Pathoanatomy kidney pathology

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Here is a comprehensive overview of kidney pathoanatomy, covering all four major compartments with detailed histology and mechanisms.

Kidney Pathoanatomy — Comprehensive Overview

Biopsy Technique & Interpretation Framework

A kidney biopsy is processed with three complementary modalities:

Light microscopy (LM): H&E (cellularity/architecture), PAS (carbohydrate moieties in membranes), Jones-methenamine silver (basement membrane structure), Congo red (amyloid), Masson's trichrome (collagen/fibrosis)
Immunofluorescence (IF): IgG, IgM, IgA, C1q, C3; pattern is either "lumpy-bumpy" (immune complex deposits) or linear (anti-GBM antibody)
Electron microscopy (EM): precise location of deposits (subepithelial, subendothelial, mesangial), basement membrane integrity, foot process effacement

Biopsy reporting terms:

Focal = <50% glomeruli involved; Diffuse = >50%
Segmental = part of one tuft; Global = entire tuft
Endocapillary proliferation = cells within capillary loops; Extracapillary = cells in Bowman's space (crescents)
Crescents = fibrocellular collections in Bowman's space — indicate rapid progression
Interstitial fibrosis = ominous sign of irreversibility

Harrison's Principles of Internal Medicine 22E, p. 2459–2460

I. GLOMERULAR DISEASES

Classification by Syndrome

Syndrome	Hallmark	Examples
Nephritic	Hematuria, RBC casts, hypertension, oliguria, mild proteinuria	PSGN, IgA nephropathy, ANCA vasculitis
Nephrotic	Massive proteinuria (>3.5 g/day), hypoalbuminemia, edema, hyperlipidemia	MCD, FSGS, MN, MPGN
RPGN	Rapid GFR decline over days–weeks, often crescents	Anti-GBM, ANCA, immune complex

A. Acute Endocapillary Proliferative GN (Poststreptococcal GN — PSGN)

Prototype of the nephritic syndrome.

Mechanism: Nephritogenic strains of Group A Streptococcus (M-types); key antigens are SPEB (streptococcal pyrogenic exotoxin B) and NAPlr (nephritis-associated plasmin receptor). Circulating immune complexes + alternate complement pathway activation.

Histology (LM):

Diffuse mesangial + endocapillary hypercellularity
PMN infiltration of capillary loops
Granular subendothelial + subepithelial ("hump") immune deposits of IgG, IgM, C3

IF: Granular IgG, IgM, C3 — "starry sky" pattern

EM: Subepithelial electron-dense humps (pathognomonic)

Labs: ↓C3, ↓CH50, normal C4 (alternate pathway); streptozyme test positive in 80–95%

Schematic:

Glomerular Schematic — PSGN showing mesangial deposits and subepithelial humps. — Harrison's 22E

Harrison's Principles of Internal Medicine 22E, p. 2460–2461

B. Anti-GBM Disease / Goodpasture Disease

Mechanism: Autoantibodies against α3(IV)NC1 domain of type IV collagen (the major GBM constituent). The collagen IV network forms a "chicken wire" structure — α3:α4:α5 heterotrimers predominate in GBM.

Histology: Crescentic (extracapillary proliferative) GN; linear IgG along GBM by IF

Clinical: RPGN ± pulmonary hemorrhage (Goodpasture syndrome)

Type IV collagen α-chain networks in GBM — Comprehensive Clinical Nephrology 7e

Comprehensive Clinical Nephrology 7th Ed., p. 345

C. IgA Nephropathy (Berger Disease)

Most common primary GN worldwide. Mechanism: aberrantly glycosylated IgA1 → immune complex formation → mesangial deposition → complement activation.

Histology: Mesangial proliferative GN (most common); may show focal segmental or diffuse proliferative patterns; crescents in aggressive disease

IF: Dominant or co-dominant mesangial IgA + IgG + C3

EM: Electron-dense mesangial deposits

Progression risk: Oxford MEST-C score (Mesangial hypercellularity, Endocapillary hypercellularity, Segmental sclerosis, Tubular atrophy/interstitial fibrosis, Crescents)

D. Membranous Nephropathy (MN)

Prototype of nephrotic syndrome in adults. Primary MN: autoantibodies against PLA2R (phospholipase A2 receptor) in ~70% of cases; also THSD7A, NELL-1, DNAJB9.

