What are the components of the glomerulus?

Components of the Glomerulus

The glomerulus consists of four main cellular components: fenestrated endothelial cells, the glomerular basement membrane (GBM), podocytes (visceral epithelial cells), and mesangial cells, which together form the glomerular filtration barrier and supporting structures. 1, 2

Cellular Components

Endothelial Cells

• Fenestrated endothelial cells line the inner surface of glomerular capillaries and are in direct contact with blood 2
• These cells are coated with a glycocalyx layer that contributes to the filtration barrier 1, 2
• The fenestrations (pores) in these cells allow passage of water and small solutes while restricting larger molecules 1
• Endothelial cells may contain tubuloreticular inclusions in certain disease states 3

Glomerular Basement Membrane (GBM)

• The GBM is a trilaminar structure composed of a highly crosslinked macromolecular meshwork 1, 4
• Key structural components include type IV collagen, proteoglycans, and laminin 4
• The GBM serves as the central layer of the glomerular filtration barrier, positioned between endothelial cells and podocytes 3
• Pathologic changes include thickening, thinning, double-contour formation, spikes, and vacuoles/pinholes 3

Podocytes (Visceral Epithelial Cells)

• Podocytes are highly specialized epithelial cells that form the outermost layer of the glomerular filtration barrier 1, 5
• These cells have interdigitating foot processes that create filtration slits 1, 5
• The slit diaphragm is a specialized junction between foot processes that forms the backbone of the filtration assembly 5
• Podocytes may show hypertrophy, hyperplasia, or contain intracytoplasmic protein resorption droplets in disease states 3
• Foot process effacement (loss of normal architecture) is a key pathologic finding in many glomerular diseases 3

Mesangial Cells

• Mesangial cells sit between the capillary loops and serve as the central hub connecting all glomerular structures 2, 6
• These cells provide structural support to the glomerular tuft 2, 6
• The mesangial matrix surrounds mesangial cells and can expand in disease states 3
• Mesangial cells are involved in intra-glomerular crosstalk and signaling to maintain glomerular health 6

Additional Structural Components

Bowman's Capsule

• Parietal epithelial cells form Bowman's capsule, which surrounds the glomerular tuft 2
• Bowman's space is the area between the glomerular tuft and Bowman's capsule 3
• Disruption of Bowman's capsule can occur in severe glomerular injury 3

Supporting Elements

• The glomerular capillary tuft is a highly intricate microvascular bed that filters plasma water and solute 2
• Mesangiolysis (dissolution of mesangial matrix) can occur in certain disease states 3
• The entire structure functions as a unit to ensure high permselectivity, filtering water and small solutes while retaining large plasma proteins 5, 6

Pathologic Considerations

When evaluating glomerular pathology, key features to assess include:

• Mesangial expansion and hypercellularity (increased mesangial cells or matrix) 3
• Capillary wall thickening and endocapillary hypercellularity 3
• GBM abnormalities including thickness changes, double contours, or spikes 3
• Podocyte changes such as foot process effacement or protein resorption droplets 3
• Leukocyte infiltration into capillary lumens or Bowman's space 3