Summary. In Romanov ewes at Day 13 or 14 of the cycle, granulosa cells originating from individual follicles were studied in short-term incubations for aromatase activity and thymidine incorporation. The study was performed on 76 follicles of different sizes (2–7 mm diameter) and degree of atresia, as assesed by histological examination of smears of granulosa cells. As atresia progressed, the labelling index and aromatase activity of granulosa cells decreased. In normal follicles, when follicular diameter increased, the labelling index decreased, while aromatase activity of granulosa cells and oestradiol-17β concentration in follicular fluid increased. There was a negative relationship between oestradiol concentration in follicular fluid and the labelling index of granulosa cells in vitro (r s = −0·75; P < 0·01), suggesting an inverse relationship between growth and differentiation of granulosa cells in normal sheep follicles.
In normal small and medium-sized follicles (2–6 mm), incubation with FSH (100 ng/ml) for 2 h increased significantly the labelling index of granulosa cells. In normal medium-sized follicles (4–6 mm), incubation with FSH (50 ng/ml) for 1 h decreased the aromatase activity of granulosa cells. From these results, it is suggested that FSH acts mainly on cells in the GI phase of the cell cycle, which are steroidogenically active, and makes them move into the S phase where their steroidogenic activity is temporarily inhibited.