Morphology of the pronephros of the juvenile brown trout, Salmo trutta.

The pronephros in juvenile brown trout (Salmo trutta) consists of a large ovoid renal corpuscle and a pair of tubules. The corpuscle is retained for 11 months, after which the glomerulus regresses. The glomerular arteries come directly from the dorsal aorta. The interstitium is permeated with venous blood vessels that arise from the anterior cardinal veins and are closely apposed to the tubules. Two distinct segments of the pronephric tubular system are distinguished by the histological and ultrastructural features of their component cells: 1) a short, transitional neck in which cells change from capsular epithelium to columnar epithelium, typical of tubules; 2) the convoluted segment composed of cells similar to first proximal tubular cells of the opisthonephros with well-formed brush borders, apical vesicles that vary in size and number along this segment, and lysosomes. Pinocytosis and exocytosis are also evident in this segment. The tubular system increases in length and in its convolutions until about week 9, when the opisthonephros develops. Distally each tubule connects with a Wolffian duct, with cells marked by the absence of apical inclusions and the presence of a uniform brush border, numerous mitochondria, and elaborate infolding of the basalar membrane. Nephrostomes, which are often characteristic of pronephroi, are not present. Cells with long cilia are found throughout the tubular system but are most characteristic of the neck and Wolffian-duct segments.