Blood testis barrier : lecture note

Blood testes barrier

Definition :

The blood–testis barrier is a physical barrier between the blood vessels and the seminiferous

tubules of the testes.

The name "blood-testis barrier" is misleading in that it is not a blood- organ barrier in a strict sense,

but is formed between Sertoli cells of the seminiferous tubule and as such isolates the further

developed stages of germ cells from the blood.

A more correct term is the "Sertoli cell barrier" (SCB).

To understand blood testes barrier , you have to understand structure of seminiferous tubule of

testes first.

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Each testis possesses approximately 500 sperm-producing seminiferous tubules (30 to 70 cm long and 150 to 250 m in diameter) embedded in a loose vascular connective tissue. The connective tissue wall of each seminiferous tubule, called the tunica propria, is surrounded by a basal lamina.

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  ltrastructural 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

The thick seminiferous epithelium (germinal epithelium) is composed of two different epithelial types: Sertoli (supporting) cells and spermatogenic cells that are in the process of differentiation to form spermatozoa. Sertoli cells (Fig. 21.3) are tall columnar cells that possess large clear indented nuclei, abundant mitochondria, well-developed smooth endoplasmic reticulum, Golgi bodies, endolysosomes, and many

cytoskeletal elements. The occluding junctions formed between adjacent Sertoli cells subdivide the lumen of the  eminiferous tubule into:

• A basal compartment, basal to the tight junctions, which is exposed to the underlying vascular connective tissue

• An adluminal compartment, which is isolated from the vascular connective tissue, establishing a blood-testis barrier and protecting the developing gametes from being exposed to the immune system, which would otherwise mount an

immune response against the developing gametes

The functions of Sertoli cells are to:

• S upport, protect, and nourish developingspermatogenic cells

• Phagocytose cell remnants (residual bodies)discarded during the process of spermiogenesis

• Facilitate the release of mature spermatids into the lumen of the seminiferous tubules via actin-mediated contraction (spermiation)

• S ecrete:• Androgen binding protein (ABP) into the seminiferous tubule lumen, increasing testosterone concentration in the seminiferous tubules

• Inhibin, which hinders the release of FSH

• Fructose-rich fluid, which nourishes and transports spermatozoa along the genital ducts

• Testicular transferrin to assist in providing iron to maturing gametes

• Antimüllerian hormone, during embryonic development, which prevents the formation of the female reproductive system and permits the development of the male reproductive system

The Sertoli cell–to–Sertoli cell junctional complex is the site of the blood–testis barrier.

The existence of a barrier between the blood and the interior of the seminiferous tubules accounts for the fact that few substances from the blood are found in the testicular fluid. The testicular capillaries are fenestrated and permit passage of large molecules. Spermatogonia have free access to materials found in blood. However, occluding junctions between the Sertoli cells form a barrier to the transport of large molecules along the space between Sertoli cells. Thus, the more advanced stages of spermatogenesis are protected from blood-borne products protecting male germ cells against blood-borne noxious agents.

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 spermantibodies 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.