How does a rat's labor begin? - briefly
Labor in rats initiates when rising estrogen levels stimulate the pituitary to secrete prolactin and oxytocin, which together trigger uterine contractility. The first observable sign is a series of rhythmic contractions that lead to cervical dilation and delivery.
How does a rat's labor begin? - in detail
The process of parturition in laboratory rodents starts with a coordinated hormonal cascade that prepares the uterus, cervix, and offspring for birth. Rising levels of fetal cortisol stimulate the maternal adrenal cortex to increase progesterone conversion into estradiol. The surge in estradiol triggers a decline in circulating progesterone, removing its inhibitory effect on uterine contractility. Concurrently, the hypothalamus releases oxytocin‑releasing factor, prompting the posterior pituitary to secrete oxytocin. Oxytocin binds to receptors on myometrial cells, inducing rhythmic contractions that progress from mild tone to strong, coordinated waves.
Physical signs become evident shortly after hormonal changes. The abdomen may appear enlarged due to fetal growth, but the first observable indicator is a softening and dilation of the cervical canal. Vaginal mucus production increases, providing a lubricated pathway for the pups. Behavioral alterations include nesting activity, decreased locomotion, and a preference for quiet, secluded areas. These preparatory actions often commence 12–24 hours before the first pup is expelled.
The uterine activity follows a three‑phase pattern. In the latent phase, contractions are infrequent and low‑intensity, allowing fetal positioning. The active phase features high‑frequency, high‑amplitude contractions that propel each neonate through the birth canal. Each pup is typically delivered within a 5‑minute interval, accompanied by a brief pause for placental expulsion. The final stage, involution, involves a gradual reduction in contractile frequency and the restoration of uterine tone, mediated by prostaglandin release and the re‑establishment of progesterone dominance.
Key physiological markers that confirm the onset of delivery include:
- A measurable drop in serum progesterone concentration.
- A rise in plasma oxytocin levels detectable by immunoassay.
- Increased uterine electromyographic activity correlating with contraction strength.
- Elevated concentrations of prostaglandin F2α in uterine tissue.
Understanding these mechanisms enables precise timing of interventions, such as anesthesia administration or experimental manipulation, ensuring animal welfare and experimental consistency.