrine, promoting detrusor relaxation and internal sphincter closure during storage. Sensory afferents convey stretch information to the spinal cord, regulating reflex pathways.

Physiologically, the bladder functions as a reservoir with high compliance. At low intravesical pressures (<5 cm H₂O), the organ can expand from a resting volume of approximately 30 µL to a maximum capacity of 250–300 µL without significant pressure increase. Void initiation involves a rapid rise in detrusor pressure (>20 cm H₂O) and relaxation of the internal sphincter, resulting in urine expulsion within 0.2–0.4 seconds.

Key parameters commonly measured in experimental studies include:

• Resting pressure
• Maximum capacity
• Compliance (Δvolume/Δpressure)
• Peak voiding pressure
• Residual volume post‑void

The mouse bladder’s small size and well‑characterized neuro‑urogenic control make it a standard model for investigations of urinary disorders, pharmacological testing, and genetic manipulation. Histological analyses routinely employ hematoxylin‑eosin staining for muscle architecture and immunohistochemistry for urothelial markers (e.g., uroplakin III) and neuronal proteins (e.g., choline acetyltransferase).

Urethra

The urethra in the laboratory mouse is a short tubular conduit that conveys urine from the urinary bladder to the exterior. In males, the urethra extends through the penis and is divided into prostatic, membranous, and spongy segments; in females, it is a single, straight tube terminating at the vestibule. Average lengths are approximately 1.2 cm in males and 0.6 cm in females, with diameters of 0.2–0.3 mm, reflecting the small body size of the species.

Histologically, the urethral wall consists of an inner urothelial layer, a lamina propria rich in connective tissue, and an outer smooth‑muscle layer. The smooth muscle is arranged in circular and longitudinal bundles, providing peristaltic propulsion of urine and contributing to sphincteric closure. Adjacent glands, such as the bulbourethral glands in males, secrete fluid that lubricates the urethral lumen.

Physiological control involves autonomic innervation: parasympathetic fibers stimulate detrusor contraction and urethral relaxation, while sympathetic fibers maintain tonic contraction of the internal sphincter. Voluntary control of the external sphincter is mediated by somatic motor neurons originating in the spinal cord.

Key characteristics of the mouse urethra:

• Length: 0.6 cm (female), 1.2 cm (male)
• Diameter: 0.2–0.3 mm
• Wall composition: urothelium, connective tissue, smooth muscle
• Segmental division (male): prostatic, membranous, spongy
• Innervation: parasympathetic (cholinergic), sympathetic (adrenergic), somatic (pudendal)

Understanding these structural and functional details is essential for interpreting urinary physiology, evaluating disease models such as infection or obstruction, and designing surgical or pharmacological interventions in murine research.

Reproductive System

Male Reproductive Organs

The male reproductive system of the laboratory mouse consists of paired testes, epididymides, vas deferens, seminal vesicles, prostate, bulbourethral glands, and the penis. Each component contributes to sperm production, maturation, storage, and delivery.

The testes are enclosed by a fibrous tunica albuginea and contain seminiferous tubules where spermatogenesis occurs. Sertoli cells support germ cell development, while Leydig cells in the interstitium synthesize testosterone under luteinizing hormone regulation. Hormonal feedback maintains steady plasma androgen levels essential for sexual maturation.

Spermatozoa exit the seminiferous tubules into the rete testis, travel through the efferent ducts, and enter the epididymis. The epididymis is a highly coiled duct divided into caput, corpus, and cauda regions. During passage, sperm acquire motility and fertilization capacity through exposure to epididymal secretions rich in proteins, lipids, and enzymes.

The vas deferens transports mature sperm from the cauda epididymis to the ejaculatory ducts. Muscular layers generate peristaltic contractions that propel sperm during ejaculation. The seminal vesicles, positioned laterally to the vas deferens, secrete a viscous fluid containing fructose, prostaglandins, and coagulating proteins that provide energy and facilitate sperm motility.

