Podocyte

Podocytes are cells in Bowman's capsule in the kidneys that wrap around capillaries of the glomerulus. Podocytes make up the epithelial lining of Bowman's capsule, the third layer through which filtration of blood takes place. Bowman's capsule filters the blood, retaining large molecules such as proteins while smaller molecules such as water, salts, and sugars are filtered as the first step in the formation of urine. Although various viscera have epithelial layers, the name visceral epithelial cells usually refers specifically to podocytes, which are specialized epithelial cells that reside in the visceral layer of the capsule.

The podocytes have long primary processes called trabeculae that form secondary processes known as pedicels or foot processes (for which the cells are named podo- + -cyte). The pedicels wrap around the capillaries and leave slits between them. Blood is filtered through these slits, each known as a filtration slit, slit diaphragm, or slit pore. Several proteins are required for the pedicels to wrap around the capillaries and function. When infants are born with certain defects in these proteins, such as nephrin and CD2AP, their kidneys cannot function. People have variations in these proteins, and some variations may predispose them to kidney failure later in life. Nephrin is a zipper-like protein that forms the slit diaphragm, with spaces between the teeth of the zipper big enough to allow sugar and water through but too small to allow proteins through. Nephrin defects are responsible for congenital kidney failure. CD2AP regulates the podocyte cytoskeleton and stabilizes the slit diaphragm.

A podocyte has a complex structure. Its cell body has extending major or primary processes that form secondary processes as podocyte foot processes or pedicels. The primary processes are held by microtubules and intermediate filaments. The foot processes have an actin-based cytoskeleton. Podocytes are found lining the Bowman's capsules in the nephrons of the kidney. The pedicels or foot processes wrap around the glomerular capillaries to form the filtration slits. The pedicels increase the surface area of the cells enabling efficient ultrafiltration.

Podocytes secrete and maintain the basement membrane.

There are numerous coated vesicles and coated pits along the basolateral domain of the podocytes which indicate a high rate of vesicular traffic.

Podocytes possess a well-developed endoplasmic reticulum and a large Golgi apparatus, indicative of a high capacity for protein synthesis and post-translational modifications.

There is also growing evidence of a large number of multivesicular bodies and other lysosomal components seen in these cells, indicating a high endocytic activity.

Podocytes require a significant amount of energy to preserve the structural integrity of their foot processes, given the substantial mechanical stress they endure during the glomerular filtration process.