mesangial cell

**Location & structure:** Mesangial cells sit in the glomerular mesangium, between capillary loops, surrounded by mesangial matrix (type IV collagen, laminin, fibronectin, proteoglycans). They lie on the inner side of the GBM, are NOT separated from the capillary lumen by the GBM, and are continuous with extraglomerular mesangial (lacis/Goormaghtigh) cells at the juxtaglomerular apparatus. Derived from vascular smooth muscle/pericyte lineage (stromal origin), though a subset has monocyte/macrophage-like phagocytic properties.

**Functions:** 1. Structural support of the glomerular capillary tuft. 2. Contractility (via actin/myosin) – regulate capillary surface area and thus GFR (Kf). Contract in response to angiotensin II, ADH, norepinephrine, and thromboxane; relax with ANP, PGE2, NO, dopamine. 3. Phagocytosis/clearance of immune complexes, apoptotic cells, and macromolecules (including those trapped in the mesangium) – a "macrophage-like" function. 4. Secretion of mesangial matrix, cytokines (IL-1, IL-6, PDGF, TGF-β), prostaglandins, and growth factors. Respond to injury with proliferation and matrix expansion. 5. Produce renin-related signaling interactions at the JG apparatus (extraglomerular mesangial cells).

**Glomerular cell layers (inside → out):** fenestrated capillary endothelium (negative charge, heparan sulfate) → GBM (type IV collagen, heparan sulfate) → podocyte foot processes with slit diaphragms (nephrin, podocin). Mesangial cells are in the central stalk between the capillaries.

**Clinical significance:** - **IgA nephropathy (Berger disease):** IgA-containing immune complexes (galactose-deficient IgA1) deposit in the mesangium → mesangial proliferation and matrix expansion. Presents with episodic gross hematuria 1–3 days after (synpharyngitic) URI/GI infection, in young adults. LM: mesangial widening; IF: mesangial IgA (± C3); EM: mesangial deposits. Associated with Henoch-Schönlein purpura (IgA vasculitis) in children. - **Diabetic nephropathy:** Hyperglycemia → nonenzymatic glycosylation of GBM and efferent arteriole, mesangial expansion with increased matrix (TGF-β–mediated) → diffuse glomerulosclerosis and nodular glomerulosclerosis (Kimmelstiel-Wilson nodules). Presents with progressive proteinuria. ACE inhibitors/ARBs slow progression by reducing efferent arteriolar tone/intraglomerular pressure. - **Lupus nephritis:** Class I (minimal mesangial) and class II (mesangial proliferative) involve mesangial immune complex deposits. Class IV (diffuse proliferative) is the most common and most severe ("wire-loop" lesions, subendothelial deposits; "full house" IF). - **Post-infectious GN, MPGN, and others:** Mesangial proliferation can accompany many forms of GN. MPGN shows "tram-track" GBM due to mesangial interposition into the capillary wall. - **Amyloidosis/light chain disease:** Mesangial deposition (Congo red positive with apple-green birefringence; light chain deposition disease gives nodular sclerosis). - **Alport/others:** Not primarily mesangial (GBM type IV collagen defect).

**Histology/pathology pearls:** Mesangial proliferation = increased cells per mesangial area (>3 nuclei per mesangial region on PAS stain). Mesangial matrix stains with PAS and silver. Immune complexes in mesangium deposit by IF as granular "mesangial" pattern (central stalk pattern).

**High-yield mnemonic/associations:** "Mesangium = IgA." Mesangial contraction ↓ Kf → ↓ GFR. Nephritic syndrome with mesangial IgA → hematuria without significant edema/hypertension in many cases; usually normal complement (C3) levels.

**Treatment tie-ins:** IgA nephropathy – ACEi/ARB for proteinuria; corticosteroids/immunosuppression in selected progressive cases; targeted therapies for high-risk disease (e.g., budesonide targeted-release formulation, endothelin/angiotensin receptor antagonists, APRIL/BAFF inhibitors in recent practice). Diabetic nephropathy – glycemic control, ACEi/ARB, SGLT2 inhibitors, finerenone.