FIGURE 22.14 • Diagram of organization of seminiferous epithelium in humans and other species. a. In subhuman species, a particular cellular association occupies varying lengths along the tubule. Therefore, in a typical cross section only a single cellular association is observed. b. In humans, cellular associations occur in irregularly shaped areas along the tubule, and therefore, a cross section typically shows two or more cellular associations. (Modified from Dym M. In: Weiss L, ed. Cell and Tissue Biology: A Textbook of Histology, 6th ed. Baltimore: Urban & Schwarzenberg, 1988.)
Sertoli cells constitute the true epithelium of the seminiferous epithelium.
Sertoli cells (sustentacular cells) are tall, columnar, non-replicating epithelial cells that rest on the thick, multilayered basal lamina of the seminiferous epithelium (Fig. 22.15). They are the supporting cells for the developing sperm that attach to their surface after meiosis. Sertoli cells contain an extensive sER, a well-developed rER, and stacks of annulate lamellae. They have numerous spherical and elongated mitochondria, a well-developed Golgi apparatus, and varying numbers of microtubules, lysosomes, lipid droplets, vesicles, glycogen granules, and filaments. A sheath of 7to 9-nm filaments surrounds the nucleus and separates it from other cytoplasmic organelles.
The euchromatic Sertoli cell nucleus, a reflection of this very active cell, is generally ovoid or triangular and may have one or more deep infoldings. Its shape and location vary. It may be flattened, lying in the basal portion of the cell near and parallel to the base of the cell, or it may be triangular or ovoid, lying near or some distance from the base of the cell. In some species, the Sertoli cell nucleus contains a unique tripartite structure that consists of an RNA-containing nucleolus flanked by a pair of DNA-containing bodies called karyosomes (Fig. 22.16).
In man, characteristic inclusion bodies (of Charcot-Btcher) are found in the basal cytoplasm. These slender fusiform crystalloids measure 10 to 25 m long by 1 m wide and are visible in routine histologic preparations. With transmission electron microscopy, they are resolved as bundles of poorly ordered, parallel or converging, straight, dense 15-nmdiameter filaments (see Fig. 22.15). Their chemical composition and function are unknown; however, recent studies detected an accumulation of lipoprotein receptor (CLA-1) proteins. This suggests that inclusion bodies might be involved in lipids transport and their utilization by the Sertoli cells.
FIGURE 22.15 • Electron micrograph of a human Sertoli cell.
This electron micrograph shows characteristic crystalloid inclusion bodies of Charcot-Btcher in the basal cytoplasm of the Sertoli cell. The basal lamina (BL) is indicated for orientation. 9,000. Inset. This higher magnification shows filaments of the crystalloid. 27,000. (Courtesy of Dr. Don F. Cameron.)
The Sertoli cell–to–Sertoli cell junctional complex consists of a structurally unique combination of membrane and cytoplasmic specializations.
Sertoli cells are bound to one another by an unusual Sertoli cellto-Sertoli cell junctional complex (Fig. 22.17). This complex is characterized, in part, by an exceedingly tight junction (zonula occludens) that includes more than 50 parallel fusion lines in the adjacent membranes. In addition, two cytoplasmic components characterize this unique junctional complex:  A fattened cisterna of sER lies parallel to the plasma membrane in the region of the junction in each cell. Actin flament bundles, hexagonally packed, are inter posed between the sER cisternae and the plasma membranes.