The Blood–Epididymis Barrier: Morphological, Physiological, Immunological and Seasonal Aspects and the Impact of Destabilizing Factors

Spermatozoa entering the epididymis from the testis are unable to actively move and do not possess fertilizing ability. These functions are acquired within the lumen of the epididymal ducts, where the components of the blood–epididymis barrier create a specialized environment. The blood–epididymis barrier restricts paracellular transport and stimulates receptor-mediated transport of macromolecules across the epididymal epithelium. The blood–epididymis barrier consists of a pseudostratified columnar epithelium resting on a basement membrane, loose connective tissue of the lamina propria, and capillary endothelium located on its own basement membrane. Apical tight junctions and adherens junctions between adjacent principal cells of the pseudostratified epithelium play a key role in the blood–epididymis barrier’s function. Tight junctions are composed of various families of transmembrane proteins. The vascular component of the blood–epididymis barrier features continuous endothelium on an uninterrupted basement membrane. Alongside the epithelial and vascular components, interactions among dendritic cells, macrophages, and lymphocytes are critical in regulating blood–epididymis barrier permeability. In many species, the epididymis consists of 5 to 9 segments, each with distinct morphofunctional and biochemical characteristics. It has been shown that the barrier function becomes progressively more pronounced from the caput toward the cauda of the epididymis. Impaired function of intercellular junctions in the blood–epididymis barrier is considered a factor contributing to male infertility. This review aimed to analyze the data on the morphofunctional organization of the blood–epididymis barrier.