116 A CYTOLOGICAL STUDY OF THE KIDNEY CELL 



the brush border. He believes that in what he calls the "resting stage" 

 (when the cells are high and the lumen narrow) the brush border is low 

 and possesses a very distinct striation, while in the active stage the brush 

 border increases in thickness and appears indistinct in its markings. 



Whatever may be the significance and affiliation of the brush border 

 with other cell structures, it undoubtedly is one of the finest architectural 

 tissue structures and therefore exposed to the greatest variations in fixing 

 artefacts. In these studies this was confirmed by the careful examination 

 of pieces of tissue where for unknown reasons penetration had lagged. 

 Here, in sections cut deeper in the piece of the tissue, the brush border 

 was severely damaged and often had a "glued-together" appearance. 

 Such a condition may easily lead to misinterpretations. And as coinci- 

 dence occasionally will produce such an artefact in a group of tubules 

 where the mitochondrial content is low, it is easy to associate the two and 

 regard them, as Kolster(6) does, as indicative of a distinct phase in se- 

 cretion. I admit that there are dift'erences in the thickness of the brush 

 border, and this is regulated somehow by the condition of the plasma of 

 the cell of the convoluted tubule, since the brush border appears clearest 

 whenever the cell cupola is most transparent. This is true of vitally 

 stained as well as fixed tissues. Just because it is so, it may be an optical 

 illusion, because of the fact that more light is permitted to strike the 

 brush border from all sides under this condition, while it can strike the 

 brush border only from three sides when the plasma is dense, and there- 

 fore the striations may not be seen as clearly. Consequently the distinct- 

 ness of the brush border striations must vary normally. The actual 

 height of the brush border varies very little within the individual section 

 of the proximal tubule. In well fixed tissues I have never seen a place 

 where the brush border was shed or being lifted off. I am unable to pre- 

 sent any photomicrographic proof for this statement, as I have not suc- 

 ceeded in taking a picture which will show the finer inner cell structure 

 clearly and at the same time do justice to the brush border. In fact, with 

 the mitochondrial stains brush-border photography is almost impossible. 



Occasionally one sees granular structures in the brush border (see 

 Figs. 4, 8, 12). They take the same stain as do the granular structures 

 of the cell itself. These granular bodies, which vary greatly in size and 

 which are commonly larger than mitochondria when there is a dissolu- 

 tion of the rods, may constitute a direct or indirect product of the mito- 

 chondrial apparatus and merely represent a physical combination of more 

 than one of the large granules seen in the cell cupola in certain phases 

 in the life cycle of the cell of the convoluted tubule, or, in fact, in any of 

 the cells of the tubular structure in general (see Fig. 8). Only occasion- 

 ally are similar granules seen in the cell lumen or within castlike 

 formations. Also, here they do not differ morphologically from the cell 



