178 



B. V. HALL AND L. E. ROTH 



Fig. 2. Photomontage of electron micrographs of a later stage in the development of the rat, renal glomerulus, demonstrat- 

 ing the early formation of the lobular organization, and initial stages of capillary formation. These incompletely differen- 

 tiated capillaries are characterized by a very limited and apparently discontinuous capillary lumen, and by large, connecting 

 masses of still undiflerentiated endothelial cells, and by the presence of few differentiated endothelial cells. The capillary 

 spaces appear to develop by the expansion and subsequent fusion of intra-cytoplasmic spaces which develop within the 

 differentiating endothelial cells. (A) Capsular epithelium, and basement membrane, still characteristically thin. {B) Much 

 enlarged capsular space. (If invagination played an important role in glomerular formation, this space should be reduced, 

 not enlarged.) (C) Maturing podocytes, developing discrete processes, pedicels and trabeculae, and greatly increasing in 

 size. (D) The cell surface membrane of the podocyte, which folds to form the numerous pedicels and trabeculae, while the 

 definitive capillary basement membrane forms in intimate relationship with the surface membrane of the endothelial cells. 

 (£) Prospective and differentiated endothelial cells. (F) Maturing arteriole. (G) Connection with tubule. Magnification x 2025. 



and processes in prospective podocytes. The mature 

 podocyte (2, 3) is characterized by an extensive and 

 complex system of relatively large processes, the 

 trabeculae, and numerous, minute, interdigitating, 

 terminal processes, the pedicels (foot-processes) 

 which arise bilaterally from more or less parallel 

 trabeculae radiating from the central mass of each 

 podocyte. Differentiating podocytes acquired first, 

 short, relatively broad processes, like those in fig. 1, 

 which appear to fit into irregular spaces between the 

 peripheral prospective endothelial cells. In later 

 stages, it appears that these apparent processes have 

 developed into ridges, or processes (the trabeculae), 

 which are 30-40 // long in some instances. They seem 

 to penetrate into the compact inner mass of prospec- 

 tive endothelium and to separate prospective endo- 

 thelial cells into connecting groups of primordial 

 capillaries that differentiate later into definitive capil- 



laries (fig. 2), The first, short, crude podocytic pro- 

 cesses seemed to have few or to lack completely 

 discrete pedicels. The elongated processes, however, 

 showed numerous, typical, bilaterally arranged pedi- 

 cels. Podocytic processes appeared to develop by 

 folding of the cell surface, and by extension of dis- 

 crete, cytoplasmic processes. The entire elaborate 

 system of large and small processes forms only from 

 the surface of a podocyte opposed to the surface 

 membrane of the differentiating endothelium. Al- 

 though the present study does not establish with 

 certainty that growth of podocytic processes into 

 the unorganized mass of prospective endothelium 

 plays an active role in organizing the primordial 

 capillaries and lobules, the evidence of the electron 

 micrographs is in full accord with such a view. 



Capillary lumina or enlarged cytoplasmic vesicles 

 were not found in prospective endothelial cells in 



