Urinary System Pathology

1 Normal Kidney Anatomy & Histology

Gross Anatomy & Functional Unit (Nephron) ▼

The Nephron — Functional Unit of the Kidney

The nephron is the functional unit of the kidney and has two main components:

The Glomerulus — filters blood
The Tubular System — modifies filtrate (reabsorption & secretion)

Structure of the Glomerulus

The glomerular tuft = network of capillaries + mesangium + podocytes
The tuft is enclosed inside the Bowman's capsule (basement membrane + parietal epithelial cells)
Bowman's space = gap between the tuft and Bowman's capsule

Glomerular Filtration Barrier (3 layers)

Layer	Feature
Endothelial cells	Fenestrated (have pores for fluid passage)
Glomerular Basement Membrane (GBM)	Central electron-dense zone flanked by two electron-lucent layers; contains nephrin proteins
Podocytes (Visceral epithelial cells)	Have numerous tiny foot processes; connected by filtration slits

Mesangium

Mesangial cells lie in a mesangial matrix between capillaries; they support the glomerular tuft
They are contractile, can proliferate, lay down matrix/collagen, and secrete biologically active mediators

Methods for Evaluating a Renal Biopsy ▼

🧠 Memory Aid — "LIE"

L — Light Microscopy | I — Immunofluorescence | E — Electron Microscopy

🔬 Light Microscopy (LM)

H&E
PAS (Periodic acid-Schiff)
PASM (Periodic acid-silver methenamine)
Masson trichrome
Immunohistochemistry (IHC)

⚡ Electron Microscopy (EM)

Detects ultra-structural changes
Shows subepithelial/subendothelial deposits
Foot process effacement

🌟 Immunofluorescence (IF)

Detects Ig & complement deposition
Granular pattern = immune complex
Linear pattern = anti-GBM Ab

2 Glomerular Disease — Fundamentals

Pathogenesis of Glomerular Injury ▼

Key Principle

Immune mechanisms underlie the MAJORITY of glomerular diseases.

Two Basic Forms of Antibody-Associated Injury

🔵 Circulating Immune Complex Deposition

Soluble antigen-antibody complexes circulate in blood
They deposit in the glomerulus
Activate complement → injury
Pattern on IF: GRANULAR

🔴 In Situ Antibody Reaction

Antibodies react directly with GBM antigens
Anti-GBM antibody disease
Pattern on IF: LINEAR

Other mediators of injury

Complement activation, autoantibodies, and leukocyte recruitment all contribute to glomerular injury.

Basic Histologic Alterations & Terminology ▼

4 Histologic Alterations

1. Hypercellularity

Proliferation of endothelial, mesangial or epithelial cells
Leukocytic infiltration (neutrophils, monocytes, lymphocytes)
Endocapillary = within capillary lumen
Extracapillary (Crescents) = parietal epithelial cell proliferation in Bowman's space

2. GBM Thickening

Deposition of immune complexes on endothelial side, epithelial side, or within GBM
Example: Membranous nephropathy

3. Exudation & Necrosis

Exudate = inflammatory fluid/cells
Fibrinoid necrosis = severe acute injury

4. Hyalinosis & Sclerosis

Hyalinosis: homogeneous eosinophilic material; plasma protein deposition; obliterates capillary lumens; end result of glomerular damage
Sclerosis: extracellular collagenous matrix; obliterates lumens; forms fibrous adhesions to Bowman's capsule

Important Descriptive Terms

Term	Definition	Memory tip
Diffuse	Involves ALL or nearly all glomeruli	"D for all" — Diffuse = Dominant involvement
Focal	Involves only a PROPORTION of glomeruli	"F for Few" — only some glomeruli affected
Global	Involves the WHOLE glomerulus	"G for whole Globe"
Segmental	Involves only PART of each glomerulus	"S for Segment" — like a segment of an orange

Clinical Syndromes of Glomerular Diseases ▼

🔵 Nephrotic Syndrome

Heavy proteinuria (>3.5 g/day)
Hypoalbuminemia
Severe edema
Hyperlipidemia
Lipiduria

Mnemonic

PHEHL — Protein, Hypo-albumin, Edema, Hyperlipid, Lipiduria

🔴 Nephritic Syndrome

Hematuria (hallmark)
Oliguria
Proteinuria (mild-moderate)
Edema (generalized)
Azotemia (↑ urea/creatinine)
Hypertension

Mnemonic

HOPAAH — Hematuria, Oliguria, Proteinuria, Azotemia, edema (Anasarca), Hypertension

Morphology

🔬 Gross

Kidneys are enlarged and red. Fine punctuated petechiae scattered over the surface.

