Male Reproductive System
This chapter will expand upon what we learned about the testis in the Endocrine System chapter.
Two of the major functions of the male reproductive system are hormone production (mainly testosterone) and gamete (sperm) production, which both occur in the testes (plural). Each testis (singular) consists of an elaborate network of tubes called the seminiferous tubules. Between the seminiferous tubules are testosterone-producing cells called interstitial cells or Leydig cells (blue cells in Figure 1). The testosterone produced by the interstitial cells stimulates spermatogenesis (sperm production) in the seminiferous tubules.
The overall structure of the seminiferous tubule is governed by Sertoli cells or sustentacular cells (brown cells in Figure 1 are the nuclei of Sertoli cells), which are large columnar cells that divide the tubule into compartments and provide an anchor point for cells in various stages of mitosis and meiosis.
Figure 1: seminiferous tubules of the testis with and without illustration overlay
Spermatogenesis
The outermost wall of the seminiferous tubule is lined with stem cells called spermatogonia (plural). When stimulated to divide, the spermatogonium (singular) undergoes mitosis. One of the daughter cells produced remains at the tubule wall to continue stem cell function. The other daughter cell migrates slightly towards the tubule lumen and differentiates to become a primary (1°) spermatocyte (2n). The diploid primary spermatocyte (2n) begins meiosis. Following meiosis I, two haploid secondary (2°) spermatocytes (n) are formed. Each secondary (2°) spermatocyte (n) undergoes meiosis II. The level of magnification utilized in Figure 1 does not make it possible to distinguish between primary and secondary spermatocytes. Following the events of meiosis II, a total of 4 spermatids (2 for each secondary spermatocyte) are formed.
The spermatids, anchored into the tubule wall, undergo a process of cellular events called spermiogenesis. The spermatids becomes more stream-lined and elongated cells called a spermatozoa and are released into the tubule lumen. While the spermatozoa have flagellum, they do not gain the ability to “swim” until they passes through the epididymis.
Sperm Maturation
Maturation of the spermatozoa into motile sperm takes place as the spermatozoa travel through the male reproductive tract. From the seminiferous tubules, the spermatozoa travel to the epididymis. The pseudostratified columnar epithelium of the epididymis contains stereocilia on their apical surface (Figure 2). While they look like cilia, the stereocilia actually function like microvilli to absorb excess cytoplasm from the sperm, which increases their motility. During their time in the epididymis, sperm gain locomotion and learn to “swim”.
Figure 2: Epididymis with and without illustration overlay
Parts of a Sperm
Figure 3 illustrates the parts of a sperm cell. The tail is composed of a flagellum (yellow), which undergoes a whip-like motion to propel the sperm through the female reproductive tract. The midpiece (green) contains numerous mitochondria which supply energy needed for locomotion. The head of the sperm contains the nucleus (blue) and cap-like acrosome (purple). The acrosome contains hydrolytic enzymes that are released to help the sperm penetrate the outer layer of the oocyte during fertilization.
Figure 3: Parts of a sperm with and without illustration overlay
Chapter Illustrations by:
Juan Manuel Ramiro Diaz, Ph.D.
Edna Martinez Sanchez, B.A.
