The Glomerulus-Revisited

Abstract

In the embryologic development of the nephron, the glomerular tuft (renal corpuscle) and the excretory tubules develop from two distinct embryologic origins or anlagen. Nephrotomes constitute the earliest formation of the excretory tubules and appear first as segmentally arranged cell clusters, which grow and become canalized, forming nephric tubules. Formation of the glomerulus begins when the dorsal aorta gives rise to small branches, consisting of tufts of capillaries. Bowman’s capsule forms as a blind terminal expansion at the proximal end of the nephron. in the final stage of development, Bowman’s capsule becomes deeply indented by the glomerular tuft. Which leads to a differentiation of it into 2 layers: 1) an inner, visceral layer and 2) an outer, or external, parietal layer. The two layers are separated by a crescent shaped space, known as Bowman’s or the urinary space.

The inner, visceral layer encloses, and, comes into close apposition with the glomerular capillaries. At the vascular pole of the glomerulus, the visceral layer is reflected off these vessels and becomes continuous with the external, or parietal layer of Bowman’s capsule, which is lined by a very flat polygonal epithelium. Traditionally, it has been depicted that there is a direct opening of the urinary space into the tubule and that the flat cells of the parietal layer of Bowman’s capsule become continuous with the cuboidal epithelium in the neck of the proximal convoluted tubule.

This investigation shows that the junction of the renal corpuscle and the proximal convoluted tubule is a more complicated one. Scanning electron microscopic studies of microdissected nephrons enabled the visualization of the glomerulus, with the two vessels at the vascular pole and the proximal convoluted tubule at the urinary pole. An unexpected finding, was the presence of a gourd-like out-pouching at the urinary pole. (Fig. 1, “A”) Higher magnification showed a saucer shaped depression (50 μm × 25 μm) on its free border. (Fig. 2). This was a consistent finding in the material that was studied.