🔬 Light Microscopy

Diffuse — nearly all glomeruli affected
Increased cellularity (endothelial + mesangial proliferation)
Neutrophilic and monocytic infiltrate
Red blood cell casts in tubules

🔬 IF & EM

IF: Scattered granular deposits of IgG and complement in capillary walls and mesangium
EM: Subepithelial "humps" (electron-dense deposits) — PATHOGNOMONIC

Clinical Features

Acute nephritic syndrome
Serum complement levels ↓ (C3, C4 consumed)
Anti-streptolysin O (ASO) antibody ↑
Prognosis: excellent in children; worse in adults

Rapidly Progressive GN (RPGN / Crescentic GN) ▼

Definition

RPGN is a clinical syndrome (not a specific disease) characterized by:

Presence of crescents in most glomeruli on biopsy
Rapid loss of renal function (weeks to months)
Most cases are immunologically mediated

What is a Crescent?

Formation of Crescents

GBM rupture → fibrin leaks into Bowman's space
Parietal epithelial cells proliferate
Macrophages migrate into Bowman's space
These form a crescent shape that compresses the glomerulus
Obliterates Bowman's space → diminishes glomerular filtration

Morphology

🔬 Gross

Kidneys are enlarged and pale.

🔬 LM (PAS/PASM)

Crescents visible in Bowman's space compressing the glomerular tuft.

GBM disruption (arrows).

🔬 Immunofluorescence — Anti-GBM type

Linear distribution of IgG along the GBM (characteristic of anti-GBM antibody disease).

Goodpasture Syndrome

Special Scenario

When anti-GBM antibodies cross-react with alveolar basement membranes:

Pulmonary hemorrhages occur
Patient has BOTH lung and kidney lesions
This combination = Goodpasture Syndrome

Clinical Course

Onset like nephritic syndrome, but more pronounced oliguria and azotemia
Proteinuria may approach nephrotic range
May become anuric → requires dialysis or transplantation
Prognosis correlates with number of crescents (more = worse)
Treatment: Plasma exchange benefits anti-GBM type

Membranous Nephropathy (Membranous Glomerulopathy) ▼

Key Feature

Most frequent cause of nephrotic syndrome in ADULTS
Caused by subepithelial immune complex deposits along the GBM

Morphology

Technique	Findings
PAS stain (LM)	Diffuse thickening of GBM without hypercellularity or inflammation
PASM stain (LM)	"Spike and dome" pattern — spikes are GBM projections between deposits
IF	Typical granular deposits of Ig and complement along GBM
EM	Subepithelial deposits (white arrows) separated by GBM spike-like protrusions (blue arrows)

🧠 Pathognomonic Pattern

"SPIKE AND DOME" on PASM/EM = Membranous Nephropathy

Clinical Features

Age: 30–50 years
Sudden onset, full-blown nephrotic syndrome
Proteinuria is non-selective
Fails to respond to corticosteroid therapy
40% progress to renal failure over 2–20 years

Minimal Change Disease (MCD / Nil Disease / Lipoid Nephrosis) ▼

Key Feature

Most frequent cause of nephrotic syndrome in CHILDREN

Morphology

Technique	Findings
Light Microscopy (LM)	Normal glomeruli — normocellular, normal GBM thickness (hence "minimal change")
Immunofluorescence (IF)	No immune deposits
Electron Microscopy (EM)	Diffuse effacement of podocyte foot processes — THE diagnostic finding

🧠 Pathognomonic

Foot process effacement on EM = Minimal Change Disease

Clinical Features

Abrupt nephrotic syndrome in healthy child
Selective proteinuria (only albumin lost, not large globulins)
Responds to corticosteroid therapy — hallmark
Favorable prognosis

Membranous vs Minimal Change — Key Differences

	Membranous	Minimal Change
Age	Adults (30–50y)	Children
LM	GBM thickening	Normal
EM	Subepithelial deposits + spikes	Foot process effacement only
IF	Granular deposits	No deposits
Steroids	Does NOT respond	RESPONDS

Chronic GN — End-Stage Glomerular Disease ▼

Gross Morphology

Symmetrically contracted kidneys
Diffusely granular surfaces
Thinned cortex
Increased peripelvic fat

🧠 Gross Appearance Comparison

Chronic GN: Symmetrically small + granular | Chronic Pyelonephritis: Asymmetrically scarred + deformed calyces

Microscopic Morphology

Majority of glomeruli show hyalinization and sclerosis
Marked atrophy of tubules
Irregular interstitial fibrosis
Mononuclear leukocytic infiltration
Arterial and arteriolar sclerosis (due to hypertension)

