Stage-specific distributions of inhibin alpha-subunit in bovine ovarian follicles and various hormones levels in follicular fluid

The changes in localization of inhibin alpha-subunit in bovine ovarian follicles during follicular development were assessed by immunohistochemical staining with monoclonal anti-inhibin alpha-subunit antibody. The changes in inhibin, estradiol-17beta (E2), progesterone (P) and testosterone (T) levels in the follicular fluid during follicular development were also investigated. The follicles obtained from the bovine ovaries in mid-luteal stage were histologically classified into 7 stages. Stages I, II and III were growing healthy follicles, and stages IV, V, VI and VII were atretic follicles. In both growing and atretic follicles, inhibin alpha-subunit were histochemically demonstrated in the granulosa cells. Strong or moderate immunostaining for inhibin alpha-subunit were found only in the granulosa cells of stages II and III follicles (the strongest staining was found in stage III), and trace staining was seen in a small number of theca interna cells of stages II and III follicles. The granulosa cells of early atretic follicles (stage IV) also have the ability to produce and secrete inhibin, but finally they lost the ability. No positive staining was demonstrated in any cells of progressed atretic follicles (stage V, VI and VII). Inhibin level in the follicular fluid was higher in the growing follicles (highest level at stage II) than in the atretic follicles. Follicular fluid E2 level increased from stages I to III, but decreased from stage IV onward. The P levels remained low from stages I to VI, but increased only in stage VII. The T levels in stages III and VII were lower than in the other stages. Follicular fluid inhibin level were related to E2 level but not P or T levels. In conclusion, the inhibin alpha-subunit is produced by follicular granulosa cells in growing follicles of the bovine ovaries, and inhibin may stimulate E2 secretion in growing but not in atretic follicles