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Spermatogenesis in adult mammals is highly organized, with the goal being continual sperm production. Vertebrate testes are arranged into recurring cellular associations that vary with time and distance along the tubule. These changes over time and distance are designated the cycle of the serniniferous epithelium and the spermatogone wave, respectively. In this Review, we briefly outline the roles that follicle-stirnulating hormone (FSH) and testosterone play in regulating spermatogenesis and describe our current understanding of how vitamin A regulates germ cell differentiation and how it may lead to the generation of both the cycle of the seminiferous epithelium and the spermatogenic wave.
Introduction
Normal fertility in males of most heterogametic species (i.e., species in which either males or females have nonidentical sex chromosomes; e.g., in humans where males have an X and a Y chromosome) requires the constant production of sperm over a long time period; in humans, production begins at puberty and usually continues until death. Spermatogenesis is a very complex, highly organized and regulated process that takes place in the seminiferous epithelium of testis tubules (Figure 1) and involves three major fundamental biological processes: the renewal of stem cells and the production and expansion of progenitor cells (mitosis); the reduction, by one- half, of the number of chromosomes in each progenitor cell (meiosis); and the unique differentiation of haploid cells (spermiogenesis) (Figure 1). In humans, each of these processes initiates at puberty and continues throughout life. Early progenitor cells, which are designated A spermatogonia in the mouse and A-darkspermatogonia in humans, are defined as "undifferentiated." These cells have the potential to become gametes but have not yet committed to the process. Undifferentiated spermatogonia divide mitotically to both repopulate the testicular stem cell population and provide progenitor cells that undergo spermatogenesis. Once spermatogonia enter the "differentiation" pathway, they become known as Al spermatogonia in the mouse and A-paieSpermatogonia in humans and begin a series of irreversible differentiation steps leading to meiosis and spermiogenesis (1) (Figure 1). Differentiating spermatogonia in mice undergo five mitotic divisions before converting to preleptotene spermatocytes. This conversion represents the initiation of meiosis. From this point forward, the steps and cell types of spermatogenesis are conserved between mice and humans. Meiosis occurs in spermatocytes, and these cells can be divided into...