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# Glomerulus and the filtration barrier: podocytes, mesangium, JGA

- Organ

The glomerulus is a tuft of capillaries that filters blood; its three-layer barrier lets water and salts through but holds back proteins.

Updated 2026-11-01

**See it in 3D:** [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Key points

- Three-layer barrier: fenestrated endothelium, glomerular basement membrane (type IV collagen and negatively charged proteoglycans) and the podocyte slit diaphragm (nephrin).
- The barrier filters by size and by charge: albumin, large and negative, normally does not pass.
- Foot process effacement on electron microscopy means podocyte injury, typical of minimal change nephrotic syndrome.
- Mesangial cells support the tuft, phagocytose and contract; in IgA nephropathy the deposits are mesangial.
- Juxtaglomerular apparatus: juxtaglomerular cells (renin, in the afferent arteriole), macula densa (chloride sensor in the distal nephron) and extraglomerular mesangial cells.

## What it is

The renal corpuscle is a tuft of capillaries, the glomerulus, between the afferent and efferent arterioles, wrapped by Bowman's capsule. The filtration barrier has three layers: fenestrated endothelium, glomerular basement membrane and slit diaphragms between podocyte foot processes. Nearly every glomerular disease is explained by the layer that is damaged, which makes it a core exam topic.

## Structure

- On H&E: a rounded tuft of capillaries surrounded by Bowman's space and the flat parietal layer of the capsule.
- Vascular pole: the arterioles enter and leave here; the macula densa sits against it, a cluster of tall cuboidal cells with crowded nuclei.
- Urinary pole: Bowman's space continues into the proximal tubule.
- PAS or silver stains outline the basement membrane; electron microscopy shows foot processes and where immune deposits sit.

## Function

- Makes the glomerular filtrate, an ultrafiltrate of plasma without cells or large proteins.
- The different calibre of afferent and efferent arterioles keeps pressure high inside the tuft.
- The macula densa and juxtaglomerular cells adjust filtration and release renin, which starts the renin-angiotensin-aldosterone system.

## Barrier layers and what happens when they fail

| Component | What it is like | Typical disease |
| --- | --- | --- |
| Endothelium | Fenestrated capillary with a negative glycocalyx | Thrombotic microangiopathy (hemolytic uremic syndrome) |
| Glomerular basement membrane | Thick sheet of type IV collagen, laminin and heparan sulfate | Alport syndrome (abnormal type IV collagen); anti-GBM disease (Goodpasture) |
| Podocytes | Cells with interdigitating foot processes joined by the slit diaphragm | Minimal change disease and focal segmental glomerulosclerosis (foot process effacement) |
| Mesangium | Contractile, phagocytic cells in a matrix between capillaries | IgA nephropathy (mesangial deposits); nodules of diabetic nephropathy |

## Markers

- PAS (basement membrane)
- Methenamine silver
- Immunofluorescence (IgA, IgG, C3)
- Electron microscopy

## Clinical notes

- Nephrotic syndrome: podocyte or barrier damage with heavy proteinuria, edema and low albumin.
- Nephritic syndrome: glomerular inflammation with hematuria, red cell casts and hypertension; red cell casts point to glomerular bleeding.
- Prerenal acute kidney injury: if perfusion pressure falls, filtration falls even when the glomerulus is healthy.

## Memory aid

Three gates in a row: the fence with holes (endothelium), the charged mesh (basement membrane) and the finest gate, the podocytes'.

## Test yourself

Think of the answer, then tap the question to see it.

## What are the three layers of the glomerular filtration barrier?

The fenestrated endothelium, the glomerular basement membrane and the slit diaphragm between podocyte foot processes.

## Which electron microscopy finding defines minimal change nephrotic syndrome?

Diffuse effacement of podocyte foot processes, with near-normal light microscopy.

## Which cells release renin, and where are they?

The juxtaglomerular cells, modified smooth muscle cells in the wall of the afferent arteriole.

## What does the macula densa sense?

The sodium chloride reaching the end of the thick ascending limb and the start of the distal tubule; it uses that signal to adjust filtration and renin.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/histology/glomerulus-and-the-filtration-barrier-podocytes-mesangium-jga/)

## See it in 3D

The structures of Durgical's 3D anatomy where it is found.

- [Kidneys](https://durgical.com/en/anatomy/kidney/)

## Related

- [Kidney: cortex, medulla and the nephron](https://durgical.com/en/histology/kidney-cortex-medulla-and-the-nephron/)
- [Basement membrane](https://durgical.com/en/histology/basement-membrane/)
- [Adrenal cortex zones and medulla](https://durgical.com/en/histology/adrenal-cortex-zones-and-medulla/)

## Related conditions

- [Acute kidney injury](https://durgical.com/en/conditions/acute-kidney-injury/)
- [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/)

[← All of histology](https://durgical.com/en/histology/)

Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.

Sources: [25.4 Microscopic Anatomy of the Kidney](https://openstax.org/books/anatomy-and-physiology-2e/pages/25-4-microscopic-anatomy-of-the-kidney) (OpenStax, Rice University, 2022); [25.5 Physiology of Urine Formation](https://openstax.org/books/anatomy-and-physiology-2e/pages/25-5-physiology-of-urine-formation) (OpenStax, Rice University, 2022); [Kidneys](https://training.seer.cancer.gov/anatomy/urinary/components/kidney.html) (National Cancer Institute, SEER Training Modules, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/histology/glomerulus-and-the-filtration-barrier-podocytes-mesangium-jga/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/histology/glomerulus-and-the-filtration-barrier-podocytes-mesangium-jga/