The prostate gland surrounds the urethra at the neck of the bladder. Its secretions contribute to the seminal plasma, adjusting pH and supplying additional nutrients. Bulbourethral glands, located near the urethral opening, release a clear mucus that lubricates the urethra and neutralizes residual acidity.

The penis comprises a corpus cavernosum and a glans. During copulation, erectile tissue engorges with blood, allowing intromission. The urethra conveys the mixed ejaculate from the accessory glands and vas deferens to the exterior.

Key physiological features:

• Testosterone production by Leydig cells regulates spermatogenesis and secondary sexual characteristics.
• Sperm maturation is completed in the cauda epididymis, where motility and membrane remodeling occur.
• Accessory gland secretions modify seminal plasma composition, supporting sperm viability and transport.
• Neural and hormonal signals coordinate vas deferens contraction, prostate secretion, and penile erection during mating.

Understanding the architecture and function of these organs provides a foundation for experimental studies on fertility, endocrine disruption, and reproductive toxicology in murine models.

Female Reproductive Organs

The female mouse reproductive system consists of paired ovaries, oviducts (fallopian tubes), a uterus, cervix, and vagina, each adapted for rapid estrous cycles and high fecundity.

The ovaries are almond‑shaped, located near the dorsal abdominal wall. They contain follicles at various developmental stages, from primordial to pre‑ovulatory, and a corpus luteum that forms after ovulation to secrete progesterone. Granulosa and theca cells surround the oocyte, providing hormonal regulation and nutrient support.

Oviducts extend from the ovaries to the uterine horns. Their luminal epithelium features ciliated cells that generate directed flow, facilitating transport of oocytes and embryos. Muscular layers provide peristaltic contractions that aid movement and timing of fertilization.

The uterus is bicornuate, comprising two elongated horns that converge at a single cervix. The endometrium undergoes cyclic remodeling under estrogen and progesterone influence, preparing for implantation. Myometrial smooth muscle contracts during parturition, expelling neonates.

The cervix connects the uterine lumen to the vagina and functions as a barrier, regulating sperm entry and protecting the uterine environment. Its epithelium secretes mucus whose viscosity changes across the estrous cycle.

The vagina is a short, muscular tube lined by stratified squamous epithelium. It serves as the copulatory tract and birth canal, providing a protective environment for sperm and newborns.

Key physiological features:

• Estrous cycle length: 4–5 days, driven by pulsatile gonadotropin release.
• Ovulation: typically one to two oocytes per cycle, released from the dominant follicle.
• Hormonal control: hypothalamic‑pituitary‑ovarian axis regulates estrogen, progesterone, LH, and FSH.
• Litter size: average of 6–8 pups, reflecting efficient ovulation and implantation processes.

Understanding these structures and their coordinated functions is essential for interpreting experimental outcomes in reproductive biology, toxicology, and genetic studies involving laboratory mice.

Immune System

Lymph Nodes

Lymph nodes in the laboratory mouse are encapsulated, bean‑shaped organs distributed along the major lymphatic vessels. Each node consists of a cortex containing densely packed lymphoid follicles, a paracortex populated by T‑lymphocytes, and a medulla with cords of plasma cells and macrophages. The capsule, composed of dense connective tissue, is penetrated by afferent lymphatic vessels that deliver interstitial fluid, antigens, and immune cells from peripheral tissues.

The internal architecture supports antigen surveillance and immune activation. Follicles undergo germinal‑center formation following antigen exposure, leading to somatic hypermutation and class‑switch recombination in B cells. The paracortex provides a site for T‑cell priming by dendritic cells that have migrated from peripheral sites. Medullary sinuses facilitate the passage of filtered lymph toward efferent vessels, which drain into the thoracic duct or other central lymphatic pathways.

Key physiological parameters in mouse lymph nodes include:

• Cellular composition: B cells (~60 % of total lymphocytes), T cells (~30 %), macrophages and dendritic cells (~10 %).
• Size range: 0.5–2 mm in short axis, varying with strain, age, and immunological status.
• Blood supply: High‑density high endothelial venules (HEVs) permit selective lymphocyte entry from the bloodstream.
• Cytokine milieu: Interleukin‑6, interferon‑γ, and tumor necrosis factor‑α dominate during active immune responses, influencing cell proliferation and differentiation.

