Relative to follicle wave dynamics, there appears to be a temporal relationship between the expression of follistatin (present study), estradiol production, and morphological features of the dominant and subordinate follicles of cattle. Immediately after divergence in the diameter profiles of dominant and subordinate follicles (i.e., selection of dominant follicle), differential expression of both follistatin (present study) and estradiol production was observed. The granulosa layer of only morphologically healthy follicles produced follistatin, and maxima in follistatin expression in granulosa cells were coincident with maxima in in-trafollicular estradiol concentrations.
Follistatin and estradiol may simply be increasing coincidentally as a result of accelerated cell function, or perhaps follistatin is involved in the process of selection of the dominant follicle by enhancing differentiation of the granulosa and theca cells. In contrast to the in vivo temporal relationship with estradiol, follistatin has been shown to enhance progesterone production in vitro from bovine granulosa cells obtained from 5-to 10-mm follicles, but not from the preovulatory follicles. Follistatin also increased LH-induced progesterone production, but not androstenedione production, from in vitro-cultured bovine theca cells obtained from 5- to 10mm follicles. In the rat and the human, activin has an inhibitory effect on androgen synthesis by theca cells in vitro; hence, follistatin was expected to have a stimulatory role.