This electron micrograph demonstrates a Sertoli–to–Sertoli junctional complex and, in close proximity, a Sertoli–to–spermatid junctional specialization. Condensation and shaping of the spermatid nucleus (N) are well advanced. The acrosome (A) of the spermatid appears as a V-shaped profile, and in close association with it is the Sertoli cell junctional specialization characterized by bundles of microfilaments that are cut in cross section (arrows). The associated profile of endoplasmic reticulum resides immediately adjacent to the microfilament bundles. The Sertoli–to–Sertoli junction lies below, joining one Sertoli cell (S1) to the adjacent Sertoli cell (S2). The arrowheads indicate the limits of the junction. Note that the junction here reveals the same elements, the microfilament bundles (arrows) and a profile of endoplasmic reticulum, as are seen in the Sertoli–to–spermatid junctional specialization. Not evident at this magnification is the tight junction associated with the Sertoli–to–Sertoli junctional complex. 30,000.
creates a permeability barrier called the blood–testis barrier. This barrier is essential in creating a physiologic compartmentalization within the seminiferous epithelium with respect to ionic, amino acid, carbohydrate, and protein composition. Therefore, the composition of the fluid in the seminiferous tubules and excurrent ducts differs considerably from the composition of the blood plasma and testicular lymph.
Plasma proteins and circulating antibodies are excluded from the lumen of the seminiferous tubules. The exocrine secretory products of the Sertoli cells (particularly the androgen-binding protein (ABP), which has a high binding affinity for testosterone and DHT) are highly concentrated in the lumen of the seminiferous tubules and maintain a high concentration of testosterone, which provides a favorable microenvironment for the differentiating spermatogenic cells.
Most important, the blood–testis barrier isolates the genetically different and therefore antigenic haploid germ cells (secondary spermatocytes, spermatids, and sperm) from the immune system of the adult male. Antigens produced by, or specific to, the sperm are prevented from reaching the systemic circulation. Conversely, -globulins and specific sperm antibodies found in some individuals are prevented from reaching the developing spermatogenic cells in the seminiferous tubule (Folder 22.3). Therefore, the blood–testis barrier serves an essential role in isolating the spermatogenic cells from the immune system.
Sertoli cells have both exocrine and endocrine secretory functions.
In addition to secreting fluid that facilitates passage of the maturing sperm along the seminiferous tubules to the intratesticular ducts, Sertoli cells produce critical factors necessary for the successful progression of spermatogonia into spermatozoa. They secrete a 90-kilodalton androgen-binding protein (ABP). ABP concentrates testosterone in the luminal compartment of the seminiferous tubule, where high concentrations of testosterone are essential for normal maturation of the developing sperm.