Chemical and morphological differences in the kidney zones of the elasmobranch, Raja erinacea mitch

The histological investigation of the kidney of the skate Raja erinacea revealed a thin cap of dorsal bundles, which contain segments of single nephrons that are arranged separately in a countercurrent manner, and a large ventral zone, where the second proximal segments (PII) and parts of the lower nephron are located. This zonation is apparent in fresh, unfixed material and makes it possible to separate small tissue samples under a dissecting microscope. The osmolality in both zones does not differ. The dorsal bundle zone had a lower urea concentration and a higher sodium concentration than the ventral zone. The differences in the mean concentrations of the tissue samples indicate a gradient for the two substances along the bundles. Determinations of amounts of water and solutes per mg solute-free, dry tissue of the two zones, showed that the amounts of water, total osmolytes, Na and K were greater in the bundle zone than in the ventral zone, while the amount of urea was identical in the two zones. This indicates that the lower urea concentration in the bundle zone is established through an accumulation of Na and water in the interstitium. The countercurrent arrangement of very early and late segments of single renal tubules supports the concept of passive reabsorption of urea in the kidney of the marine elasmobranch.