Recruitment and development of the follicle; the roles of the transforming growth factor-β superfamily

Peripheral endocrine hormones and local paracrine and autocrine factors contribute, in a coordinated fashion, to the processes of recruitment, development or atresia, selection and ovulation of follicles. Among the local ovarian factors, there is growing evidence from genetic and experimental data that many members of the transforming growth factor (TGFβ) superfamily have a biological role to play in folliculogenesis. These members include activin, inhibin, TGFβ, BMP, GDF9 and perhaps MIS. In this review, we discuss the potential roles of the TGFβ superfamily members, in particular activin, during folliculogenesis. Since the actions of these factors are determined by ligand availability, receptor expression and modulation of their signal transduction pathways, we also collate information on the expression of their signalling components in the follicle. We conclude that the TGFβ superfamily signalling pathways, in particular activin's pathway, reside in the ovary. Furthermore, follistatin and β-glycan-components of the accessory binding protein system that modifies activin action-are also present in follicles. In the post-natal rat ovary, the changes in receptor/Smad expression coincide with granulosa cell proliferation and antrum formation. We hypothesise that these pathway components are expressed in a temporal and cell-specific manner to meet the changing demands of cells during follicular development. The analysis of the components of the signal transduction pathways of the TGFβ family members in populations of defined follicles and the identification of activated pathways in individually stimulated follicles should help clarify the roles of the TGFβ members in folliculogenesis.