The central vessel of the renal countercurrent bundles of two marine elasmobranchs--dogfish (Scyliorhinus caniculus) and skate (Raja erinacea)--as revealed by light and electron microscopy with computer-assisted reconstruction.

The renal countercurrent bundles of elasmobranch fish were studied by light and electron microscopy. The kidneys of the lesser spotted dogfish, Scyliorhinus caniculus Blainville, and the little skate, Raja erinacea Mitchill, were investigated. Three-dimensional reconstruction with computer assistance revealed the spatial association of the renal tubular segments and their relationships to each other, as well as to the microvasculature. Regular association between structures was assessed by quantification of contact points on histological sections. The bundles contain a hairpin loop of neck segment and the beginning of the proximal tubule, PIa. The limbs of this loop closely adhere to each other, and a second loop (the early distal tubule) coils around the first loop at the tip of the bundle. The collecting tubule runs between the two loops, and merges with the collecting duct inside the end portion of the bundle. A single lymph capillary-like vessel originates from a few blind-ended rami at the tip of the bundle and runs in close contact with the collecting tubule along the entire bundle. This central vessel merges via several side branches with the venous sinusoid capillaries of the peritubular blood circulation. Thereby the central vessel provides a channel for convective flow of NaCl-rich fluid unidirectionally to the venous portal system of the mesial tissue zone of the kidney. By the close spatial arrangement of the collecting tubule and the central vessel countercurrent exchange of urea from the collecting tubule urine to the fluid in the central vessel is feasible. Thus, the spatial organisation of renal tubular segments and the central vessel is considered to represent the morphological correlate to urea retention by the kidney of Elasmobranchii.