Mesenchymal cell precursors of peritubular smooth muscle cells of the mouse testis can be identified by the presence of the p75 neurotrophin receptor

In the mouse embryo, at approximately 11.5 days postcoitum (dpc), cells mig rate from the mesonephros into the developing testis to contribute to the s omatic population of the interstitial compartment (i.e., peritubular myoid cells, Leydig cells, and endothelial cells). Studies from this laboratory h ave shown that the interstitial population of mesenchymal cells in fetal an d newborn mouse testis express the p75 neurotrophin receptor (p75NTR, forme rly known as the low-affinity nerve growth factor receptor); part of the ce ll population progressively congregates around testis cords, later to be re placed by contractile peritubular myoid cells, which express smooth muscle cell markers. In the present study, we show that the migrating cells and th e p75NTR-expressing cells are the same population. We also show that the ne urotrophin receptor is a useful endogenous marker to follow cell migration within the urogenital ridge and to identify and isolate mesenchymal precurs ors of myoid cells. A time-course immunolocalization study of the location of p75NTR-bearing cells within the urogenital ridge of mouse embryos betwee n 10.5 and 12.5 dpc showed that the interstitium of the fetal testis was pr ogressively occupied by p75NTR(+) cells. The progressive increase of p75NTR expression within the developing testis was confirmed by immunoblot analys is of proteins isolated from the fetal gonads. Organ cultures of isolated t estes or testis-mesonephros grafts confirmed that p75NTR(+) cells do not ap pear in the testis unless a mesonephros is attached to it. Cells bearing th e p75NTR receptor, purified from 12.5-dpc male mouse mesonephroi by immunom agnetic sorting, were able to differentiate in vitro into myoid cells. Immu nofluorescence analysis of postnatal testis sections confirmed the presence around the tubules of cells coexpressing p75NTR and or-smooth muscle actin . The ability to identify and purify precursors of myoid cells may be of co nsiderable help for studying the mechanisms regulating their differentiatio n.