KIDNEY ULTR VSTRUCTURE 



to the basement membrane of the tubule, convokition is less coiled than the proximal 

 They extend from the basement membrane one. The cells become small and a wide lumen 

 to a level of about one micron from the cell opens up (Fig. 3). The surface of the cells 

 surface. The mitochondrial fine structure is usually has longer microvilli than is the case 

 of the same appearance as elsewhere in the in the thick limb, but the length varies from 

 nephron with the exception that the inner cell to cell. The mitochondria decrease no- 

 membranes (or plates) are more frequently ticeably in number and size, as does the 

 seen arranged parallel with the long axis of number of RXA-particles, features which all 

 the mitochondrion. contribute to a light appearance of the cell 



The free surface of the cells displays a cytoplasm. A few shallow and narrow in- 

 number of small and scattered microvilli. The foldings of the basal plasma membrane can 

 luminal portion of the cells, more or less de- be recorded, but they are too narrow to ad- 

 void of mitochondria, is pervaded by abun- mit any mitochondria to be enclosed in the 

 dant microvesicles bounded by a smooth small cytoplasmic strands they give rise to 

 single membrane. Some of the vesicles are (Fig. 5). A fair number of small vesicles is 

 connected with the surface plasma mem- still to be found in the luminal part of most 

 brane. The Golgi apparatus of the cells oc- of the cells, but cells can be recorded which 

 cupies a restricted area around the upper part are completely devoid of these structures, 

 of the nucleus. Its fine structure is identical When the distal convolution approaches 

 with the one recorded in the proximal con- the neighborhood of the vascular pole of the 

 volution. The basal plasma membrane is glomerulus, it becomes attached to the angle 

 deeply infolded between the elongated mito- between the afferent and efferent arterioles, 

 chondria leaving but a narrow strip of cyto- This part of the distal convolution has been 

 plasm in between. The RNA-particles are called the macula densa because it can be 

 numerous and scattered among the small seen that the cells of the distal convolution 

 vesicles in the apical cytoplasm. here assume a more columnar shape than 



It is obvious that the work performed by elsewhere, thus causing the nuclei to stand 



the thick limb requires a big supply of en- out very clearly in close juxtaposition (Fig. 



zymes judging from the great number of 1). Apart from their extreme columnar shape, 



mitochondria present in the cells. Sodium is the cells of the macula densa do not show any 



taken up by these cells by an active process peculiarities as far as their fine structure is 



which also leads to a simultaneous retention concerned which would discriminate them 



of water. Furthermore, formation of ammonia from other cells of the distal convolution, 



and the acidification of the urine seem to take Cortical Collecting Duct. This part of 



place in this portion of the nephron. It may the nephron immediately follows the distal 



be reasonable to assume that the numerous convolution and serves only as a short con- 



microvesicles are structural evidence for one nection between this part and the collecting 



or both of these processes. Similar micro- tubule. It is characterized by two cell types, 



vesicles are a prominent feature of the pari- the first' identical with the one occurring in 



etal cells of the gastric mucosa. These cells the distal convolution, the second, with the 



supposedly are responsible for the production appearance of the cells found in the collect- 



of the hydrochloric acid of the stomach. ing tubule. As the cortical collecting duct is 



Distal Convolution. The thick ascend- gradually transformed into the collecting 



ing limb of Henle's loop is gradually trans- tubule, the cell type reminiscent of that of 



formed into a convoluted position, the dis- the distal tubule disappears. Because these 



tal convolution, when the nephron again cells seem to be interposed between the main 



reaches the cortex of the kidney. The distal cell type, they have been called intercalated 



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