The mature mesonephric nephron of the rabbit embryo. III. Freeze-fracture studies.

In freeze-fracture replicas of the entire cross-fractured mesonephros of 18 day rabbit embryos the basolateral and luminal cell faces of the different nephron segments were studied and compared with their metanephric counterparts. In the proximal tubule, the shallow zonula occludens exhibited only 1-2 strands and resembled the corresponding metanephric zonula, a very "leaky" type which was found with a considerable paracellular flow component in sites of isotonic reabsorption. Gap junctions were restricted to the proximal tubule and were seen more frequently in its terminal segment. The distal tubule harboured two types of tight junctions. The most common type, a band of 5-8 closely parallel strands, matched the zonula occludens of the metanephric straight distal tubule. The observed particle density of the basolateral membrane (2,500 +/- 306). In addition, the collecting tubule exhibited a zonula occludens of the "tight" variety similar to that which occurred in the metanephric collecting duct. Rod-shaped particles of the luminal membrane were mainly concentrated in some of the intercalated cells but also had developed on principal cells, and occasionally, in the distal tubule. The Wolffian duct, with a deep "tight" zonula occludens, had an obviously rather inactive epithelium with no conspicuous transport-linked membrane specializations.