Origin and Role of Testicular Macrophages in Testis Development, Steroidogenesis, and Spermatogenesis.

Testicular macrophages are the principal immune cells in the testis. In addition to their classical immune roles, they regulate male hormone synthesis by Leydig cells, regeneration of Leydig cells, spermatogonia proliferation and differentiation, maintenance of testis-specific environment for sperm formation, and testis development. The juvenile and adult testes contain two distinct macrophage populations with unique tissue localization, genetic markers, morphology, and function. The interstitial macrophages are physically and functionally connected to Leydig cells, while the peritubular macrophages localize around the seminiferous tubules and are crucial for spermatogonia differentiation. Macrophages in the fetal testes regulate testis vasculature formation and clearance of mislocated cells. The origin of testicular macrophages is unclear. Some studies suggest their origin from the yolk sac and others from the bone marrow-derived monocytes. We discuss this issue at the end of this review article.