Epithelial-mesenchymal crosstalk in Wolffian duct and fetal testis cord development

Interactions between adjacent epithelial and mesenchymal tissues represent a highly conserved mechanism in embryonic organogenesis. In particular, the ability of the mesenchyme to instruct cellular differentiation of the epithelium is a fundamental requirement for the morphogenesis of tubular structures such as those found in the kidneys, lungs, and the developing male reproductive system. Once the tubular structure has formed, it receives signals from the mesenchyme, which can control proliferation, patterning, and differentiation of the epithelium inside the tube. However, the epithelium is not a "silent partner" in this process, and epithelium-derived factors are often required for proper maintenance of the mesenchymal compartment. Although much emphasis has been placed on the characterization of mesenchymally-derived signals required for epithelial differentiation, it is important to note that epithelial-mesenchymal interactions are a two-way street wherein each compartment requires the presence of the other for proper tubule morphogenesis and function. In this review, we discuss epithelial-mesenchymal interactions in the processes of Wolffian duct and fetal testis cord development using the mouse as a model organism and propose inhibin beta A as a conserved mesenchyme-derived regulator in these two male-specific tubular structures.