FXECTKON MKHOSCOI'Y 



The capillaries of the f^lomerulus are not saccliaridcs which represent the main con- 

 freely suspended within the space of Bow- tent of the basement membrane may have a 

 man's capsule, l)ut are attached to each other deiinite influence upon the filtration of the 

 by the so called mesangium (Fig. 1). This is urine. 



formed by a number of cells which lie in the The innermost layer of the capillary wall 



center of the capillary bed and attach to the is made up of the endothelial cells. These cells 



sides of the main capillaries which face the are extremely flat and the cytoplasm dis- 



center of the capillary stem. The cytoplasm plays a number of pores regularly distributed 



of the mesangial cells is quite electron dense (Fig. 2). The width of each pore ranges be- 



and contains a fair number of fibrillar struc- tween 200 and 900A. Except for the area 



turcs, possibly indicating that the cells are of where the nucleus is located, the cytoplasm 



mesodermal (connective tissue) origin. has an average thickness of about 800A. 



The cells of the visceral layer of Bowman's In considering the complexity of the 

 capsule are squamous but quite different glomerular filtering membrane, it is believed 

 from the squamous cells of the parietal layer, that the size of the substances filtered is de- 

 The cytoplasm of the cell is pulled out into termined not only by one of the components 

 long and coarse extensions, the podocytes, of this membrane, but rather by a combined 

 each of which sends out numerous small effortof all three together. This would explain 

 processes, the pedicels, which surround the why pathologic damages to one or several 

 capillary and attach to its basement mem- components of the filtering membrane may 

 brane, frequently interdigitating with each sometimes result in a similar functional im- 

 other (Fig. 2). The interdigitation occurs pairment of the filtration process, 

 with pedicels of neighboring podocytes, be Proximal Convolution. The first part 

 they podocytes of the same cell body or those of the tubule is called the proximal convolu- 

 of a neighboring cell. It is frequently seen tion because it is highly coiled and inter- 

 that the pedicels not only interdigitate with twined with neighboring tubules as long as 

 each other but also penetrate underneath the tubules are located within the cortex of 

 other pedicels. The foot processes (pedicels) the kidney and are in the vicinity of the 

 leave a small, slit -formed space free in be- glomerulus. The cells of the proximal con- 

 tween each other, the width of this space volution are of a cuboidal or slightly col- 

 ranging between 70 and lOOA. These spaces umnar type. Their cytoplasm is quite dense 

 have been called slit-pores. The pedicels and in the electron micrographs because of a 

 the slit-pores are believed by some to con- large number of mitochondria and also be- 

 stitute the structures w^hich are responsible cause of the fact that abundant RNA-parti- 

 for the filtration mechanism of the glomeru- cles are densely packed within the cells. In 

 lar filtering membrane. By a certain swelling a cross section of the tubule, two to three 

 or shrinkage of the pedicels, the width of the nuclei are seen as compared to three to five 

 slit-pores could be regulated, thus, either nuclei in a cross section of a distal convolu- 

 hindering or letting through various amounts tion. In the light microscope, it is virtually 

 of fluid or substances (particles) of various impossible to see the cell borders. They can 

 sizes. be seen in the electron micrograph, but they 



The basement membrane of the capillaries are difficult to trace throughout because of 



has the usual ultrastructural appearance the high degree of interdigitation that exists 



found in mammalian tissues (Fig. 2). It is between adjacent cells. 



quite homogeneous and only occasionally In well preserved tissue the lumen is usu- 



can there be detected fibrillar units as com- ally closed (Fig. 3). The reasons for this may 



ponents. It is believed that the mucopoly- be that a certain collapse of the tubule or 



166 



