This paper (link opens a .pdf) points out the importance of testosterone levels in men’s health, erectile functioning, and the roll that endothelial cell biology plays. “Besides stimulating the production of endothelial cell mediators, testosterone is also thought to promote the vasculogenic reendothelialization process, mediated by bone marrow-derived endothelial progenitor cells.” The balancing of these factors is important for healthy male functioning.