Lymphatic drainage patterns are species‑specific; in mice, the cervical, mesenteric, inguinal, and popliteal nodes serve as primary collection points for head‑neck, gastrointestinal, and hind‑limb tissues, respectively. Surgical removal or genetic ablation of specific nodes alters systemic immune dynamics, demonstrating their essential function in coordinating adaptive immunity while maintaining fluid homeostasis.

Spleen

The spleen of the laboratory mouse is a compact, ovoid organ situated in the left dorsal abdominal cavity, adjacent to the stomach and beneath the diaphragm. Its average length is 12–15 mm, width 5–7 mm, and thickness 3–4 mm; the capsule is composed of dense connective tissue that encloses the parenchyma.

Microscopically, the organ consists of white pulp and red pulp. White pulp is organized around central arterioles, forming periarteriolar lymphoid sheaths (PALS) populated by T‑lymphocytes, with adjacent lymphoid follicles containing B‑cells. Red pulp comprises a network of splenic cords (cords of Billroth) interspersed with sinusoids lined by endothelial cells and supported by reticular fibers. Macrophages line the sinusoidal walls and populate the cords, facilitating phagocytosis of aged erythrocytes.

Blood enters the spleen via the splenic artery, which branches into central arterioles within the white pulp. Outflow occurs through the splenic vein, draining into the portal venous system. The arterial–venous architecture permits both open circulation, where blood percolates through the cords before re‑entering sinusoids, and closed circulation, in which erythrocytes pass directly through endothelial channels.

Functional responsibilities include:

• Removal of senescent or damaged red blood cells and platelets.
• Phagocytic clearance of blood‑borne pathogens and debris.
• Sequestration and recycling of iron from hemoglobin.
• Initiation of adaptive immune responses via antigen presentation in the white pulp.

In experimental protocols, the mouse spleen serves as a source of lymphocytes for in‑vitro culture, a site for assessing splenomegaly in disease models, and a target organ for evaluating splenic irradiation or surgical removal. Standard fixation for histology employs 10 % neutral‑buffered formalin, and cryosectioning preserves antigenicity for immunofluorescence studies.

Thymus

The thymus of the laboratory mouse is a bilobed organ situated in the ventral mediastinum, anterior to the heart and between the sternum and the trachea. Each lobe is encapsulated by a dense connective tissue sheath that merges with the pericardial fascia. The organ measures approximately 8–10 mm in length in adult C57BL/6 mice and exhibits a pale, lobulated appearance due to its high lymphoid cellularity.

Microscopically, the thymus is divided into an outer cortex and an inner medulla. The cortex contains densely packed immature thymocytes interspersed with epithelial reticular cells, dendritic cells, and macrophages. The medulla houses fewer thymocytes, mature medullary epithelial cells, and Hassall’s corpuscles—concentric keratinized structures unique to the organ. A distinct vascular network supplies the cortex, while the medulla receives blood through post‑capillary venules.

Primary physiological functions include:

• Induction of T‑cell lineage commitment from bone‑marrow progenitors.
• Positive selection of thymocytes capable of recognizing self‑MHC molecules.
• Negative selection eliminating autoreactive T‑cells to prevent autoimmunity.
• Generation of regulatory T‑cells that modulate peripheral immune responses.

During post‑natal development, the thymus reaches maximal cellularity around 4–6 weeks of age, after which involution commences, characterized by a gradual replacement of lymphoid tissue with adipocytes. In aged mice, the organ retains residual epithelial compartments that continue to support low‑level T‑cell output.

Experimental studies frequently utilize the mouse thymus to investigate thymic epithelial cell biology, T‑cell repertoire formation, and age‑related immune decline. Its accessibility, defined architecture, and well‑characterized genetic background make it a central model for dissecting mechanisms of central tolerance and lymphoid organogenesis.