Histologic Transformation of the Ovary of the Lactating Hamster

During the latter half of pregnancy, the hamster ovary contains an average of 22 large vesicular follicles. At parturition (day 1 of lactation), all vesicular follicles undergo atresia and luteolysis destroys the corpora lutea of pregnancy. The ovary assumes a “brainlike”a ppearance and consists of extensive interstitial tissue with follicles developing only to the multilayered granulosa stage. Treatment of hypophysectomized hamsters with LH results in similar histologic modifications of the ovary. The ovary of lactation maintains this appearance until day 26, when antral follicles develop, followed by ovulation by day 30. The ovary of lactation is maintained by as few as 1-2 suckling young. Injection of 30 IU pregnant mare's serum on day 2 of lactation results in development of numerous follicles which ovulate spontaneously; however, the resultant corpora lutea are not functional and regress 4-5 days after their formation. Removal of all young on days 2 or 14 of lactation results in ovulation exactly 4 days later (the duration of the normal estrous cycle). On removing all young, pituitary FSH and LH are depleted, paralleling the reappearance of vesicular follicles, which ultimately ovulate. Despite the morphologic changes in the lactating ovaries, they are not essential after day 7 for lactation and normal growth of the young. These observations indicate that the neurogenic stimulus of suckling alters the gonadotropic output of the hamster pituitary; FSH release is diminished, whereas LH discharge apparently continues. (Endocrinology77: 641, 1965)