KIDNEY ULTRASTRUCTURE 



reached, the access to the surrouncUng capil- 

 laries is easily achieved. 



The Golgi zone of these cells is quite re- 

 stricted and it is believed that it is mainly- 

 involved in secretory processes, particularly 

 in the synthesis of protein used either by the 

 cell itself or for extracellular purposes. This 

 hypothesis is supported by the fact that 

 rough-surfaced endoplasmic reticulum is ab- 

 sent in these cells, but the cytoplasm is filled 

 by numerous RNA-particles which presuma- 

 bly are the structural evidence of cytoplasmic 

 proteins. 



The Loop of Henle. When the nephron 

 leaves its convoluted portion in the cortex 

 of the kidney, it assumes a straight course 

 down into the medulla and papilla. The first 

 part of this portion is called the straight de- 

 scending loop of Henle. Once down in the 

 papilla, it bends back again and the turning 

 part is called the thin segment of HenWs loop. 



PROXIMAL CONVOLUTED TUBULE 



Fig. 8. Schematic representation of the basic 

 structures of the proximal tubule cells of the 

 mouse kidney: nucleus (N), mitochondria (M), 

 microbodies (m), Golgi apparatus (Go), auto- 

 fluorescent vacuoles (V), large dense granules 

 (D), ribonucleoprotein particles (R), brush border 

 extensions (B), tubular invaginations (Ti), plasma 

 membrane (PM) with infoldings, terminal bars 

 (TB), and basement membrane (BM). (After 

 Rhodin, 1954). 



cell borders 

 bcsame^f rnembrane 



secfioma " cytoplasmic laimllat 



cytoplasmic lamtliaz frcm eel Is pulled auaq 



terminal 

 bars 



rnifochondria 



nucleus 



cell 

 borderi 



■ THIN SEGMENT OF HENLE'S LOOP 



Fig. 9. Three-dimensional reconstruction of 

 cells of the proximal convolution and thin seg- 

 ment of the mouse kidney. The reconstruction is 

 not based on serial sections. (After Rhodin, 1958). 



From there on, it approaches again the cortex 

 and the neighborhood of the glomerulus. This 

 portion is called the ascending (thick) limh of 

 Henle's loop. It should be stressed that in 

 most nephrons, the thin segment comprises 

 a considerable part of the descending loop, 

 the hairpin turn itself, and for some distance, 

 the ascending loop. 



Straight Descending Loop. The cells of the 

 straight descending loop of Henle have a 

 light cytoplasm due to a certain scarcity in 

 mitochondria and RNA-particles. The sur- 

 face of the cells shows scattered brush border 

 extensions, but these are shorter and coarser 

 than those found in the proximal convoluted 

 portion of the nephron (Fig. 10). The lumen 

 of the tubule is frequently open, presumably 

 because of the relatively few brush border 

 extensions. Tubular invaginations of the 

 surface plasma membrane are few and the 

 basal infoldings of the plasma membrane, so 

 numerous in the proximal convoluted part, 

 are shallow and few, often completely absent. 



173 



