A photomicrograph of a cross section of the adult male seminiferous tubule is shown in Figure 1A. The seminiferous tubule walls are composed of germinal epithelial cells and Sertoli cells. The germ cells undergo meiotic divisions, differentiation, and maturation (spermatogenesis, discussed in section V.D) as they move towards the center of the lumen, as shown in Figure 1B. Mature spermatozoa are transported through the lumen of the seminiferous tubules to the highly convoluted network of ducts known as the rete testis and then through the efferent ductules to the epididymis, where they are stored.
Fig1. The seminiferous tubule and surrounding interstitium. A. The photomicrograph of a testicular section from a normal human male shows portions of two seminiferous tubules (ST) and the relationship between them and the Leydig cells (LC) in the interstitium (IT). The interstitium also contains myoid cells and capillaries which, along with the Sertoli cells in the tubules, are not clearly visible in this picture. Reprinted from Swerdloff, R.S., Wang, C., and Kikim, A.P.S., 2nd ed. (2009). Hypothalamic-Pituitary-Gonadal Axis in Men in Hormones, Brain and Behavior ed. Pfaff, D.W., Arnold, A.P., Farbach, S.E., Etgen, A.M., Rubin, R.T. Academic Press, p. 2378. B. As depicted in this schematic diagram, the seminiferous tubule is defined by the basement membrane. Outside of the tubules are clusters of Leydig (interstitial) cells, contractile myoid cells, and capillaries. Within the seminiferous tubule the spermatogonia (green), including the subpopulation of spermatogonial stem cells, are in contact with the basement membrane. As the germ cells develop and undergo meiosis to become secondary spermatocytes and spermatids, they move towards the lumen of the tubule. Early to late stages of spermatids are shown from right to left on the luminal side of the tubule. The mature spermatozoa travel from the seminiferous tubule through the duct system as described in Figure 2. The Sertoli cells are in direct contact with the developing germ cells, which are surrounded by thin cytoplasmic extensions of the Sertoli cells. Tight junctions (not shown) between Sertoli cells divide the testis into the basal (containing the spermatogonia) and adluminal compartments.
Fig2. The testis and epididymis. In the scrotal sac, the epididymis lies posterior to the testis, which is covered with the thick tunica albuginea. The spermatic cord is the conduit for blood vessels, nerves, and lymphatics. The sperm produced by the seminiferous tubules are collected in the rete testis and travel through the epididymis to the ductus (vas) deferens.
The Sertoli cells (Figure 1B) line the basement membrane of the seminiferous tubules; they are in close contact with the innermost layer of the basement mem brane and, with their thin cytoplasmic extensions, they surround the germ cells as these undergo their maturation process. Tight junctions between Sertoli cells, along with desmosomes and gap junctions, form the blood–testis barrier. One important function of this barrier, which is formed during puberty, is to prevent exposure of more mature germ cells to antibodies from the interstitial compartment. Local disassembly of the tight junctions allows the passage of primary spermatocytes from the basal to the adluminal compartment.
Sertoli cells provide important nurturing for devel oping sperm and are sometimes referred to as “nurse cells.” Recently it has become clear that Sertoli cells play a crucial role in maintaining the stem cell niche, or microenvironment, required for the self-renewal of progenitor spermatogonia at the basement membrane of the seminiferous tubule. In rodents Sertoli cells also secrete androgen-binding protein (ABP; same as sex hormone binding globulin, SHBG, section III.D) into the seminiferous lumen, where it helps maintain the high testosterone levels necessary for normal spermatogenesis. In both rodents and humans, Sertoli cells secrete inhibin as part of the control of FSH secretion.
