Regional differentiation of the blood-epididymis barrier in stallion (Equus caballus)

The occurrence, localization and ultrastructural characteristics of a blood-tissue barrier throughout the stallion proximal seminal excurrent duct system were studied by the exclusion of electron-dense tracers and freeze-fracture techniques. Striking differences were observed in the distribution of lanthanum tracer and in the geometrical organization of the zonulae occludentes along the ductus efferentes, epididymides and vas deferens. The zonulae occludentes domain, the principal structural component of the blood-epididymis barrier, differed in permeability, width and strand numbers along the ductus. The flow of tracer was not impeded by the vascular endothelium, the peritubular myoid layer or other surface membrane specialization. The tight junctions of the ductuli efferentes are poorly developed but unlike those of rats, guinea pigs or man they are not associated with gap junctions. The result of the tracer experiments and the low number of tight junctional strands in the ductuli efferentes suggests that the barrier of the ductuli efferentes corresponds to the 'leaky type'. In the epididymis the zonulae occludentes are well developed throughout the duct. The greatest number of strands, especially in the cauda epididymidis regions, correlates well with a decreased junctional permeability in this area. Another evidence for the existence of the stallion blood-epididymis barrier are the differences in the proteins electrophoretic profiles between blood plasma as compared with the fluid inside the seminal ductus. This junctional complexes contribute to create a highly defined luminal fluid microenvironment that ensures the sperm maturation and survival.