KIDNEY ULTRASTRUCTURE 



urine seems to be passed along the proximal 

 convolution is probably more like the slow 

 filtration through a finely porous sieve than 

 like the rapid drainage of a sink with a wide 

 open plumbing system. The "pores" of this 

 particular sieve are then represented by the 

 narrow slits between the millions of brush 

 border extensions. It can be i^ecorded in high 

 resolution micrographs that the plasma 

 membrane which covers the extensions has a 

 thickness of about 50A. However closely 

 packed, the extensions do not get in closer 

 contact than lOOA apart. The intervening 

 space is occupied by a substance with less 

 density than the plasma membrane. It has 

 been suggested that this substance represents 

 an additional layer of possibly lipid mole- 

 cules with a depth of 50 A, identical with the 

 intervening layers between adjacent cell bor- 

 ders. When the urine passes along the proxi- 

 mal convolution, it expands slightly the two 

 lipid layers of adjacent brush border exten- 

 sions, thus creating the narrow slits men- 

 tioned above. By this mechanism is estab- 

 lished a much closer contact between the 

 urine and the surface membrane of the brush 

 border extensions than would be the case 

 with a wide open lumen. 



Some substances, as for instance proteins, 

 which cannot be absorbed through the sur- 

 face membrane, are taken up by the tubular 

 invaginations. These structm'es are located 

 between the bases of the brush border ex- 

 tensions and represent minute tubules which 

 are in open connection with the tubular lumen 

 (Fig. 8). The tubular invaginations are ex- 

 panded to vacuolar profiles when fluid and 

 substances are taken in. A certain condensa- 

 tion of the vacuolar content is noted con- 

 comitantly with a breaking-off of the con- 

 nection between the expanded tubular 

 invagination and the lumen of the nephron. 

 The condensed vacuole can then be found 

 anywhere in the cell and as the condensation 

 of its contents proceeds, the vacuole is trans- 

 formed into what has been called a large 

 granule in electron microscopy. This whole 



mechanism of taking in fluid-dissolved sub- 

 stances has been called micropinocytosis or 

 membrane flow. 



The mechanism by which substances other 

 than protein are taken up by the cells of the 

 proximal convolution is structurally not 

 clear. It involves substances and ions such 

 as glucose, sodium, potassium, phosphate, 

 sulfate, amino acids, urea, and creatinine 

 to mention some of them. The reabsorption 

 of water is a passive process, but most of the 

 substances listed involve an active process 

 which requires energy. The enzymes recjuired 

 for these active processes are supplied by the 

 multitude of mitochondria present in the cells 

 of the proximal convolution (Fig. 4). The 

 ultrastructure of the mitochondria has been 

 thoroughly investigated (cross reference: cell 

 ultrastructure) and it is believed that the 

 efficiency of the mitochondrial work is in- 

 creased by the number of mitochondria and 

 the presence of structurally intact internal 

 membranes. Organelles have been recorded 

 in these cells which presumably constitute 

 mitochondrial precursors. They have been 

 called microbodies, and represent small spher- 

 ical bodies without internal membranes and 

 with a single membranous capsule as com- 

 pared with the double-contoured mitochon- 

 drial outer membrane. 



Structures have been found in the basal 

 portion of the cells of the proximal convolu- 

 tion which probably facilitate the flow of re- 

 absorbed fluid and substances through the 

 cell body. They represent infolding s of the 

 basal plasma membrane which extend to a 

 varying degree into the cell (Fig. 4). The 

 mitochondria have a close relationship with 

 these infoldings and it is tempting to as- 

 sume that this facilitates a certain interac- 

 tion between the enzymes carried by the 

 mitochondria and the infolded plasma mem- 

 brane (Fig. 8). Once the reabsorbed fluid and 

 substances have obtained contact with the 

 basal plasma membrane, they may penetrate 

 it by means of an enzymatic activation (Fig. 

 9). And when the extracelhilar space is 



169 



