Inhibin, Activin, and Growth Factors: Paracrine Regulators of Testicular Function

In the last decade there have been rapid advances in our understanding of the importance of paracrine factors in regulating testicular function (Mather 1984; Bergh 1983; Parvinen et al. 1986; Jegou et al. 1988; Skinner 1991). These factors include substances known only through their effects on the gonads, such as Müllerian inhibiting substance (MIS), seminiferous growth factor and P-Mod-S; and those more widely implicated in cell-cell communication including: fibroblast growth factor, nerve growth factor, insulin-like growth factors, TGF-α, TGF-β and interleukins. Due to their nature, paracrine interactions are difficult to identify and verify in vivo. Thus, the majority of support for the hypothesis that these factors play an important paracrine role in regulating testicular function comes from in vitro experiments or circumstantial evidence (e.g. the presence of mRNA in a cell). The use of established cell lines both as assay systems and as producers of growth factors has been valuable in elucidating the factors which may play a role in paracrine regulation of the testis (Mather et al. 1982a; Zheng et al. 1990).