Further Studies on the Nephron Ultrastructure in Mouse: 

 Terminal Part of Proximal Convolution 



J. Rhodin 



The Lahoratory for Biological Ultrastructure Research of the Department of Anatomy, 



Karolinska Institiitet, Stockholm 



Light and fluorescent microscopic studies (4) 

 proved structural differences between different parts 

 of the proximal convoluted tubule of the mouse 

 nephron. Analyses of the nephron by means of 

 electron microscopy (3, 7) dealt with the first part 

 of the proximal convolution. The present report 

 gives some data concerning the terminal part of the 

 proximal convolution, and demonstrates characteris- 

 tic features of its tubular cells, proving differences 

 also at an ultrastructural level. 



White female mice were decapitated and within 2 

 minutes small pieces of kidney tissue were immersed in 

 1 per cent isotonic osmium tetroxide bufiered at pH 7.2 

 (1, 3, 5). The duration of the fixation was 60 minutes. 

 After dehydration in alcohols and embedding in nietha- 

 crylate, a Sjostrand Ultra Microtome was used for sec- 

 tioning and an RCA EMU 2c electron microscope for 

 examination of the sections. 



The tubular cells of the terminal part of the proxi- 

 mal convolution have a height of 5-8 //, the value 

 decreasing towards the first straight part of Henle's 

 loop. In most cases, a free lumen is present which 

 tallies with light microscopic findings after fixation 

 with freeze-drying method. The luminal part of the 

 cell is fitted with brush border extensions, all free 

 from each other. This general arrangement of brush 

 border extensions was described and illustrated with 

 micrographs for the first part of the proximal con- 

 volution (3). With the tubule dilated this arrange- 

 ment is easier to interpret in the terminal part. The 

 extensions are covered with a 60 A thick cell mem- 

 brane. Between the bases of the brush border exten- 

 sions are tubular invaginations of the surface mem- 

 brane. However, they are less abundant than in the 

 first part of the proximal convolution. The invagina- 

 tions reach 0.1-0.2 /n into the cell. The bottom of the 

 invaginations is usually slightly extended. The basal 

 part of the cell faces the basement membrane with a 

 50 A thick plasma membrane. This membrane shows 

 but few of the infoldings, /i-cytomembranes (6), so 

 characteristic of the first part of the nephron. It 

 has been noticed that there is a tendency for these 

 folds to decrease both in number and height from 

 the neck of the nephron towards the first straight 

 part of Henle's loop. The scarcity of infoldings is a 

 typical feature of the terminal part of the proximal 

 convolution and facilitates the recognition of its 

 tubular cells. 



The basement membrane is a homogeneous, 800 A 

 thick, structureless layer. It is separated from the 

 adjacent plasma membrane by a 180 A wide space. 

 The mitochondria are few and of oval shape with a 



diameter some times exceeding 0.5 //. Their ultra- 

 structure is identical with what has earlier been 

 described in tubular cells (3,7). Microbodies with a 

 diameter of 0.1-0.3 // are present with a single outer 

 membrane, the thickness of which is 50 A. They 

 lack inner membranes and thus can easily be distin- 

 guished from mitochondria. A small Golgi apparatus 

 can be found with pairs of membranes as the main 

 component. Large opaque granules and large vacu- 

 oles are present to the same extent as in the first part 

 of the nephron. Their size is usually larger than 0.5 

 micron. Both large granules and the vacuoles are 

 surrounded by a single membrane with a thickness of 

 50 A. No connections have been traced between 

 large vacuoles and tubular invaginations. Thus, it 



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Fig. I. Brush border zone with a free lumen (L). At the bases 

 of the brush border extensions (B) are widened tubular in- 

 vaginations (V). In close connection a mitochondrion (M). 

 Magnification 20,000. 



