Proximal Tubule

The proximal tubule consists of three histologically distinct parts, the S1, S2 and S3 segments.

These only roughly correspond with the appellations of proximal convoluted and proximal straight tubule (pars convoluta and pars recta). 

These appelations will not be discussed in depth here; it is sufficient to understand that the cells of the proximal tubule become shorter and less eosinophilic, and the tubule straightens out as it approaches the loop of Henle (the pars recta = the descending thick limb of the loop of Henle). 

Proximal tubule cells are simple cuboidal. 

They are also acidophilic due to a high content of long mitochondria concentrated in the base of the cell. 

They have a microvillous brush border that extends into the lumen of the nephron. 

The brush border is coated with a glycocalyx. In this PAS stained micrograph of the proximal convoluted tubule, the glycocalyx stains a reddish-pink. 

Localized along the basolateral membranes is the Na+/K+ ATPase, which pumps sodium out of the cells. 

On electron microscopy, or using special stains with light microscopy, apical vesicles and lysosomes can also be seen. 

This specially prepared LM shows lysosomal vesicles. 

Because the proximal tubule is the only section of the nephron involved in protein uptake, these vesicles can only be seen here. 

A good way to identify the proximal tubule by normal light microscopy is to determine the shape of the lumen. 

The lumen is star shaped and appears to contain debris (the glycocalyx) in most H&E sections. 

There are generally 5-7 cells per cross section.