"Vertebra" - what is it, definition of the term
A spinal segment in rodents is a cylindrical ossicle comprising a central body that bears compressive forces, a posterior arch that encloses the neural canal, and paired transverse and spinous processes that serve as muscle‑attachment sites; it connects with neighboring segments through intervertebral discs, forming the flexible column that supports the body and protects the spinal cord.
Detailed information
The spinal skeleton of rodents consists of a series of articulated bony units that protect the neural tube and support the axial musculature. Each unit is composed of a body, a neural arch, and processes that serve as attachment sites for ligaments and muscles. In both rats and mice, the total count ranges from 26 to 30 elements, divided into cervical, thoracic, lumbar, sacral, and caudal regions. The cervical series contains seven segments, identical to the human pattern, while the thoracic region comprises twelve to thirteen segments, each bearing a pair of ribs. Lumbar elements number five to six, providing the primary load‑bearing function. The sacrum fuses into a single block of three to four segments, and the caudal series varies from 20 to 30 unfused pieces that form the tail.
Morphologically, the anterior portion of each element is a cylindrical body with a slightly concave dorsal surface. The posterior arch encloses the spinal canal and gives rise to transverse and spinous processes. In rats, the spinous processes of thoracic and lumbar segments are relatively short and blunt, facilitating flexibility during locomotion. Mice display proportionally longer spinous projections in the lumbar area, which contribute to greater dorsoventral curvature. The articular facets on each side of the arch articulate with adjacent elements, permitting limited rotation and extension while restricting excessive bending.
Developmentally, the vertebral precursors arise from somites that segment along the embryonic axis. Gene expression patterns, such as Hox clusters, define regional identity. In laboratory rodents, mutations in Hox10 genes lead to transformation of lumbar elements into thoracic‑like structures, illustrating the genetic control over segmental differentiation. Ossification begins with primary cartilage models that undergo endochondral conversion; the timing differs between species, with mice completing ossification of lumbar bodies by embryonic day 16, whereas rats reach the same stage around day 18.
Biomechanical properties are quantifiable through compressive testing of isolated segments. Rat lumbar units exhibit an average compressive strength of 12 MPa and a Young’s modulus near 1.2 GPa. Mouse lumbar elements show slightly lower values, approximately 10 MPa and 1.0 GPa, reflecting differences in size and mineral density. Histological analysis reveals a higher proportion of trabecular bone in the caudal series of mice, correlating with the greater flexibility required for tail movements.
Research relevance includes the use of these small mammals as models for spinal injury, degeneration, and genetic disorders. The accessibility of the lumbar region allows for surgical implantation of devices and for in vivo imaging of bone remodeling. Comparative data between the two species assist in selecting the appropriate model for specific experimental objectives.
Key comparative points:
- Total segment count: rat ≈ 26–28, mouse ≈ 27–30.
- Spinous process morphology: rat = short/blunt, mouse = longer in lumbar region.
- Ossification timing: rat lumbar bodies ≈ embryonic day 18, mouse ≈ day 16.
- Mechanical strength: rat lumbar ≈ 12 MPa, mouse lumbar ≈ 10 MPa.
These details provide a comprehensive overview of the spinal skeletal elements in the two rodent species, supporting their application in biomedical investigations.