Testis Cord Maintenance in Mouse Embryos: Genes and Signaling.

Testis cords, embryonic precursors of the seminiferous tubules, are fundamental for testis structure and function. Delay or disruption of testis cord formation could result in gonadal dysgenesis. Although mechanisms regulating testis cord formation during sex determination have been well-studied, the genes and signaling pathways involving in testis cord maintenance after the cords have formed are not well characterized. It is now clear that the maintenance of cord structure is an active process. In this review, we summarize the recent findings regarding the regulation of testis cord integrity by a series of Sertoli cell transcription factors, including the WT1-SOX8/SOX9-beta-CATENIN-DHH network, GPR56, STIM1, and NR0B1 (also known as DAX1). In particularly, we emphasize the underappreciated role of peritubular myoid cells in testis cord maintenance and their cooperation with Sertoli cells. The regulation of the size, shape, and number of testis cords by Sertoli cell proliferation (e.g., SMAD4, GATA4, and TGF-beta signaling), Leydig cell products (e.g., ACTIVIN A), vascular development (a lesson learned from PDGF signaling), and available gonad space (as observed in Ift144 mutant mice) is also addressed. Further efforts and new genetic models are needed to unveil the gene networks and underlying mechanisms regulating testis cord integrity and morphology after sex determination.