Developmental origin and maintenance of distinct testicular macrophage populations

Testicular macrophages (tM phi) are the principal immune cells of the mammalian testis. Beyond classical immune functions, they have been shown to be important for organogenesis, spermatogenesis, and male hormone production. In the adult testis, two different macrophage populations have been identified based on their distinct tissue localization and morphology, but their developmental origin and mode of homeostatic maintenance are unknown. In this study, we use genetic lineage-tracing models and adoptive transfer protocols to address this question. We show that embryonic progenitors give rise to the interstitial macrophage population, whereas peritubular macrophages are exclusively seeded postnatally in the prepuberty period from bone marrow (BM)-derived progenitors. As the proliferative capacity of interstitial macrophages declines, BM progenitors also contribute to this population. Once established, both the peritubular and interstitial macrophage populations exhibit a long life span and a low turnover in the steady state. Our observations identify distinct developmental pathways for two different tM phi populations that have important implications for the further dissection of their distinct roles in organ homeostasis and testicular function.