What term describes the pores found in the visceral layer of the glomerular capsule?

Bowman's capsule (or the Bowman capsule, capsula glomeruli, or glomerular capsule) is a cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form urine. A glomerulus is enclosed in the sac. Fluids from blood in the glomerulus are collected in the Bowman's capsule.

Structure[edit]

Outside the capsule, there are two "poles":

Inside the capsule, the layers are as follows, from outside to inside:[citation needed]

• Parietal layer—A single layer of simple squamous epithelium. Does not function in filtration.
• Bowman's space (or "urinary space", or "capsular space")—Between the visceral and parietal layers, into which the filtrate enters after passing through the filtration slits.
• Visceral layer—Lies just above the thickened glomerular basement membrane and is made of podocytes. Beneath the visceral layer lie the glomerular capillaries.[citation needed]
• Filtration barrier—The filtration barrier is composed of the fenestrated endothelium of the glomerular capillaries, the fused basal lamina of the endothelial cells and podocytes, and the filtration slits of the podocytes. The barrier permits the passage of water, ions, and small molecules from the bloodstream into the Bowman's space. The barrier prevents the passage of large and/or negatively charged proteins (such as albumin). The basal lamina of the filtration barrier is composed of three layers. The first layer is the lamina rara externa, adjacent to the podocyte processes. The second layer is the lamina rara interna, adjacent to the endothelial cells. The final layer is the lamina densa which is a darker central zone of the basal lamina. It consists of the meshwork of type IV collagen and laminin which act as a selective macromolecular filter.[citation needed]

Function[edit]

The process of filtration of the blood in the Bowman's capsule is ultrafiltration (or glomerular filtration), and the normal rate of filtration is 125 ml/min, equivalent to 80 times the daily blood volume.[citation needed] It is a major site for blood filtration (including Glomerulus)

Any proteins under roughly 30 kilodaltons can pass freely through the membrane, although there is some extra hindrance for negatively charged molecules due to the negative charge of the basement membrane and the podocytes.[citation needed]

Any small molecules such as water, glucose, salt (NaCl), amino acids, and urea pass freely into Bowman's space, but cells, platelets and large proteins do not.[citation needed]

As a result, the filtrate leaving the Bowman's capsule is very similar to blood plasma (filtrate or glomerular filtrate is composed of blood plasma minus plasma protein i.e. it contains all the components of blood plasma except the proteins) in composition as it passes into the proximal convoluted tubule.[citation needed]

Clinical significance[edit]

Micrograph of proteinaceous material in Bowman's space, which is unspecific. It is present in about 5% of people aged over 60 years, but also in for example hypertensive kidney disease.

Measuring the glomerular filtration rate (GFR) is a .

A decreased GFR may be a sign of kidney failure.[citation needed]

A number of diseases can result in various problems within the glomerulus. Examples include acute proliferative (endocapillary) glomerulonephritis, mesangioproliferative glomerulonephritis, mesangiocapillary (membranoproliferative) glomerulonephritis, acute crescentic glomerulonephritis, focal segmental glomerulonephritis, and diabetic glomerulosclerosis.[citation needed]

History[edit]

Bowman's capsule is named after Sir William Bowman (1816–1892), a British surgeon and anatomist. However, thorough microscopical anatomy of kidney including the nephronic capsule was first described by Ukrainian surgeon and anatomist from the Russian Empire, Prof. Alexander Schumlansky (1748–1795), in his 1782 doctoral thesis "De structura renum" ("About Kidney Structure", in Latin); thus, much prior to Bowman.

Together with the glomerulus it is known as a renal corpuscle, or a Malpighian corpuscle, named after Marcello Malpighi (1628–1694), an Italian physician and biologist. This name is not used widely anymore, probably to avoid confusion with Malpighian bodies of the spleen.[citation needed]

Blood flows in the afferent arteriole at the top, and out the efferent arteriole at the bottom. Blood flows through the capillaries of the glomerulus, where it is filtered by pressure. The podocytes (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 blood passes out the proximal tubule (yellow) on the right.

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. One type of specialized epithelial cell is podocalyxin.

The podocytes have long foot processes called pedicels, for which the cells are named ( + ). 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.

Structure[edit]

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.

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.

Function[edit]

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

Podocytes have primary processes called trabeculae, which wrap around the glomerular capillaries. The trabeculae in turn have secondary processes called pedicels. Pedicels interdigitate, thereby giving rise to thin gaps called filtration slits. 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, CD2AP. and FAT1.

Small molecules such as water, glucose, and ionic salts are able to pass through the filtration slits and form an ultrafiltrate in the tubular fluid, 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[edit]

Pedicels of podocytes interdigitating to create numerous filtration slits around glomerular capillaries in 5000x electron micrograph

A loss of the foot processes of the podocytes (i.e., podocyte effacement) is a hallmark of minimal change disease, which has therefore sometimes been called foot process disease.

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 kidney failure. This disease has been found to be caused by a mutation in the nephrin gene.

What term describes the pores found in the glomerulus?

What is found in the inner layer of the glomerular capsule?

What is the visceral layer of the kidney?

What is found in the inner layer of the glomerular capsule quizlet?