Histology (LM):

Diffuse thickening of GBM without increased cellularity
"Spike and dome" pattern on Jones silver stain (silver-negative deposits between silver-positive spikes)

IF: Granular IgG + C3 along capillary walls

EM: Subepithelial deposits with overlying GBM spikes ("spikes and domes"); foot process effacement

Stages (Ehrenreich-Churg): I → IV (progressive incorporation of deposits into GBM)

E. Focal Segmental Glomerulosclerosis (FSGS)

Leading cause of nephrotic syndrome in adults (especially Black patients).

Primary FSGS: Circulating permeability factor (podocyte injury). Variants: NOS, Tip, Cellular, Collapsing (worst prognosis — associated with HIV, COVID-19), Perihilar (secondary to hyperfiltration)

APOL1 risk alleles (G1, G2) dramatically increase risk in patients of West African ancestry.

Histology: Focal (<50% glomeruli), segmental sclerosis with hyalinosis; foot process effacement on EM; tubular atrophy proportional to sclerosis

IF: Non-specific IgM, C3 in sclerosed segments (trapped, not immune complex)

F. Membranoproliferative GN (MPGN)

Histology hallmark: Mesangial interposition into GBM → "tram-track" or double-contour appearance on PAS/Jones silver

Two pathogenic mechanisms:

Immune complex–mediated (IF: IgG, IgM, C3, C4) — HCV-associated cryoglobulinemia, SLE, bacterial infections
Complement-mediated (IF: C3 dominant, no Ig) — C3GN, Dense Deposit Disease (DDD); associated with C3 nephritic factor, CFH mutations

EM: DDD shows dense osmiophilic deposits within the GBM lamina densa ("sausage-shaped")

G. Lupus Nephritis (ISN/RPS Classification)

Class	Morphology	Clinical Significance
I	Normal LM, mesangial deposits by IF/EM	Minimal disease
II	Mesangial proliferative	Mild
III	Focal proliferative (<50%)	Moderate — treat
IV	Diffuse proliferative (≥50%)	Severe — aggressive therapy
V	Membranous	Nephrotic syndrome
VI	Advanced sclerosis	ESKD

IF: Full-house pattern — IgG, IgM, IgA, C3, C1q

EM: Subendothelial + mesangial deposits; "fingerprint" deposits in cryoglobulinemia; tubuloreticular inclusions in endothelium (interferon signature)

H. Diabetic Kidney Disease (DKD)

Histology hallmarks (by LM):

GBM thickening — first lesion identifiable by EM (within 2 years of onset)
Mesangial expansion — first lesion by LM (PAS stain; 4–5 years after T1DM onset)
Nodular glomerulosclerosis (Kimmelstiel-Wilson nodules) — PAS-positive acellular mesangial nodules; pathognomonic
Arteriolar hyalinosis (both afferent AND efferent — specific for diabetes)
Tubular basement membrane thickening

IF: Linear GBM staining for IgG (non-immune, trapping)

Diabetic kidney disease biopsy — Kimmelstiel-Wilson nodules

KW nodules (asterisks), arteriolar hyalinosis (arrowhead), Jones silver stain — Brenner & Rector's The Kidney

Brenner and Rector's The Kidney, p. 1778

I. Monoclonal Immunoglobulin Deposition Disease (MIDD)

Includes LCDD (light chain), HCDD (heavy chain), LHCDD.

LM: Nodular mesangial sclerosis mimicking DKD (2/3 of LCDD cases)

Key feature: Ribbon-like eosinophilic deposits along tubular basement membranes (predominantly distal tubules)

IF: Diffuse linear TBM and GBM staining for monotypic κ (LCDD) or a specific heavy chain (HCDD)

EM: Finely granular "powdery" electron-dense deposits along inner GBM and outer TBM

Comprehensive Clinical Nephrology 7e, p. 394

II. TUBULAR & INTERSTITIAL DISEASES

A. Acute Tubular Necrosis (ATN)

Most common cause of intrinsic AKI. Caused by ischemia/reperfusion or nephrotoxins (aminoglycosides, contrast, cisplatin, myoglobin).

