THE CENTRAL VESSEL OF THE RENAL COUNTERCURRENT BUNDLES OF 2 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 dogfi sh, Scyliorhinus caniculus Blainville, and the little skate, Raja erin acea Mitchill, were investigated. Three-dimensional reconstruction wit h computer assistance revealed the spatial association of the renal tu bular segments and their relationships to each other, as well as to th e microvasculature. Regular association between structures was assesse d by quantification of contact points on histological sections. The bu ndles contain a hairpin loop of neck segment and the beginning of the proximal tubule, PIa. The limbs of this loop closely adhere to each ot her, and a second loop (the early distal tubule) coils around the firs t loop at the tip of the bundle. The collecting tubule runs between th e two loops, and merges with the collecting duct inside the end portio n 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 cont act with the collecting tubule along the entire bundle. This central v essel merges via several side branches with the venous sinusoid capill aries of the peritubular blood circulation. Thereby the central vessel provides a channel for convective flow of NaCl-rich fluid unidirectio nally to the venous portal system of the mesial tissue zone of the kid ney. By the close spatial arrangement of the collecting tubule and the central vessel countercurrent exchange of urea from the collecting tu bule urine to the fluid in the central vessel is feasible. Thus, the s patial organisation of renal tubular segments and the central vessel i s considered to represent the morphological correlate to urea retentio n by the kidney of Elasmobranchii.