The transforming growth factor-b (TGF-b) superfamily is a large group of structurally related growth factors that play prominent roles in a variety of cellular processes. The importance and prevalence of TGF-b signaling are also reflected by the complex network of check points that exist along the signaling pathway, including a number of extracellular antagonists and membranelevel signaling modulators. Recently, a number of important TGF-b crystal structures have emerged and given us an unprecedented clarity on several aspects of the signal transduction process. This review will highlight these latest advances and present our current understanding on the mechanisms of specificity and regulation on TGF-b signaling outside the cell.

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The role of follistatin as an activin-binding protein has dominated the study of this molecule for the last 10 years. However, there is emerging evidence that follistatin has a role in modulating the biology of other members of the transforming growth factor beta (TGF-beta) superfamily. This review summarizes the current concepts encompassing follistatin biochemistry as well as molecules with which it is functionally associated. Moreover, the importance of the two follistatin isoforms (follistatin-288 and follistatin-315) is discussed with particular emphasis on the regulation of the ovary. In addition to activin, this review discusses the functions of other members of the TGF-beta superfamily, for example growth differentiation factor 9 (GDF-9), bone morphogenetic protein 15 (BMP-15), BMP-6, BMP-4 and BMP-7, in the ovary, and the potential interactions between follistatin and these growth factors. The complex network of TGF-beta superfamily growth factor members involved in the modulation of ovarian function and the interactions of follistatin with these proteins is highlighted.

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