Clinical Features

Insidious onset, slowly progressive
Death in uremia over years to decades
Management: Dialysis and kidney transplantation

📊 Quick Comparison Table — All Glomerular Diseases ▼

Disease	Age	Syndrome	LM	IF	EM	Steroids
Poststreptococcal GN	Children	Nephritic	Diffuse proliferative; RBC casts	Granular (IgG + C3)	Subepithelial humps	Supportive
RPGN	Any	Rapidly progressive nephritic	Crescents	Linear (anti-GBM) or granular	GBM disruption	Plasma exchange
Membranous Nephropathy	Adults 30–50y	Nephrotic	GBM thickening (PAS); Spikes (PASM)	Granular (subepithelial)	Subepithelial deposits + spikes	No response
Minimal Change Disease	Children	Nephrotic	Normal	No deposits	Foot process effacement	Responds
Chronic GN	Adults	CKD/ESRD	Hyalinization + sclerosis	Variable	Variable	Dialysis/Transplant

4 Pyelonephritis

Pathways of Renal Infection & Causative Organisms ▼

Route 1: Ascending Infection (More Common)

Urinary bladder infection (cystitis)
Vesicoureteral reflux (VUR)
Intrarenal reflux
Reach kidney via ureter

Route 2: Hematogenous (Less Common)

Bacteremic spread from distant site
Bacteria seed the kidney via blood

Causative Organisms

Enteric gram-negative bacilli
E. coli — by far the most common
Other: Proteus, Klebsiella, Pseudomonas

Acute Pyelonephritis ▼

Morphology

🔬 Gross

Cortical surface studded with focal yellowish abscesses
Dark congestion between abscesses

🔬 Microscopy

Liquefactive necrosis / abscess formation
Suppuration in tubular lumina and interstitial tissue
White cell casts (pus casts)
Glomeruli are NOT affected

⚠️ Complications

Pyonephrosis: pus fills renal pelvis, calyces, ureter (when obstruction present)
Papillary necrosis: ischemic + suppurative necrosis of pyramid tips

Predisposing Conditions for Papillary Necrosis (Triad)

Diabetes mellitus + Urinary tract obstruction + Sickle cell anemia

Gross: Gray-white to yellow necrosis of apical 2/3 of pyramids

Clinical Features

Predominantly females
Sudden onset
Systemic infection signs (fever, chills, malaise)
Localizing signs (loin pain, costovertebral angle tenderness, dysuria)
Initially unilateral; may become recurrent, chronic, and bilateral
Papillary necrosis = poor prognosis

Chronic Pyelonephritis ▼

Key Features (Definition)

Scarring and deformity of the pelvicalyceal system
History of urinary tract infections
Two forms: Chronic obstructive and Chronic reflux-associated

Morphology

🔬 Gross

Kidneys NOT equally contracted (uneven scarring)
Scarring involves pelvis/calyces → papillary blunting and calyceal deformities

🔬 Microscopy

Tubules: Thyroidization (dilated tubules filled with eosinophilic casts resembling thyroid follicles)
Uneven interstitial fibrosis and inflammatory infiltrate
Glomeruli: uninvolved OR periglomerular fibrosis OR sclerosed
Arteriosclerosis

🧠 Key Gross Difference

Chronic GN → Symmetrically shrunken + granular surface (no calyceal deformity)

Chronic Pyelonephritis → Asymmetrically scarred + deformed calyces (papillary blunting)

Clinical Features

Gradual onset of renal insufficiency
Hypertension, polyuria, nocturia, proteinuria
Recurrent acute pyelonephritis episodes
Characteristic radiologic image (asymmetric scarring + deformed calyces)

5 Renal Cell Carcinoma (RCC)

RCC — Overview & Risk Factors ▼

Basics

Arise from renal tubular epithelium
Represent 80–85% of all primary renal malignant tumors
Male:Female = 2:1

Risk Factors

Mnemonic — SHOPC

S — Smoking | H — Hypertension | O — Obesity | P — Polycystic disease | C — Cadmium exposure + genetic (Cancer genes)

Clear Cell RCC (Most Common — 65–70%) ▼

Molecular Basis

95% sporadic; others familial or associated with VHL disease
Loss or inactivation of both copies of VHL tumor suppressor gene
VHL protein normally limits angiogenesis → its absence → increased angiogenesis → tumor growth

Gross Morphology

Usually solitary (5% multiple)
Well-defined margins; centered on cortex
Cut surface: solid; yellow to orange to gray-white
Variegated appearance: hemorrhage, necrosis, calcification, cystic change
May form tumor thrombus in the renal vein

