Mixed signals: development of the testis.

Induction and patterning of the testis occurs over a brief window of time. Before male-specific morphogenesis, the gonad primordium is bipotential and capable of developing into either an ovary or testis. However, expression of the transcription factor SRY initiates male development and induces patterning, proliferation, and epithelialization specific to the testis. Male sex determination begins with commitment of Sertoli cells via autonomous and nonautonomous mechanisms. These mechanisms have recently been shown to both promote the male fate and simultaneously repress ovarian development. A second critical event in the development of the testis is the epithelialization of testis cords. After their specification, Sertoli cells epithelialize and surround the male germ line to form large looping structures bound by extracellular matrix. Cells excluded from cord structures are called interstitial cells and comprise several different cell types, including steroidogenic cells, endothelial cells, and a smooth muscle cell that directly surround the cords. Numerous male-specific signaling pathways influence testis cord morphogenesis and specification of distinct cell types, although a coherent progression of events is unclear. In this article we focus on signals in the male gonad that first are responsible for the specification of Sertoli cells, and second for the specification and patterning of interstitial cells.