Follistatin immunostaining was localized only in the granulosa layer of all follicles in the cattle studied, a finding consistent with the results of earlier studies in the rat and human. Follistatin was not detected in the theca interna regardless of follicle type or phase, consistent with an earlier report that theca interna tissues obtained from bovine follicles did not produce follistatin in vitro.
Although follicle wave-related changes in follistatin production have apparently not been reported previously, mRNA for follistatin in rat granulosa cells was higher in growing secondary and tertiary follicles than in other follicles. The mRNA signal for follistatin apparently decreased in mature preovulatory follicles of the rat, but im-munostained follistatin was most intense in graafian stage follicles. In bovine granulosa cells in vitro, the strongest mRNA signal for follistatin was found in preovulatory follicles. Ovaries of cows given superstimulatory treatment also contained high levels of follistatin mRNA. Results of the present in vivo study, however, revealed that follistatin expression was equally high in the mid-growing phase and preovulatory phase of dominant follicle development.