Renal corpuscle structure
Blood flows in the afferent arteriole (9) at the top, and out the efferent arteriole (11) at the bottom. Blood flows through the capillaries of the glomerulus (10), where it is filtered by pressure. The podocytes (3a and 3b, green) are wrapped around the capillaries. Blood is filtered through the slit diaphragm (or filtration slit), between the feet or processes of the podocytes. The filtered urine passes out the proximal tubule (B, yellow) on the right.
Podocytes (or visceral epithelial cells) are cells in the Bowman's capsule in the kidneys that wrap around the capillaries of the glomerulus. The Bowman's capsule filters blood, holding back large molecules such as proteins, and passing through small molecules such as water, salts, and sugar, as the first step in forming urine.
The long processes as foot projections called pedicels, of the podocytes wrap around the capillaries, and leave slits between them. Blood is filtered through these slits, each known as a filtration slit or slit diaphragm. Several proteins are required for the foot projections 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. Nephron defects are responsible for congenital kidney failure. CD2AP regulates the podocyte cytoskeleton and stabilizes the slit diaphragm.
Adjacent podocytes interdigitate to cover the basal lamina which is intimately associated with the glomerular capillaries. The pedicels of the podocytes interdigitate and leave gaps or thin filtration slits between them.The slits are covered by slit diaphragms which are composed of a number of cell-surface proteins including nephrin, podocalyxin, and P-cadherin, which restrict the passage of large macromolecules such as serum albumin and gamma globulin and ensure that they remain in the bloodstream. Proteins that are required for the correct function of the slit diaphragm include nephrin, NEPH1, NEPH2, podocin, and CD2AP.
Small molecules such as water, glucose, and ionic salts are able to pass through the filtration slits and form an ultrafiltrate which is further processed by the nephron to produce urine.
Podocytes are also involved in regulation of glomerular filtration rate (GFR). When podocytes contract, they cause closure of filtration slits. This decreases the GFR by reducing the surface area available for filtration.
Structural features of podocytes indicate a high rate of vesicular traffic in these cells. Many coated vesicles and coated pits can be seen along the basolateral domain of the podocytes.
"Pedicels" (or "foot processes") extend from the podocyte and increase the surface area which is crucial for the efficiency of ultrafiltration.
Disruption of the filtration slits or destruction of the podocytes can lead to massive proteinuria where large amounts of protein are lost from the blood.
An example of this occurs in the congenital disorder Finnish-type nephrosis, which is characterised by neonatal proteinuria leading to end-stage renal failure. This disease has been found to be caused by a mutation in the nephrin gene.
- "Podocyte" at Dorland's Medical Dictionary
- Wickelgren, I. (1999). "CELL BIOLOGY: First Components Found for Key Kidney Filter". Science 286 (5438): 225–6. doi:10.1126/science.286.5438.225. PMID 10577188.
- . doi:10.1007/s00431-009-1017-x. Missing or empty
- Jarad, G.; Miner, J. H. (2009). "Update on the glomerular filtration barrier". Current opinion in nephrology and hypertension 18 (3): 226–232. doi:10.1097/mnh.0b013e3283296044. PMC 2895306. PMID 19374010.
- Wartiovaara, J.; Ofverstedt, L. G. R.; Khoshnoodi, J.; Zhang, J.; Mäkelä, E.; Sandin, S.; Ruotsalainen, V.; Cheng, R. H.; Jalanko, H.; Skoglund, U.; Tryggvason, K. (2004). "Nephrin strands contribute to a porous slit diaphragm scaffold as revealed by electron tomography". Journal of Clinical Investigation 114 (10): 1475–1483. doi:10.1172/JCI22562. PMC 525744. PMID 15545998.
- Neumann-Haefelin, E.; Kramer-Zucker, A.; Slanchev, K.; Hartleben, B.; Noutsou, F.; Martin, K.; Wanner, N.; Ritter, A.; Gödel, M.; Pagel, P.; Fu, X.; Müller, A.; Baumeister, R.; Walz, G.; Huber, T. B. (2010). "A model organism approach: Defining the role of Neph proteins as regulators of neuron and kidney morphogenesis". Human Molecular Genetics 19 (12): 2347–2359. doi:10.1093/hmg/ddq108. PMID 20233749.
- Fukasawa, H.; Bornheimer, S.; Kudlicka, K.; Farquhar, M. G. (2009). "Slit Diaphragms Contain Tight Junction Proteins". Journal of the American Society of Nephrology 20 (7): 1491–1503. doi:10.1681/ASN.2008101117. PMC 2709684. PMID 19478094.
- Physiology at MCG 7/7ch04/7ch04p08
- Anatomy photo: Urinary/mammal/vasc1/vasc1 - Comparative Organology at University of California, Davis - "Mammal, renal vasculature (EM, High)
- Histology image: 22401loa – Histology Learning System at Boston University - ". Ultrastructure of the Cell: podocytes and glomerular capillaries"
- UIUC Histology Subject 1400
- podocyte.ca at Samuel Lunenfeld Research Institute
- Physiology at MCG 7/7ch04/7ch04p09
- Histology image: 22402loa – Histology Learning System at Boston University
- Histology image: 22403loa – Histology Learning System at Boston University