Bowmen’s capsule | Visceral |parietal | glomerulus| Agrin| sialoglycoprotein-heparan sulphate| sulfated polysaccharides glycosaminoglycans| filtration slits| nephrin
Table of Contents
A detailed structure of the renal corpuscle- it has two components:
The renal corpuscle is the Bowman’s capsule and the glomerulus present in it.
Structure of the Glamorous
1. The glomerulus is a tuft of freely branching and anastomosing capillaries of the afferent arteriole. The capillaries join to form an efferent arteriole. About 30 loops of capillaries are present in one glomerulus.
This is one of the sites where an arteriole drains into arteriole. So blood from one arteriole passes into another arteriole.
The capillaries are fenestrated. The endothelial cell of the capillaries has multiple thousands upon thousands.’ The size of these pores ranges from 70 to 90 nanometers, allowing only particles of that size to pass through. Larger particles cannot pass through these pores. The capillaries rest on a basement membrane known as the ‘glomerular basement membrane.
Structure of the Glomerular Basement Membrane
The glomerular basement membrane is composed of three layers as recognized by the electron microscope:
Lamina densa is the thick, dense layer, surrounded by ‘lamina rara externa’ on the Bowman’s space side and ‘lamina rara interna’ towards the capillary side.
In other words, lamina densa is present in the center of the glomerular basement membrane.
Structure of Lamina Densa
Lamina densa comprises three peptide chains of the collagen type 4 collagen. There are six crucial isoforms of collagen Type 4. The isoforms are α-1 to α-6. Each isoform composes a triple helix structure in which the other two isoforms organize the collagen Type-4 network. α-3,4,5 isoforms form the triple helix. At the same time, α-1 and α-2 chains include mesangium.
The mesangium is essential for maintaining normal glomerular basement membrane integrity and becomes highly active in response to increased mechanical demands.
Many components are present in this highly cross-linked network, each with a specific function. For example, some structures include fibronectin, nidogen I &II, α-actinin, and laminin.
Laminin is an extracellular matrix glycoprotein.
The terminal parts of the three peptide chains are known as the ‘collagen component of collagen type 4. Antibody formation against these antigens may occur in some individuals, triggering the development of autoantibodies, specifically anti-glomerular basement membrane antibodies (anti-GBM).
This type of basement membrane is present in the basement membrane of other organs. The alveolar basement membrane structure is like that of the glomerular basement membrane.
Lamina rara externa and lamina rara interna
Lamina rara externa
The epithelial cells of the visceral layer of the Bowman’s capsule are present on the lamina rara externa. The lamina is composed of sialoglycoprotein-heparan sulfate. In addition, the lamina is rich in glycosaminoglycans -sulfated polysaccharides attached to a core protein known as Agrin.
Lamina rara interna
The lamina rara interna, lamina densa, and lamina rara externa are electronegative, repelling electronegative particles and attracting electropositive particles.
The epithelial cells have podocytes known as pedicles. The foot processes glue on the lamina and interdigitate to form the filtration slit. The size of the filtration slits ranges from 10 to 20 nanometers, preventing the passage of larger molecules.
The glomerular basement membrane is the main component of the glomerular filtration barrier and is responsible for ultrafiltration by the renal corpuscles. The glomerular basement membrane provides a barrier in two ways:
- Size barrier, and
- Charge selective barrier.
However, recent studies report that charge-selective permeability of the glomerular filtration barrier is not a significant barrier in glomerular filtration.
Types of collagen
Type 1 is present in bone, tendon, skin, and other organs
Type 2 is present in cartilage
Type 3 is widely present in the body as a reticulate
Type 4 is present in the basement membrane
Type 5 is present in cell surfaces and the Placenta.
The structure of the alveolar basement membrane is similar to that of the glomerular basement membrane. So the antibody developed against the glomerular basement membrane damages both the glomerular and alveolar basement membranes. This causes hematuria and hemoptysis. This occurs in Goodpasture syndrome along with other features.
Bowman’s Capsule
Bowman’s capsule is the initial dilated part of the nephron. It has two epithelial layers:
1. Visceral layer, and 2. Parietal layer.
The visceral layer is a part of the glomerular basement membrane. It is continuous with the parietal layer of the Bowman’s capsule at the entrance site of the afferent arterioles and exit of the efferent arterioles.
Parietal epithelial cell forms the outer lining of the Bowman’s capsule and are continuous with the visceral layer and the proximal convoluted tubule.
Bowman’s space is between the two layers of the Bowman’s capsule and is continuous with the proximal convoluted tubule lumen. The ultrafiltrate is collected in this space before passing to the proximal convoluted tubule.
The epithelial cells of the visceral layer of the Bowman’s capsule are present over the lamina rara externa. However, the epithelial cells are not continuous and give food processes known as pedicles. The pedicles interdigitate upon the lamina rara externa and form the filtration slit of 7-10 nm in size.
The filtration slit is guarded by a protein known as nephrin, allowing the passage of particles only 7-8 nanometers in size.
The renal corpuscle plays a crucial role in the ultrafiltration process, also known as hydrostatic filtration, of the plasma. Additionally, slight changes in the structure and electrical charges of the glomerular basement membrane can cause various diseases.
Size-dependent barriers
Layers of filtration:
1st: Fenestrated capillaries: 70-80 nanometers.
2nd filtration slits: 10-20 nanometers.
3rd nephrin 7-8 nanometer size.
Charge-related barriers
The lamina rara interna, lamina densa, and lamina rara externa are electronegative, repelling electronegative particles and attracting electropositive particles.
Therefore, electronegative plasma proteins are not filtered by normal glomerular basement membrane despite their small size.
The glomerular basement membrane is crucial for ultrafiltration, and any defect in this membrane results in the leakage of blood components.
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