Fig 15

(A) Intrafollicular distribution of the activin-inhibin-follistatin system. All activin/inhibin β subunits (inhbaa, inhbab and inhbb) are exclusively expressed in the somatic follicle cells together with inhibin α subunit (inha). In contrast, follistatin a (fsta) is exclusively expressed by the oocyte whereas fstb is expressed in both cell types. Activins from the follicle cells may act directly on the oocyte to control its gene expression. In addition to antagonizing activin, inhibin from the follicle cells also feeds back to the pituitary to control gonadotropin expression and secretion especially FSH. (B) Roles of activin/inhibin β subunits in folliculogenesis. Activin plays important roles in controlling follicle activation and maintaining normal folliculogenesis. Activin βA (inhbaa and inhbab) appears more important than βB (inhbb), especially inhbaa whose functions can be partially compensated by inhbab. The βAa (inhbaa) may also be the main β subunit that contributes to the formation of inhibin (αβ).