A study of the population of primordial and small follicles in the sheep

The population of primordial and small follicles in adult ewes of high (3.1) and low (1.6) ovulation rate was studied by histological methods. The small follicle population which has a skewed frequency distribution in relation to either follicle or oocyte size can be divided according to morphological features into dormant, transitory and growth phases. It is suggested that initiation of follicle growth involves firstly the passage of follicles from the dormant to the transitory category and (if it occurs) is independent of gonadotrophins; and secondly, the passage from the transitory category to the growth phase. The second step is dependent on gonadotrophins and the rate of such passage is higher in ewes with a high ovulation rate. The overall mean follicle size of small follicles is larger in ewes with a low ovulation rate, suggesting that either they have structurally different small follicles or their reserve of small follicles is less depleted than in ewes with a high ovulation rate.