Inhibin in individual ovine follicles in relation to diameter and atresia

Inhibin activity was measured by bioassay in follicular fluid of 99 individual ovine follicles ranging from 1 . 4 to 6 . 8 mm diameter (used to calculate volume) and in various stages of atresia. Treatment of samples before assay with charcoal concentrations of greater than 1 mg/ml resulted in significant loss of inhibin activity. The inhibin content of follicular fluid from individual follicles varied with follicular fluid volume but not with the degree of atresia, as assessed by morphological criteria. Inhibin concentration was not related to atresia, but was correlated with follicular fluid volume. However, aromatase activity in granulosa cells and oestradiol-17 beta concentration of follicular fluid, considered to be good indices of atresia, were highly correlated with both inhibin content and concentration in follicles greater than or equal to 3 . 5 mm diameter. Inhibin in ovine follicular fluid shows marked variation between follicles and it is suggested that this reflects a combination of the number and activity of granulosa cells within the follicle and the exit rate of inhibin from the follicle.