Development of the renal corpuscle during metamorphosis in the lamprey

The renal corpuscle of the adult lamprey, Petromyzon marinus L., is formed during the programmed period of metamorphosis. Development is initiated early in this metamorphic period and is marked by the synchronous formation and growth of rudimentary nephron units (RNU) from longitudinal cord of nephrogenictissue extending from the posterior tip of the degenerating larval kidney to the cloaca and connected to the peritoneal epithelium. Detachment of the RNU from the peritoneum involves autolysis and cell death and is accompanied by their branching into five or six hexagonally-arranged nephrons which radiate from the original point of attachment. Differentiation of the epithelial cells at the proximal ends of the nephrons is preceded by the widening of lateral intercellular spaces, the formation of tubular lumina (primitive urinary spaces), the loss of apical cell junctions, and the development of a capillary network with its associated mesangium. With the extension of the capillaries and mesangium between the proximal ends of adjacent undifferentiated nephrons, visceral epithelial cells (podocytes), with long cell processes (trabeculae) and slit membranes, make their appearance. The urinary spaces resulting from this form of development are lined by the epithelium of the dilated ends of the nephrons (nephric capsules). The cells of these capsules differentiate mainly into podocytes, but a few parietal cells connect to the draining tubule. This method of development explains the unique form of the renal corpuscle in the adult lamprey. Despite the type of morphogenesis, this renal corpuscle possesses the fine-structural features seen in the renal corpuscles of other vertebrates.