THE STRUCTURE OF THE FUNCTIONAL PRONEPHROS IN LARVAE OF AMBYSTOMA OPACUM AS STUDIED BY LIGHT AND ELECTRON MICROSCOPY.

Abstract

AbstractThe functional pronephros in early larvae of Ambystoma opacum, studied in serial sections, consists of an anterior and posterior nephrostome, respective proximal tubules which fuse into a common proximal tubule, a ciliated intermediate segment, and a distal tubule, which leaves the pronephros posteriorly as the nephric duct. In the coelom medial to each pronephros is a glomus, arising from the dorsal aorta.In electron micrographs of osmium‐fixed, methacrylate‐embedded pronephros and glomus, the fine structure generally resembles that seen in adult kidneys. Proximal tubule cells show a brush border of irregular microvilli, tubular invaginations, resorption granules, and very few basal infoldings. Cells of the distal tubule have numerous radially‐oriented mitochondria, and the basal infoldings are generally well developed. The fine structure of the nephric duct seems to resemble that of the distal tubule. Cells of the nephrostomes and intermediate segment bear cilia of usual fine structure, and contain melanin granules, lipid droplets and slender mitochondria. The glomus resembles a glomerulus in having an endothelium with sparse pores, a basement membrane, and epithelial cells with numerous interdigitating foot processes.The similarity at the fine structure level between the pronephros and the more posterior mesonephros and metanephros of adult vertebrates suggests the serial homology of vertebrate nephrons, and thus constitutes further evidence for the archinephros or holonephros theory of the origin of vertebrate kidneys.