Pathophysiology:

Tubular cell vacuolation → loss of brush border → disruption of epithelial polarity
Intratubular obstruction by cellular debris
Backleak of filtrate through denuded epithelium
Afferent arteriolar vasoconstriction (tubuloglomerular feedback)
"Trading away" GFR for medullary oxygenation preservation

Site of injury: Outer medulla (most severe) — S3 segment of proximal tubule (ischemia); S1/S2 (aminoglycosides)

Histology:

Tubular cell vacuolation, brush border loss, epithelial disruption
Intratubular casts (muddy brown granular casts on urinalysis)
Focal patchy necrosis (may be missed on cortical biopsy)
Patchy interstitial mononuclear infiltrates (without tubulitis)
Evidence of regeneration (mitoses, flattened epithelium) alongside fresh injury

Urinalysis: Muddy brown coarse granular casts; FENa >1%, FEurea >35%

Tubulointerstitial nephritis histology H&E

H&E of tubulointerstitium — lymphocytic infiltration, tubulitis, interstitial edema — compatible with AIN/ATN pattern

Comprehensive Clinical Nephrology 7e, p. 990

B. Acute Interstitial Nephritis (AIN)

Accounts for 15–27% of biopsies done for AKI. Most common cause: drugs (antibiotics > PPIs > NSAIDs).

Histology:

Interstitial inflammatory infiltrate — predominantly T lymphocytes, plasma cells, eosinophils (especially drug-induced)
Interstitial edema
Tubulitis — lymphocytes infiltrating tubular epithelium (distinguishes AIN from ATN)
Tubular atrophy and fibrosis if chronic

IF: May show linear or granular IgG along TBMs (drug-induced)

Clinical triad (classic but uncommon): Fever + rash + eosinophilia (present in <10–30% of drug-induced AIN)

Mean latency: ~10 days after drug exposure; NSAIDs may take months

Comprehensive Clinical Nephrology 7e, p. 990–991

C. Chronic Pyelonephritis / Reflux Nephropathy

Mechanism: Vesicoureteral reflux → ascending bacterial infection → recurrent cortical scarring

Gross pathology: Coarse cortical scars, irregular surface, dilated calyces (clubbing) overlying scars

Histology:

Interstitial fibrosis + chronic inflammatory infiltrate
Thyroidization of tubules — dilated tubules packed with pink colloid-like casts resembling thyroid follicles
Periglomerular fibrosis and global glomerulosclerosis
Vascular intimal thickening

D. Light Chain Cast Nephropathy (Myeloma Kidney)

Histology: Large, fractured, laminated, bright eosinophilic intratubular casts in distal tubules/collecting ducts; multinucleated giant cell reaction; tubular cell injury

Pathogenesis: Monoclonal free light chains (Bence Jones) overwhelm proximal tubule reabsorption → precipitate with Tamm-Horsfall protein in distal tubule (low pH, high NaCl, diuretics)

III. VASCULAR DISEASES

A. Hypertensive Nephrosclerosis (Benign)

Second most common cause of ESKD after diabetic nephropathy in the US (~27% of ESKD cases).

Histology:

Arteriolar hyalinosis (arteriolosclerosis) — pink homogeneous material replacing media of afferent arterioles
Interlobular artery intimal hyperplasia — fibrous thickening ("onion-skin" mild form)
Ischemic glomerulosclerosis — global sclerosis, wrinkling/retraction of GBM (ischemic collapse)
Tubular atrophy, interstitial fibrosis
No immune deposits (key distinguishing feature)

Genetic risk: APOL1 G1/G2 alleles markedly increase ESKD risk in African Americans

Benign hypertensive nephrosclerosis — sclerotic glomerulus, arteriolar hyalinosis

Globally sclerotic glomerulus, arteriolar hyalinosis, tubular atrophy — chronic nephrosclerosis. H&E.

Harrison's 22E, p. 2462

B. Malignant Hypertension (Malignant Nephrosclerosis)

Mechanism: Extreme hemodynamic stress → endothelial injury → fibrinoid necrosis of vessel walls

Histology:

Fibrinoid necrosis of arterioles (MSB stain: fibrin = red)
Onion-skin lesion (hyperplastic arteriolosclerosis) — concentric laminated smooth muscle + collagen proliferation in interlobular arteries; markedly narrows lumen
Thrombotic microangiopathy (TMA) — fibrin thrombi in glomerular capillaries, fragmented RBCs (schistocytes)
Acute glomerular capillary injury → FSGS-like injury

Labs: Microangiopathic hemolytic anemia (MAHA), thrombocytopenia, AKI

Malignant hypertension — fibrin thrombi, glomerular capillary thrombosis

Fibrin deposition in glomerular capillary loops (MSB stain) — malignant nephrosclerosis. Compatible with TMA morphology.

