**1. Introduction**

Mammalian spermatogenesis is a cell-organized differentiation process of male germ cells in the testis. This process includes spermatogonium mitosis, spermatocyte meiosis, and spermatid morphogenesis. Throughout spermatogenesis, Sertoli cells tightly embrace differentiating germ cells in the seminiferous epithelium and create a microenvironment essential for germ cell differentiation. In addition to physical support, Sertoli cells provide nutrition for developing germ cells and take up apoptotic components. During spermatogenesis, most of male germ cells undergo apoptosis, and those that finalize differentiation process will shed their most cytoplasmic components as residual bodies (RB). Apoptotic germ cells (AGC) and RB must be timely eliminated by Sertoli cells via phagocytosis.

The phagocytic elimination of AGC and RB by Sertoli cells has been proposed to contribute to spermatogenesis in several ways: (1) reducing space competition for enormous male germ cells to finalize differentiation process, (2) preventing noxious cellular contents that may be released by necrosis of apoptotic germ cells, (3) removing autoantigens that may induce an autoimmune response, and (4) recycling of AGC and RB as an energy source for Sertoli cells.

Various mechanisms are involved in the regulation of Sertoli cell phagocytosis of AGC and RB. The interaction of class B scavenger receptor type I (SR-BI) expressed

on phagocytes and phosphatidylserine (PS) exposed on apoptotic cell surfaces is a universal mechanism by which phagocytes engulf apoptotic cells [1]. This mechanism is also involved in the regulation of the phagocytosis of AGC and RB by Sertoli cells [2]. Tyro3, Axl, and Mer (TAM) tyrosine kinase receptors and their functional common ligand, growth arrest specific gene 6 (Gas6), are also essential for optimal phagocytosis of AGC by Sertoli cells. Several other genes that regulate Sertoli cell phagocytosis of AGC have been recognized. The mechanisms underlying phagocytic removal of AGC by Sertoli cells are the main focus of this article.

Impairment of Sertoli cell phagocytosis is associated with pathogenesis and dysfunction of the testis, thus impairing male fertility. The inhibition of Sertoli cell phagocytic ability disrupts spermatogenesis [3]. Gene mutation that impairs Sertoli cell phagocytosis may lead to autoimmune orchitis and male infertility [4]. The pathogenic conditions due to impaired Sertoli cell phagocytosis are mentioned in the text.
