Structure of the complex basement membrane underlying the epithelium of the vas deferens in the rat.

Underlying the epithelium of the vas deferens there is a complex basement membrane showing a thick lamina densa separated from the plasma membrane of epithelial cells by a lamina lucida. On the connective tissue side of the lamina densa, there are plaques composed of a material that is similar to that of the lamina densa but is more compact and has a greater electron density. This material also forms plaques at a short distance from the lamina densa, where it appears as irregular nodular masses. The plaques are bridged by striated anchoring fibrils (SAF) that are variable in structure. Some SAF are long (0.5-0.6 micron) and bilaterally symmetrical, with a central fusiform segment and, on each side, coarsely banded segments. While the fusiform segment presents 5 or 6 diffuse cross striations, the coarsely banded segments show distinct bands labeled B1-B4. Shorter SAF show a coarsely banded segment alone or a coarsely banded segment plus a fusiform segment. Some SAF also branch at the level of the fusiform segments, in which case they form star-shaped structures with three or more branches that have their extremities inserted into plaques. The plaques, as well as the lamina densa, are immunohistochemically reactive to type IV collagen, laminin, and heparan sulfate proteoglycan, whereas the SAF are not immunoreactive to these substances. SAF and plaques, considered as integral components of this basement membrane, form a series of arches or open tunnels traversed by collagen fibrils. It is thus apparent that these elements contribute to the attachment of the basement membrane and the overlying epithelium to the underlying dense connective tissue of the lamina propria.