EXPRESSION OF MESSENGER-RNA FOR FOLLISTATIN AND INHIBIN ACTIVIN SUBUNITS DURING FOLLICULAR-GROWTH AND ATRESIA/

The aim of the present study was to investigate the sites and time of follistatin and inhibin alpha and beta(A) subunit gene expression duri ng ovine follicular development and atresia. Prepubertal ovaries of 2- , 8- and 14-week-old ewe lambs (n=9) Regardless of age, the ovaries co ntained follicles at different stages of development up to 2 mm in dia meter, but large antral follicles were not found. Ovarian sections wer e hybridized with S-35-labelled antisense RNA probes transcribed from follistatin, inhibin alpha or inhibin beta(A) cDNA. Ovaries from matur e gonadotrophin-stimulated ewes were used as controls. All three probe s hybridized exclusively to granulosa cells, and not to other follicul ar or stromal cells. None of the probes hybridized to primordial folli cles or primary follicles with less than two layers of granulosa cells . Follistatin mRNA was expressed strongly in the granulosa cells of al l preantral follicles with two or more layers of cells, and in all non -atretic antral follicles. In addition, follistatin mRNA was found in some cells of the ovarian rete tubules. The inhibin alpha riboprobe hy bridized to the granulosa cells of most preantral and all non-atretic antral follicles. In thepreantral follicles, the strongest inhibin a e xpression was observed in the cells that were in close proximity to th e oocyte. The inhibin beta(A) riboprobe hybridized exclusively to the granulosa cells of antral follicles. The labelling was observed either in the cumulus oophorus or in the cumulus oophorus and periantral gra nulosa cells of the non-atretic antral follicles. In adult ovaries, wh ich were used as controls, the inhibin beta(A) riboprobe hybridized st rongly to all granulosa cells of non-atretic large antral follicles. D uring follicular atresia, expression of all three mRNAs progressively decreased. In early atresia, inhibin beta(A) mRNA was observed only in the cells of the cumulus, whereas inhibin alpha and follistatin mRNAs were still present in the granulosa cells. As atresia progressed mRNA for inhibin alpha, and later also follistatin, disappeared. Our resul ts suggest that there is a sequential appearance and disappearance of follistatin, inhibin alpha and inhibin beta(A) mRNAs during follicular development and atresia respectively The marked expression of inhibin beta(A) in the cumulus cells implies a role of the oocyte in the diff erentiation of these cells.