Spermatogonial Response to Somatic Cell Interactions

Abstract

The spermatogonial population must maintain a balance between self-renewal and differentiation for continuity of the spermatogenic lineage and therefore fertility. Extrinsic signals emanating from the somatic cell populations in the testes are intricately involved in regulating fate decisions in the undifferentiated and differentiating spermatogonial pools. Importantly, these signals not only originate from Sertoli cells that are considered to be a part of the SSC niche, but also from somatic cells on the surface of the seminiferous tubule and those residing in the interstitium. This review provides an overview of known interactions between spermatogonia and a number of somatic cell type in the testis; including the Sertoli, Leydig, peritubular myoid, macrophage and vascular cells. In particular, we focus on the production of growth factors by these somatic cells that have the capacity to stimulate maintenance of the undifferentiated spermatogonial population as well as self-renewal of spermatogonial stem cells (SSCs). Further, we review the capacity for somatic cell signals to regulate the differentiating transition in the spermatogonial pool via tight control over the synthesis and degradation of the differentiating signal; retinoic acid (RA). This review highlights the intricate and dichotomous role of somatic cells in regulating activities of the spermatogonial population; describing how, depending on the stage of the epithelial cycle, the same somatic cells that support maintenance of the undifferentiated population can instigate entry of these spermatogonia into the differentiating pathway that will culminate in the production of mature male gametes.