Ovarian Changes in the Guinea Pig During Various Reproductive Stages and Steroid Treatments

Follicular growth in the guinea pig ovaries has been studied at various stages of the estrous cycle, pregnancy, and on Day 3 postpartum. (4–7 animals per group). In addition, effects of progesterone (10 mg/day for 16 days beginning Day 2 of the cycle) and testosterone propionate (5 mg on Day 2 followed by 1 mg/day for 48 days) were studied. Ovaries were serially sectioned, and the number and size of vesicular follicles were determined. No statistically significant changes in the ovarian weight or follicular count were observed during the estrous cycle. In contrast, during pregnancy (particularly during the 2nd and 3rd trimester) the ovarian weight doubled but the follicular count declined from 101 ± 9 (SE) in cyclic animals to 67 ± 5 (p < 0.01). Follicles larger than 700μ in diameter (considered preovulatory) were consistently found only on Day 15 of the cycle and on Day 66 of pregnancy. Administration of progesterone blocked the development of these preovulatory follicles and ovulation but otherwise failed to decrease the follicular count, growth, or ovarian weight when compared to cyclic animals. In contrast, testosterone propionate decreased both the ovarian weight and the follicular growth, and also suppressed the formation of corpora lutea.