SUMMARY The concentration of progesterone and oestradiol-17β in peripheral plasma was determined by radioimmunoassay on each day of pregnancy in the mouse. The progesterone concentration, low on days 1 and 2, correlated with the histological development of the corpora lutea. Progesterone levels during the first half of pregnancy rose to a peak on day 6 with a significant decrease (P < 0·02) on day 7. Peripheral progesterone concentration during the second half of pregnancy rose on days 12–13 with a peak on days 14–17, and declined after day 17, 3 days before the onset of parturition. Oestradiol-17β concentrations were high on day 1, declined precipitously to low levels on days 2 and 3 recovering slightly on day 4. The levels then fell and remained low from day 5 to days 15–16, when there was a significant increase in oestradiol-17β concentration which continued until day 20.

Follicle-stimulating hormone (FSH), luteinizing hormone (LH) and prolactin concentrations were determined by radioimmunoassay (NIAMD-Rat RIA) of sera from intact hamsters, which were decapitated at selected times during the 4-day estrous cycle. Serum FSH concentrations on Day 1 of the cycle (day of ovulation) were 2-to 6-fold greater than the levels on Day 2 and 5 to 12-fold greater than the mean, "basal," level of FSH (53 ± 2 ng/ml; 80 hamsters) found during Day 3 and until 1200 hr, Day 4. There was a biphasic serum FSH pattern on Day 4 (proestrus) which was demarcated by the initiation of the preovulatory surge at 1400 hr, the peak level at 1600 hr, the decay of the surge by 2100 hr, and the subsequent elevation to the high levels present on Day 1. In marked contrast to these changes in serum levels of FSH, LH levels were virtually constant throughout the estrous cycle, except for the preovulatory surge between 1300 hr and 1900 hr on Day 4 with a peak level at 1600 hr. Prolactin concentrations varied at most by one-half to 2-fold from each other during diestrus. The preovulatory increase in serum prolactin on proestrus was at a maximum level at 1500 hr, then declined by 2000 hr. (Endocrinology 94: 1295(Endocrinology 94: , 1974 LOOD levels of luteinizing hormone (LH), determined by ovine:ovine radioimmunoassay, in the cyclic hamster have been reported by several groups of investigators (1-3). However, peripheral levels of follicle-stimulating hormone (FSH) and prolactin during the estrous cycle of the hamster have not been measured.Previous studies from this laboratory have centered on follicular (4) and luteal (5) development during the cycle and preovulatory changes in the ovary and pituitary during proestrus (6,7). To further delineate the reproductive processes occurring during the estrous cycle of the hamster, the objective of the present investigation was to determine the serum FSH, LH and prolactin during the cycle.

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An enzymatic method was developed to collect intact follicles at different stages of development from cyclic hamsters to study ovarian folliculogenesis under various circumstances. Ovaries from 6 adult hamsters on each day of the cycle (Day 1 = ovulation) were collected, corpora lutea and large preantral and antral follicles were dissected, and follicles saved. Minced ovaries were then incubated with a mixture of collagenase, DNAse and pronase at 37 degrees C for 20 min to disperse intact follicles. Histological studies with 2191 isolated follicles revealed 10 different stages of follicular development (depending on the number of granulosa cell layers surrounding the oocyte and development of the antrum). Of the total follicular population, 14% showed signs of atresia, with 50% of those having 1-3 layers of granulosa cells (Stages 1-3); a second peak of 18% was observed in antral follicles (Stages 8-10). No signs of thecal cells were evident until the follicles reached Stage 6 (7-8 layers of granulosa cells), which possibly accounts for reduced atresia in this class and beyond. Ultrastructural study revealed that there were no signs of morphological damage to the basement membrane or to other subcellular organelles in the small preantral follicles. The presence of subnuclear lipid droplets in follicles with 3 layers of granulosa cells provided evidence for potential steroidogenesis by small follicles. The number of Stage 1-10 follicles was remarkably constant throughout the estrous cycle (460 +/- 34 per animal on Day 1 vs. 492 +/- 66 on Day 4). The usefulness of this method in analyzing follicular kinetics is illustrated in experiments involving hypophysectomy and the effects of unilateral ovariectomy. This procedure offers an improved method to study the factors responsible for the growth and the differentiation of small preantral follicles in the mammalian ovary.

Male (1--60 days old) and female (1--30 days old) hamsters were decapitated and serum levels of LH, FSH, PRL, progesterone, androgens (males), and estradiol (females) were measured by RIA. Males and females had similar levels of LH until 15 days of age and of FSH until 12 days of age, at which times gonadotropin levels increased significantly in females. Peak levels for females occurred on days 19--21 for LH and on days -2--24 for FSH, later than the times reported for female rats. Adjusting female gonadotropin peaks for gestation length places these peaks for hamsters and rats at the same time in postmating age. In female hamsters, large variations occur in LH between 16--25 days of age, as reported for female rats. Males reached peak serum levels of LH and FSH on day 40, just before the first motile epididymal sperm. Serum PRL levels were identical in male and female hamsters until at least day 30. PRL levels sharply increased in both sexes after day 18 and remained elevated until at least day 30. In males, serum androgens were low until 30 days of age, in contrast to high levels reported for infantile rats. Androgens rose sharply in male hamsters after day 30 to peak levels on day 50. Progesterone in males also remained low until after day 30. Serum estradiol in females did not attain the extremely high elevations seen in rats. Some fluctuations occurred between 10--30 days of age, which presumably represent maturational changes in the ovary. Serum progesterone in females followed a pattern of development similar to estradiol.

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Preantral follicles from pro-oestrous and oestrous hamsters were isolated enzymically (Stages 1-5) and by microdissection (Stage 6) and cultured for up to 168 h in the absence or presence of 100 ng ovine FSH or LH separately or combined or 1 or 10 micrograms progesterone or estradiol-17 beta in serum-free defined medium and exposed to 1 muCi [3H]thymidine for 24 h before termination. In the presence of insulin and hydrocortisone but not gonadotrophins, the morphology of follicles from pro-oestrous animals at Stages 1-4 (1-4 layers granulosa cells; no theca) were unaffected for up to 48 h whereas for Stages 5 (5-6 layers granulosa cells and developing theca) and 6 (7-8 layers granulosa cells and theca), atresia was prominent by 24 h. FSH significantly reduced the percentage of atretic follicles in Stages 1-5 throughout the culture period; but was effective only up to 96 h for Stage-6 follicles. LH was also effective, albeit to a lesser extent. FSH increased follicular labelling indexes during every 24-h labelling period and, during a pulse-chase period, follicular DNA content and granulosa cell numbers. FSH, but not LH, induced differentiation by 96 h of preantral follicles at Stage 6 into small antral stages (Stages 7-8). FSH and LH together induced almost the same effect as FSH alone. However, neither progesterone nor oestradiol had any significant long-term effects on DNA synthesis and oestradiol induced atresia beyond 24 h. Both FSH and LH induced follicular maturation in vitro as evident from increases in progesterone, androstenedione and oestradiol production. Follicles (Stages 1-4) collected from oestrous hamsters responded to FSH to a lesser extent than did those from pro-oestrous animals, possibly because of in-vivo exposure to periovulatory changes in gonadotrophins; however, an antrum formed in Stage-6 follicles by 72 h.