The elasmobranch kidney

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Abstract
The morphology of the little skate (Raja erinacea) and spiny dogfish shark (Squalus acanthias) nephron has been investigated in sexually mature females by 1) gross observations of the kidney surfaces, 2) vascular injections, 3) scanning electron microscopy, 4) light microscopy. In the little skate, each nephron is highly complex and begins at the urinary pole of the renal corpuscle, which is located between a thin, dorsalbundle zone and a thicker, ventralsinus zone. The nephron loops back and forth, repeatedly entering and exiting each zone. In thebundle zone, segments from each nephron form a bundle of 5 tubules (tubular bundle) which are arranged in a countercurrent loop fashion. A peritubular sheath composed of closely packed, squamous cells wraps the 5 nephron segments of the tubular bundle together and separates each bundle from the next. In thesnus zone the tubules from many nephrons mix freely with each other in large blood sinuses. In the spiny dogfish, the nephron displays a complex pattern similar to that of the skate. Renal corpuscles are adjacent to abundle zone composed of tubular bundles, each wrapped by a peritubular sheath in a cell-rich connective tissue matrix. However, thebundle zone is not limited to the dorsal region of the shark kidney but extends ventrally along deep interlobular septa. Thesinus zone of the shark is like that of the skate except that it is not limited to the ventral regions of the kidney.