Follicular steroid content and oocyte meiotic status after PMSG stimulation of immature hamsters

Summary. Follicular growth was stimulated by the administration of PMSG to immature female hamsters. Changes in the number and size of follicles, their content of progesterone, testosterone, oestradiol, and oestrone (determined in whole follicular homogenates) and the meiotic status of the oocytes, were followed for 4 days to monitor follicular development and ageing. The results showed that 92% of small antral follicles (371–590 μm) were seen 24–30 h after PMSG stimulation and that the maximum number of medium- (591–740 μm) and large- (>740 μm) sized antral follicles occurred >48 h later. There was a progressive increase in the concentrations of testosterone, oestradiol and oestrone over the first 48–54 h, but maximal values of oestrone (geometric mean (limits of 1 s.d.): 326·5 (230·4–462·7) pg/follicle) lagged 24 h behind the highest values for testosterone (352·5 (242·0–456·0) pg/follicle) and oestradiol (718·4 (315·7–1634·7) pg/follicle). The progesterone content of all follicles did not change significantly during the first 72–78 h (range of geometric mean values: 0·25–1·06 ng/follicle), but at 96–102 h two populations of mediumand large-sized follicles could be distinguished on the basis of a low progesterone concentration (Group 5a: medium, 0·27 (0·13–0·55) ng/follicle; large, 0·1 (0·08–0·13 ng/follicle; P < 0·01) or a high concentration (Group 5b: medium, 7·77 (3·12–19·36) ng/follicle: large, 13·10 (8·68–19·76 ng/follicle; P < 0·01) and this grouping corresponded to the peripheral plasma progesterone values of the animals (Group 5a: 1·32 (0·51–3·42) ng/ml; Group 5b: 6·76 (5·05–9·05) ng/ml; P < 0·01). Differences in concentrations of testosterone, oestradiol and oestrone were not found in these two groupings at 96–102 h, but large antral follicles at this time contained more oestrone (x 1·5) than medium-sized follicles. Oocytes recovered from follicles up to 78 h after PMSG stimulation showed few signs of atresia (>80% viable) and had not resumed meiosis (>93% had a visible germinal vesicle). At 96–102 h oocytes from medium and large follicles with a low progesterone content maintained their viability (>88%), but 41% of oocytes recovered from medium-sized follicles showed evidence of germinal vesicle breakdown. However, in follicles containing high concentrations of progesterone the incidence of atresia was high (38–48%) and most oocytes had resumed meiosis (>97%).