Constant sperm production necessitates the maintenance and regulation of the spermatogonial stem cell (SSC) population in the testis, in which SSCs must balance self-renewal and differentiation to promote fertility ( Oatley & Brinster 2012) therefore, a strict microenvironment must be established and maintained for the SSC population. Male fertility is dependent on the continual production of sperm, starting at puberty and continuing throughout adult life. Recent work has shown that these cell types are critical for morphogenesis of the fetal testis ( Bott et al., 2006 Combes et al., 2009 Cool et al., 2011 DeFalco et al., 2014), as well as promoting spermatogenesis to ensure male fertility ( DeFalco et al., 2015 Potter & DeFalco 2017). Within the interstitial compartment of both human and rodent testes, there are various cell types, including: peritubular cells vascular endothelial cells vascular smooth muscle and other perivascular cells steroidogenic Leydig cells and their undifferentiated mesenchymal progenitors and immune cells, which under normal conditions is mostly comprised of testicular macrophages. In spite of some species-specific differences, lessons can be learned from each species regarding the diverse functions of testicular cell types. Additionally, human testes contain mast cells ( Meineke et al., 2000), which are rarely found in rodent testes. Although the basic architecture of the testis among mammals is fairly similar, there are some differences, such as the fact that human testes have a multi-cell-layered peritubular wall, in contrast to a single-cell-layered wall in rodents ( Mayerhofer 2013). While the vast majority of studies described in this review are based on rodent models, some research has also been done on human testicular cells. The tubule compartment is maintained separately from the interstitium via a collagen- and laminin-containing basement membrane, which is overlaid by peritubular cells (often called peritubular myoid cells) that physically separate the two compartments. The interstitial compartment of the testis, which constitutes the area of the organ outside of the testis cords/seminiferous tubules, is comprised of several cell types. While Sertoli cells are necessary for testicular differentiation, recent studies have revealed that other cell types are critical contributors to testicular development and function. Sertoli cells intimately associate with germ cells and ensure proper germline development throughout life. Under the influence of Sry and its target gene Sox9, during E10.5-E11.5 in the mouse and 4–6 weeks in the human, gonadal progenitor cells become Sertoli cells, which are the first male-specific cell type specified in the gonad. The mammalian testis develops in utero when the expression of the Y-linked gene Sry is expressed in a subset of bipotential progenitor cells that initially have the ability to become testicular or ovarian cells ( Albrecht & Eicher 2001 Koopman et al., 1990 Sinclair et al., 1990).
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