VASCULAR REGRESSION DURING AMPHIBIAN METAMORPHOSIS - A SCANNING ELECTRON-MICROSCOPE STUDY OF VASCULAR CORROSION CASTS OF THE VENTRAL VELUM IN TADPOLES OF XENOPUS-LAEVIS DAUDIN

We used scanning electron microscopy and vascular casting to study gro ss arterial supply, venous drainage, and microvascular patterns of the fully developed ventral velum of tadpoles of Xenopus laevis Daudin an d analyzed changes of the velar vascular bed from prometamorphosis to metamorphic climax in a qualitative and quantitative manner. The multi layered, highly secretory ventral velum is supplied bilaterally by an anterior and a posterior velar artery, branches of the external caroti d artery. Velar arterioles branch mainly dichotomously and form a flat two-dimensional capillary meshwork overlying the tops of filterplates I-IV. Thymopharyngeal veins, dorsal branches of the filter plates vei ns, and the internal jugular veins drain the velum toward the venous s inus of the heart. Location, architecture, and the drainage of the vel ar microvascular bed into the venous sinus make a significant contribu tion of the velar capillaries to gas exchange unlikely. Instead, velar capillaries rather serve the nutrition of the secretory epithelium. T he overall morphology of velar vessels from prometamorphosis to metamo rphic climax-deduced from vascular corrosion casts-points to atonic ve ssels with increased leakage indicated by adhering globular extravasat ions, and to obstructed or blind ending vessels evidenced by the taper ed and/or rounded blind ending cast vessels. The significant decrease in the size of the ventral velum during the metamorphic cycle was para lleled by a miniaturization of the velar vascular bed. We hypothetize that this miniaturization occurs by a shortening and fusion of capilla ry mesh elements. Our findings in corrosion casts, particularly the mi niaturization of the velar microvascular bed and the morphology of the regressing capillaries, point to profound morphologic and ultrastruct ural changes in velar vessels; a study on the fine structure of the mi crovascular bed of the ventral velum in metamorphic tadpoles is in pro gress.