Localization of ovine follistatin and alpha and beta A inhibin mRNA in the sheep ovary during the oestrous cycle.

The sites of follistatin and alpha and beta A inhibin gene expression were examined by in situ hybridization in sheep ovaries during the early and mid-luteal phases (days 3 and 10) of the oestrous cycle and a prostaglandin F2 alpha (PGF 2 alpha)-induced follicular phase. Follistatin mRNA was detected in the granulosa cells of preantral, antral and early atretic follicles at all stages of the oestrous cycle, and in the corpora lutea at the early and mid-luteal stages of the cycle. However, only low levels of expression of follistatin were observed in the presumptive preovulatory follicle at 56 h after treatment with PGF 2 alpha. Both alpha and beta A inhibin were shown to be expressed in ovaries at all stages of the oestrous cycle. In situ hybridization localized alpha subunit mRNA to the granulosa cells of most, but not all, healthy antral follicles, and to no other ovarian cell type. In contrast, expression of the beta A subunit was confined to a few medium-to-large healthy antral follicles. In antral follicles expressing beta A inhibin, mRNAs for alpha inhibin and follistatin were always detected, but the converse was not true. Unlike follistatin, no alpha and beta A inhibin expression was seen in preantral follicles, developing corpora lutea, or follicles undergoing atresia. These results show that, in the adult sheep ovary, follistatin gene expression is a constitutive event in all growing follicles from the early preantral stage, and also provide indirect evidence of the involvement of follistatin, but not inhibin or activin, in the early stages of ovarian follicle development in sheep.