 Spermatogenic cells, which regularly replicate and differentiate into mature sperm. These cells are derived from primordial germ cells originating in the yolk sac that colonize the gonadal ridges during early development of the testis. Spermatogenic cells are organized in poorly defined layers of progressive development between adjacent Sertoli cells (Fig. 22.6). The most immature spermatogenic cells, called spermatogonia, rest on the basal lamina. The most mature cells, called spermatids, are attached to the apical portion of the Sertoli cell, where they border the lumen of the tubule.
The tunica (lamina) propria, also called peritubular tissue, is a multilayered connective tissue that lacks typical fibroblasts. In man, it consists of three to five layers of myoid cells (peritubular contractile cells) and collagen fibrils, external to the basal lamina of the seminiferous epithelium (see Fig. 22.6). At the ultrastructural level, myoid cells demonstrate features  associated with smooth muscle cells, including a basal lamina and large numbers of actin filaments. They also exhibit a significant amount of rough endoplasmic reticulum (rER), a feature indicating their role in collagen synthesis in the absence of typical fibroblasts. Rhythmic contractions of the myoid cells create peristaltic waves that help move spermatozoa and testicular fluid through the seminiferous tubules to the excurrent duct system. Blood vessels and extensive lymphatic vasculature as well as Leydig cells are present external to the myoid layer.
As a normal consequence of aging, the tunica propria increases in thickness. This thickening is accompanied by a decreased rate of sperm production and an overall reduction in the size of the seminiferous tubules. Excessive thickening of the tunica propria earlier in life is associated with infertility.
Leydig cells (interstitial cells) are large, polygonal, eosinophilic cells that typically contain lipid droplets (Fig. 22.7).
head of
FIGURE 22.4 • Sagittal section of the human testis. a. This schematic diagram shows a midsagittal section of the human testis. The genital duct system, which includes the tubuli recti, rete testis, efferent ducts, duct of the epididymis, and ductus deferens, is also shown. Note the thick connective tissue covering, the tunica albuginea, and the surrounding tunica vaginalis. (Modified from Dym M. In: Weiss L, ed. Cell and Tissue Biology: A Textbook of Histology, 6th ed. Baltimore: Urban & Schwarzenberg, 1988.) b. Sagittal section of an H&E–stained section of the testis and the head and body of the epididymis. Again note the surrounding tunica albuginea and tunica vaginalis. Only a small portion of the rete testis is visible in this section. Its connection with the excurrent duct system is not evident in the plane of this section. 3.
Lipofuscin pigment is also frequently present in these cells as well as distinctive, rod-shaped cytoplasmic crystals, the crystals of Reinke (Fig. 22.8). In routine histologic preparations, these crystals are refractile and measure approximately 3  20 m. Although their exact nature and function remain unknown, they probably represent a protein product of the cell.
Like other steroid-secreting cells, Leydig cells have an elaborate smooth endoplasmic reticulum (sER), a feature that accounts for their eosinophilia (see Fig. 22.7). The enzymes necessary for the synthesis of testosterone from cholesterol are associated with the sER. Mitochondria with tubulovesicular cristae, another characteristic of steroid-secreting cells, are also present in Leydig cells.