RENAL VASCULATURE AND EXCRETORY SYSTEM OF THE AGAMID LIZARD, CTENOPHORUS-ORNATUS

The structure and ultrastructure of the vasculature and nephric tubule s of the kidney of the Ornate Dragon Lizard, Ctenophorus (=Amphiboluru s) ornatus, was investigated by light microscopy and scanning electron microscopy of resin casts. Compared with other agamid lizards, the ar terial supply of only two arteries per kidney is small. The number of glomeruli per kidney in C. ornatus, at between 400 and 600, is low by reptilian standards and very low for lizards. The glomeruli are not on ly comparatively few in number but are also simple in the nature of th eir capillary configuration. These features are suggestive of a greatl y reduced surface area for arterial filtration. In contrast to the sma ll arterial supply, the afferent venous supply, the renal portal syste m (RPS), is substantial and forms an extensive capillary network throu ghout the kidneys. No venous shunts were found in the kidneys, suggest ing that the venous network is functioning as a true portal system. Va lves were not identified within the RPS, implying that neural control of general vascular tone could alter velocity and perhaps the directio n of blood flow by changing pressure profiles. The nephroi are compara tively short and display features consistent with the production of ur ate precipitates as the primary nitrogenous waste product. A close ass ociation between the distal tubule (DT) and the glomerular hilus may d emonstrate a simple form of a juxtaglomerular apparatus. The finding o f aglomerular tubules indicates an important role for tubular secretio n. The results of this study indicate that these lizards may have a re duced reliance on filtration for homoiostasis and a greater reliance o n tubular secretion. (C) 1993 Wiley-Liss, Inc.