In order to clarify the process of renal development more precisely than previously, the present study observed the rat neonatal kidney by scanning electron microscopy (SEM) of KOH digested tissue as well as by light microscopy of plastic sections. In the subcapsular region, aggregation of the mesenchymal cells was closely associated with the upper side of the ureteric duct ampulla, These mesenchymal cells projected a number of fine irregular processes at the basal portion facing the ureteric duct, A spherical cluster transformed from the mesenchymal cell aggregation was found on the lower side of the terminal ampulla, and was differentiated into the renal vesicle. Some cells at the top of the renal vesicle formed a cone-shaped projection and invaded the ureteric duct ampulla, forming a, connection with it. In the advanced stage, a shallow transverse cleft appeared on the outer lateral side of the renal vesicle, and a second cleft was formed on the opposite side close to the junction between the renal vesicle and the ampulla, As the two clefts deepened, the vesicle assumed the well-known S-shaped body. In the advanced S-shaped body, the lower limb became cup-shaped, while the segment between the middle and lower limbs of the "S" elongated to form a tubular structure (i.e., the prospective proximal tubule and Henle's loop). The upper 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 lower limb (i,e,, the prospective renal corpuscle) and was considered to become the macula densa of the mature nephron, In the maturing renal corpuscle, irregularly shaped cells were observed as a sheetlike aggregation at its vascular pole and were continuous with the vascular smooth muscle cells. These findings will help toward a better understanding of the morphological complexities of nephrogenesis.
