Three-dimensional analysis of nephrogenesis in the neonatal rat kidney: Light and scanning electron microscopic studies

In order to clarify the process of renal development more precisely than pr eviously, the present study observed the rat neonatal kidney by scanning el ectron microscopy (SEM) of KOH digested tissue as well as by light microsco py of plastic sections. In the subcapsular region, aggregation of the mesen chymal cells was closely associated with the upper side of the ureteric duc t ampulla, These mesenchymal cells projected a number of fine irregular pro cesses at the basal portion facing the ureteric duct, A spherical cluster t ransformed from the mesenchymal cell aggregation was found on the lower sid e of the terminal ampulla, and was differentiated into the renal vesicle. S ome cells at the top of the renal vesicle formed a cone-shaped projection a nd invaded the ureteric duct ampulla, forming a, connection with it. In the advanced stage, a shallow transverse cleft appeared on the outer lateral s ide of the renal vesicle, and a second cleft was formed on the opposite sid e close to the junction between the renal vesicle and the ampulla, As the t wo clefts deepened, the vesicle assumed the well-known S-shaped body. In th e advanced S-shaped body, the lower limb became cup-shaped, while the segme nt between the middle and lower limbs of the "S" elongated to form a tubula r structure (i.e., the prospective proximal tubule and Henle's loop). The u pper limb of the "S" also increased its length to form a distal tubule, The middle limb of the "S", however, was attached firmly to the cup-shaped low er limb (i,e,, the prospective renal corpuscle) and was considered to becom e the macula densa of the mature nephron, In the maturing renal corpuscle, irregularly shaped cells were observed as a sheetlike aggregation at its va scular pole and were continuous with the vascular smooth muscle cells. Thes e findings will help toward a better understanding of the morphological com plexities of nephrogenesis.