Lack of between-follicle interactions in the sheep ovary.

Follicular growth over 2 mm in diameter is mainly regulated by gonadotrophins (FSH and LH). However, it is possible that between-follicle interactions may be involved in ensuring ovulation of a specific number of follicles. The present study tested specific hypotheses regarding: (1) a stimulatory role of atretic follicles on differentiation of follicles of the next wave; (2) a stimulatory role of large follicles in F-gene-carrier Booroola ewes on differentiation of follicles of the same ovulatory cohort; and (3) an inhibitory role of the dominant follicle on differentiation of follicles of the same ovulatory cohort. The end points measured were aromatase activity (all trials) and granulosa cell proliferation (Booroola). The techniques used tested effects of ovarian venous serum or conditioned media or follicle co-culture. Comparison of the effects of similar protein concentrations of serum or media conditioned by healthy versus atretic follicles could not demonstrate a stimulatory role of the atretic follicle. This was further confirmed using ovarian venous serum draining an ovary bearing an atretic follicle. Comparison of the effects of similar protein concentration of serum or media conditioned by F+ or ++ follicles could not demonstrate a stimulatory role of large follicles in F+ewes. This was supported by the observation that ovarian venous serum of F + or ++ ewes had similar effects on aromatase activity. Furthermore, coculture of small follicles with a large follicle of either genotype did not alter granulosa-cell proliferation compared with controls. This suggests that an alteration in the between-follicles regulation does not operate in Booroola sheep to generate their high ovulation rate. Comparison of the effects of similar protein concentrations of serum, follicular fluid from large follicles or media conditioned by large follicles showed that follicular fluid (but not conditioned media) contained an aromatase inhibitor. This suggests that dominant follicles do not secrete the aromatase inhibitor they contain.