Examining the reproductive system of the rat, students have the opportunity to compare male and female organs, study internal fertilization, and observe gestational changes in the female. The rat is representative of mammalian reproduction where fertilization of ova occurs inside the female, and the fertilized zygotes develop in the mother during a gestation period known as pregnancy.

Male reproductive anatomy

The testes, the male gonads, are large, oval-shaped lobes found in the scrotum slightly anterior to the anus on the ventral side of the organism. Inside the testis are seminiferous tubules where sperm cells are made. When mature, sperm cells travel to and are stored in the epididymis. During ejaculation, sperm are released from the epididymis and travel through the vas deferens to the urogenital duct. Along the sperm’s journey through the vas deferens, several glands contribute fluid to the sperm to make semen.

The rat penis is enclosed in a sheath, called the prepus. Rats have a bone in the penis called a bacula. Prior to mating, the contraction of muscles moves the bacula into the penis, stiffening it for copulation.

Figure 1 – Major structures of the male reproductive system

Female reproductive anatomy

The gonads of the female rat are the paired ovaries. The ovaries release egg cells into the oviducts. During ovulation, oviducts attached to the ovaries receive the mature eggs. In the presence of sperm, fertilization usually occurs in the upper third of the oviduct. This is followed by implantation farther along in the system, in one of the uterine horns. The length and shape of each uterine horn allows several fetuses to develop simultaneously. In rats, the vagina is formed from the union of the uterine horns. The external opening of the vagina is the vulva; this opening is separate from the urethral orifice through which urine is excreted.

Figure 2 – Internal anatomy of the pregnant female

Pregnancy and development

The gestational period averages 22 days. In the first few days post-fertilization, each zygote divides and forms a hollow ball of cells, called a blastula. By days 6 and 7, the blastulas have traveled down the oviducts and implanted in the uterine horns where each begins to differentiate into embryonic tissue and extra-embryonic tissue. As an embryo continues to develop, it takes nourishment from the mother through a complex system of connecting blood vessels, called the umbilical cord. The placenta functions to deliver oxygen and remove waste from the embryo’s environment. The amniotic sac contains and protects the embryo during pregnancy.

By day 9, the embryo is about 1 mm and has formed a neural plate, from which the brain and spinal cord will arise. The arm and leg buds are visible around day 11, and the embryo is about 3.3 mm. By day 13, the facial processes (olfactory pits) are prominent and the embryo is about 8 mm. During days 19 to 22, the rat embryo grows to 20 to 40 mm and the nervous system pathways develop.

Pregnant rats undergo labor for 1 to 2 hours. Labor is the result of muscle contractions of the uterus that push the fetuses through the vagina and out of the birth canal. Rats normally give birth to 7 to 12 offspring per litter. When the young rats are born, they are hairless and their eyelids remain sealed and ear ducts plugged. During the first 16 days of the young rats’ lives, they feed on the milk produced by their mother’s mammary glands. The mammary glands become swollen about 2 weeks into gestation as the pregnant rat begins to produce milk and colostrums. A couple of days before giving birth, the rat will lose the hair around the nipples to make it easier for her young to feed. About 45 days after birth, the young rats are fully weaned and actively foraging and feeding. At 50 to 60 days old, rats reach sexual maturity.

Figure 3a and 3b – Rat embryo enclosed in amniotic sac (left) and rat embryo with placenta and umbilical cord (right)

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