C. Thrombotic Microangiopathy (TMA)

Causes: HUS (STEC-O157:H7 → Shiga toxin → endothelial injury), TTP (ADAMTS13 deficiency), complement-mediated (atypical HUS — CFH/CFI/C3/MCP mutations), malignant hypertension, antiphospholipid syndrome, scleroderma renal crisis

Histology:

Glomerular capillary fibrin/platelet thrombi
Endothelial swelling with "bloodless" glomeruli
Mesangiolysis (dissolution of mesangial matrix)
Arteriolar intimal mucoid edema, intraluminal thrombi

D. Cholesterol Emboli (Atheroembolism)

Mechanism: Cholesterol crystals released from atheromatous plaques (post-procedure/spontaneous) → lodge in arcuate/interlobular arteries → ischemic damage + local eosinophilic inflammation

Histology:

Biconvex clefts in vessels (cholesterol dissolved during tissue processing)
Surrounding giant cell reaction and eosinophilia
Downstream cortical ischemia/infarction

E. Renal Artery Stenosis

Causes: Atherosclerosis (older, males, proximal RAS) or fibromuscular dysplasia (young women, distal/medial RAS → "string of beads" angiography)

Histology: Ischemic nephropathy — tubular atrophy, arteriolar collapse, global glomerulosclerosis, preserved cortical width (bilateral RAS) or focal cortical thinning (unilateral)

IV. INHERITED / STRUCTURAL / OTHER

A. Alport Syndrome

Mutation in COL4A3, COL4A4 (autosomal) or COL4A5 (X-linked, most common). Leads to absence of α3:α4:α5 type IV collagen network from GBM.

EM (pathognomonic): Irregular thinning and thickening, splitting/lamellation of GBM — "basket-weave" or "moth-eaten" appearance

IF: Absent staining for α3/α4/α5(IV) collagen in GBM (diagnostic)

Clinical: Hematuria → progressive CKD + sensorineural hearing loss + ocular defects (anterior lenticonus)

B. Thin Basement Membrane Disease (Benign Familial Hematuria)

Heterozygous COL4A3/A4 mutations. Diffuse uniform thinning of GBM (<150 nm; normal 300–400 nm) on EM. LM and IF: normal. Benign persistent microscopic hematuria.

C. Polycystic Kidney Disease (PKD)

	ADPKD	ARPKD
Gene	PKD1 (85%), PKD2	PKHD1
Cysts	All nephron segments	Collecting ducts + bile ducts
Age	Adult (3rd–5th decade)	Perinatal/childhood

Pathology: Massively enlarged kidneys with innumerable fluid-filled cysts of all sizes; cysts lined by flattened epithelium; interstitial fibrosis between cysts

Summary Table

Category	Key Lesion	LM Pattern	IF	EM
PSGN	Endocapillary proliferative	PMN infiltration, hypercellularity	Granular IgG, IgM, C3	Subepithelial humps
Anti-GBM	Crescentic GN	Extracapillary proliferation	Linear IgG	GBM disruption
IgA nephropathy	Mesangial proliferative	Mesangial expansion	Dominant IgA (mesangial)	Mesangial deposits
MN	No proliferation	Thick GBM, spikes	Granular IgG (subepithelial)	Subepithelial deposits
FSGS	Focal segmental sclerosis	Segmental hyalinosis	Non-specific IgM/C3	Foot process effacement
MPGN	Lobular, double contours	Tram-track	C3 ± Ig	Subendothelial/intramembranous
DKD	Nodular glomerulosclerosis	KW nodules, arteriolar hyalinosis	Linear IgG (non-immune)	GBM thickening, mesangial expansion
ATN	Tubular injury	Brush border loss, casts	—	Mitochondrial swelling
AIN	Interstitial inflammation	Tubulitis, eosinophils	± Linear TBM IgG	Normal glomeruli
Benign HTN	Arteriosclerosis	Arteriolar hyalinosis, ischemic GS	None	—
Malignant HTN	Fibrinoid necrosis, TMA	Onion-skin, thrombi	Fibrin	Endothelial swelling
Alport	GBM splitting	Normal LM	Absent α3/α5(IV)	Basket-weave GBM

Key References:

Harrison's Principles of Internal Medicine 22E (2025), pp. 2459–2463
Comprehensive Clinical Nephrology 7th Ed., pp. 344–346, 990–991
Brenner and Rector's The Kidney, pp. 1778–1780

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