ELECTRON :\lICKOSCOPY 



^^^M^^ 



6 



Fig. 10. Diagram showing the essential fea- 

 tures of the colls lining different parts of a tj'^pical 

 cortical nephron in mammalian kidney as seen 

 with the electron microscope. 



1) Collecting tubule (arched or proximal part) : 

 dark intercalated cell; 2) Collecting tubule (arched 

 or proximal part): light cell; 3) Proximal convo- 

 luted tubule: proximal part; 4) Distal convoluted 

 tubule: intercalated part (Schaltstiiek); 5) Proxi- 

 mal convoluted tubule: terminal portion; 6) Distal 

 convoluted tubule: thick (ascending) limb; 7) 

 Thin segment of Henle's loop. (After Rhodin, 1958) 



These phenomena seem to indicate that less 

 resorptive activity is performed by the 

 straight descending loop as compared with 

 the convoluted proximal part of the nephron 

 when judging from a structural point of view. 

 Thin Segment. The epithelium of the thin 

 segment of Henle's loop is of a squamous 

 type with extremely flattened cytoplasm 

 (Fig. 7). The cell surface shows only scattered 

 short microvilli and tubular invaginations 

 are not recorded. The mitochondria are 

 scarce and exceedingly small. The cytoplasm 

 is light due to a small number of RNA-par- 

 ticles. Basal infoldings are present only be- 

 neath the nucleus where the cytoplasm is of 

 greater thickness than elsewhere. The at- 

 tenuated part of the cell shows some quite 

 characteristic features of this portion of the 

 nephron. It displays a large number of cyto- 



plasmic extensions which rest on the base- 

 ment membrane. These extensions resume 

 the shape of the arms of a starfish and they 

 interdigitate frequently with similar exten- 

 sions of neighboring cells (Fig. 9). This pat- 

 tern is reminiscent of the way the epithelial 

 cells of the glomerular capillaries are ar- 

 ranged. However, in the case of the thin seg- 

 ment, the interdigitated cell processes al- 

 ways show a terminal bar close to the surface 

 which presumably secures the firm attach- 

 ment of individual cell processes to one an- 

 other. It can, therefore, be assumed that 

 there do not exist any "slit-pores" between 

 the cells of the thin segment as was indi- 

 cated to be the circumstances regarding the 

 glomerular capillary epithelial cells. 



In the papilla and deeper portions of the 

 medulla, the descending and ascending parts 

 of the thin segment are closely opposed to 

 each other as well as to the capillaries (vasa 

 recta) and the collecting ducts. It has been 

 suggested that the close juxtaposition would 

 facilitate the mutual exchange of fluid, sub- 

 stances and energy because of the close re- 

 semblance of this system to the basic princi- 

 ple of a counter current system with streams 

 moving in opposite directions. The ultra- 

 structure of the thin segment seems to sup- 

 port this hypothesis. The thin walls of its 

 loop are evidently ideal to serve this ex- 

 change of fluid and substances. 



Ascending (Thick) Limh. The ascending 

 limb of Henle's loop is slightly wider than 

 the other portions of the nephron and has, 

 therefore, been called the thick limb. The 

 cells have a cuboidal shape and a free open 

 lumen is always to be found. The extreme 

 multitude of mitochondria makes this sec- 

 tion of the nephron stand out more clearly 

 than the others, both in light and electron 

 microscopy. Not only are the mitochondria 

 numerous, but they also have a coarse and 

 elongated shape as compared to the spherical 

 form of the other parts of the nephron (Fig. 

 6). The mitochondria are densely packed and 

 arranged with their long axes perpendicular 



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