LOCALIZATION OF OVINE FOLLISTATIN AND ALPHA-INHIBIN AND BETA(A)-INHIBIN MESSENGER-RNA IN THE SHEEP OVARY DURING THE ESTROUS-CYCLE

The sites of follistatin and alpha and beta(A) inhibin gene expression were examined by in situ hybridization in sheep ovaries during the ea rly and mid-luteal phases (days 3 and 10) of the oestrous cycle and a prostaglandin F-2 alpha (PGF(2 alpha))-induced follicular phase. Folli statin mRNA was detected in the granulosa cells of preantral, antral a nd 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. Ho wever, only low levels of expression of follistatin were observed in t he 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 lo calized a subunit mRNA to the granulosa cells of most, but not all, he althy antral follicles, and to no other ovarian cell type. In contrast , expression of the beta(A) subunit was confined to a few medium-to-la rge healthy antral follicles. In antral follicles expressing beta(A) i nhibin, mRNAs for a inhibin and follistatin were always detected, but the converse was not true. Unlike follistatin, no alpha and beta(A) in hibin expression was seen in preantral follicles, developing corpora l utea, or follicles undergoing atresia. These results show that, in the adult sheep ovary, follistatin gene expression is a constitutive even t in all growing follicles from the early preantral stage, and also pr ovide indirect evidence of the involvement of follistatin, but not inh ibin or activin, in the early stages of ovarian follicle development i n sheep.