Podocyte

Podocyte
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. A - Renal corpuscle B - Proximal tubule C - Distal convoluted tubule D - Juxtaglomerular apparatus 1. Basement membrane (Basal lamina) 2. Bowman's capsule - parietal layer 3. Bowman's capsule - visceral layer 3a. Pedicels (podocyte processes) 3b. Podocyte 4. Bowman's space (urinary space) 5a. Mesangium - Intraglomerular cell 5b. Mesangium - Extraglomerular cell 6. Granular cells (Juxtaglomerular cells) 7. Macula densa 8. Myocytes (smooth muscle) 9. Afferent arteriole 10. Glomerulus capillaries 11. Efferent arteriole
Details
Latin	podocytus
Precursor	Intermediate mesoderm
Identifiers
Dorlands /Elsevier	Podocyte
Anatomical terminology

Podocytes (or visceral epithelial cells) are cells in the Bowman's capsule in the kidneys that wrap around capillaries of the glomerulus.^[1] The Bowman's capsule filters blood, retaining large molecules such as proteins while smaller molecules such as water, salts, and sugar are filtered 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. Nephrin defects are responsible for congenital kidney failure. CD2AP regulates the podocyte cytoskeleton and stabilizes the slit diaphragm.^[2][3]

Structure

Diagram showing the basic physiologic mechanisms of the kidney

Podocytes are found lining the Bowman's capsules in the nephrons of the kidney. The foot processes known as pedicels that extend from the podocytes wrap themselves around the capillaries of the glomerulus to form the filtration slits. The pedicels increase the surface area of the cells enabling efficient ultrafiltration.^[4]

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.

Function

Scheme of filtration barrier (blood-urine) in the kidney.
A. The endothelial cells of the glomerulus; 1. pore (fenestra).
B. Glomerular basement membrane: 1. lamina rara interna 2. lamina densa 3. lamina rara externa
C. Podocytes: 1. enzymatic and structural protein 2. filtration slit 3. diaphragma

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.^[5] Proteins that are required for the correct function of the slit diaphragm include nephrin,^[6] NEPH1, NEPH2,^[7] podocin, and CD2AP.^[8]

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.

Clinical significance

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.

References

External links

• 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: 7/7ch04/7ch04p09 - Essentials of Human Physiology
• Histology image: 22402loa – Histology Learning System at Boston University
• Histology image: 22403loa – Histology Learning System at Boston University