Cells and Structures of the Renal Corpuscle 



177 



Fig. 1. Photomontage of electron micrographs of an early stage of a developing, rat renal glomerulus, with prospective 

 endothelial cells but without capillaries, or capillary structures. {A) Bowman's capsular epithelium, and thin capsular 

 basement membrane characteristic of these early stages. {B) Initial capsular space, seemingly developed by separation of the 

 two epithelial layers. (C) Prospective podocytes, or Bowman's visceral epithelial layer. (D) Prominent cell surface mem- 

 branes between the outermost prospective endothelial cells and the prospective podocytes. Especially noteworthy are the 

 absence of a capillary basement membrane, and the short, broad extensions of podocytic cytoplasm between the prospec- 

 tive endothelial cells. (£") These extensions may play an important role in the organizing of prospective endothelial cells 

 into the capillary system characteristic of the glomerulus. (F) Two cells of an immature arteriole. (C) Capsular connection 

 with tubule. Note within the mass of prospective endothelial cells, the mitotic figure, ni. and the precocious erythrocytes, 

 as f, and the complete absence of formed capillaries. Osmium fixed. Magnification 2065. 



spective endothelial and podocytic cells were seen 

 bounding the narrow space which in a later stage 

 would be occupied by lamina densa. 



During these early stages of development, the 

 basement membrane of Bowman's capsule appeared 

 very tenuous. It had much the appearance of a fused, 

 thickened membrane on the external surface of the 

 prospective parietal epithelium. Flattening of the 

 outermost concentric layer of cells and the concomi- 

 tant initial definite appearance of a limited capsular 

 space was the next recognizable advance in the dif- 

 ferentiation of the Malpighian corpuscle. Pronounced 

 mitotic activity of the prospective endothelial cells 

 characterized this stage (fig. i ). The flattening of the 

 prospective parietal cells, and the enlargement, 

 perhaps even the initiation, of Bowman's capsular 

 space may depend upon the elaboration and accu- 

 mulation of fluid between the parietal and visceral 



12 — 568204 Electron Microscopy 



layers of Bowman's capsule. Capsular space appears 

 to arise from separation of the parietal and visceral 

 epithelial layers. It then enlarges, if invagination 

 played a major \o\c in glomerular development 

 capsular space should decrease. The.sc facts support 

 the concept that in the rat the renal corpuscle is 

 not developed by invagination. 



DitTerentiating podocytes, after developing pro- 

 minent surface membranes, usually appeared par- 

 tially separated from each other. It appears that 

 membrane formation was important to the separa- 

 tion process, increase in surface area of the inner 

 cell mass, afforded b\ mitotic di\ision and growth 

 of the prospective endothelial cells, may be an added 

 important, mechanical factor tending to induce 

 separation of the developing podocytes. 



The next observed advance in glomerular develop- 

 ment was initiation of formation of complex folds 



