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The first morphological event after initiation of male sex determination is seminiferous cord formation in the embryonic testis. Cord formation requires migration of pre-peritubular myoid cells from the adjacent mesonephros. The embryonic Sertoli cells are the first testicular cells to differentiate and have been shown to express neurotropin-3 (NT3), which can act on highaffinity trkC receptors expressed on migrating mesonephros cells. NT3 expression is elevated in the embryonic testis during the time of seminiferous cord formation. A trkC receptor tyrophostin inhibitor, AG879, was found to inhibit seminiferous cord formation and mesonephros cell migration. Beads containing NT3 were found to directly promote mesonephros cell migration into the gonad. Beads containing other growth factors such as epidermal growth factor (EGF) did not influence cell migration. At male sex determination the SRY gene promotes testis development and the expression of downstream sex differentiation genes such as SOX-9. Inhibition of NT3 actions caused a reduction in the expression of SOX-9. Combined observations suggest that when male sex determination is initiated, the developing Sertoli cells express NT3 as a chemotactic agent for migrating mesonephros cells, which are essential to promote embryonic testis cord formation and influence downstream male sex differentiation.