The pig mesonephros. III. Distal tubule, collecting tubule, and Wolffian duct: SEM- and TEM-studies.

The ultrastructure of the distal and collecting tubules of mature pig mesonephroi (41st gestational day) was studied in perfusion-fixed embryos. In the distal tubule, the three subsegments postulated on the basis of enzyme histochemistry show only minor differences of their luminal surfaces, mostly of cell size. TEM photographs reveal a single cell type with interdigitating basolateral processes, frequently flattened to 30-120 nm lamellae devoid of organelles. Larger interdigitating processes harbor vertically oriented mitochondria in the form of indented plates. The macula densa cells are small, do not interdigitate, and have distended intercellular spaces. The collecting tubule starts with a dorsal convolution, in which intercalated cells (with apical microfolds and numerous mitochondria) occur in addition to interdigitating cells. Further down this segment, the interdigitating cells are gradually replaced by principal cells characterized by interlocking lateral microvilli, basal infoldings, and relatively few organelles. Intercalated cells extend into the Wolffian duct. Although the pig mesonephros has the most differentiated nephron of the mammals studied so far, with metanephros-like cells, its intrinsic urinary concentrating capacity appears to be low in view of its vascularization pattern and nephron architecture.