Proximal tubule

Proximal Convoluted Tubule
Scheme of renal tubule and its vascular supply. (1st convoluted tubule labeled at center top.)
1 Glomerulus, 2 proximal tubule, 3 distal tubule
Details
Precursor	Metanephric blastema
Latin	tubulus contortus proximalis, pars contorta tubuli proximalis
Anatomical terminology []

The proximal tubule is the portion of the duct system of the nephron of the kidney which leads from Bowman's capsule to the loop of Henle. It is conventionally divided into the proximal convoluted tubule (PCT) and the proximal straight tubule (PST).

The most distinctive characteristic of the proximal tubule is its brush border (or "striated border").

The luminal surface of the epithelial cells of this segment of the nephron is covered with densely packed microvilli forming a border readily visible under the light microscope giving the brush border cell its name. The microvilli greatly increase the luminal surface area of the cells, presumably facilitating their resorptive function as well as putative flow sensing within the lumen.

The cytoplasm of the cells is densely packed with , which are largely found in the basal region within the infoldings of the basal plasma membrane. The high quantity of mitochondria gives the cells an acidophilic appearance. The mitochondria are needed in order to supply the energy for the active transport of sodium ions out of the cells to create a concentration gradient which allows more sodium ions to enter the cell from the luminal side. Water passively follows the sodium out of the cell along its concentration gradient.

Cuboidal epithelial cells lining the proximal tubule have extensive lateral interdigitations between neighboring cells, which lend an appearance of having no discrete cell margins when viewed with a light microscope.

Agonal resorption of the proximal tubular contents after interruption of circulation in the capillaries surrounding the tubule often leads to disturbance of the cellular morphology of the proximal tubule cells, including the ejection of cell nuclei into the tubule lumen.