Microscopy

Tumor cells: large, rounded/polygonal; abundant clear cytoplasm (lipid + glycogen)
Nuclei: small, round, centrally located
Growth: solid, tubular, or disorganized cord pattern
Stroma: scant but highly vascularized (prominent thin-walled blood vessels)
Oil Red O stain: shows abundant intracellular lipid

Papillary RCC (15–18%) ▼

Molecular

Familial cases: activating mutations of c-MET oncogene (chromosome 7q)
Sporadic cases: trisomy of chromosomes 7, 12, 13, 16, 17, 20; deletion of chromosomes 21 and Y

Key Features

Well-circumscribed; tend to be bilateral and multiple
Show varying papilla formation with extensive necrosis, hemorrhage, cystic degeneration
Less orange-yellow (less lipid) than clear cell RCC
May extend beyond renal capsule into renal sinus

Histology

Papillae with fibrovascular cores
Cells: cuboidal or low columnar; clear or pink cytoplasm
Interstitial foam cells (lipid-laden macrophages) in papillary cores — characteristic

Chromophobe RCC (~5%) ▼

Key Facts

~5% of all RCC
Arise from intercalated cells of collecting ducts
Multiple losses of entire chromosomes
Favorable prognosis compared to other RCC types

Morphology

Gross: Well-circumscribed, solitary; homogeneous gray-brown; no hemorrhage or necrosis
LM low power: broad "alveolar" arrangement
Classic ("plant-cell") type: sharply defined borders, abundant pale acidophilic cytoplasm, very prominent cell membranes
Eosinophilic variant: perinuclear halo; less prominent cell membranes

Clinical Features of RCC & Comparison Table ▼

Clinical Presentation

Classic Triad of RCC

Painless hematuria
Palpable abdominal mass
Dull flank pain

Note: The classic triad is found in only ~10% of patients — most present with just hematuria

Other Features

Systemic: weight loss, anemia, fever
Paraneoplastic syndromes (ectopic hormone production)
Invades renal vein → tumor thrombus (especially clear cell RCC)
Metastases: Lungs (>50%) → Bones (33%) → lymph nodes → liver → adrenal → brain

Summary Comparison Table

	Clear Cell	Papillary	Chromophobe
Frequency	65–70%	15–18%	~5%
Origin	Proximal tubule	Proximal tubule	Collecting duct intercalated cells
Gene	VHL (tumor suppressor)	c-MET (oncogene)	Chromosomal losses
Gross color	Yellow-orange (rich lipid)	Less yellow	Homogeneous gray-brown
Special feature	Renal vein invasion; highly vascular	Bilateral, multiple; foam cells	"Plant cell" appearance; best prognosis

6 Urothelial Neoplasms of the Bladder

Normal Bladder Anatomy & Histology ▼

Gross Anatomy

Empty bladder: three-sided pyramid shape
Has apex, base, superior surface, two inferolateral surfaces
Two ureters enter at upper corners of base
Urethra drains from lower corner of base
Trigone: smooth triangular area between ureter openings and urethra

Histology — Layers

Mucosa: urothelium + lamina propria
Muscularis propria (detrusor muscle)
Adventitia

Urothelium: 2–7 layers thick (varies with distension); top layer = distinctive umbrella cells

Epidemiology & Risk Factors for Bladder Cancer ▼

Demographics

Male; >50 years; White race
Urothelial tumors = 90% of all bladder tumors
Many tumors are multifocal at presentation

Risk Factors

Mnemonic — SCORSE

Genetics

Mutations in genes on chromosome 9 (including p16)
p53 mutations
FGFR3 mutations (most common in low-grade tumors)

WHO 2016 Grading of Noninvasive Papillary Urothelial Neoplasia ▼

Two Precursor Lesions of Invasive Carcinoma

Noninvasive papillary tumor
Carcinoma in situ (CIS)

Relatively simple branching papillary architecture
Urothelium histologically normal; prominent umbrella cells
Very rare, solitary, noninvasive, benign; rarely recur
Small exophytic lesions with delicate papillary fronds

Rare benign neoplasm in elderly males
Endophytic lesion — grows INTO the lamina propria (inverted = inward)
Cords and trabeculae of urothelial cells; peripheral palisading of basal cells; mature cells in center

Papillary fronds lined by thickened urothelium (7–10 layers) but lacks cytologic malignancy features
Orderly arrangement, no loss of polarity
Nuclei crowded, minimally enlarged; no mitoses; no prominent nucleoli

Rarely invasive, but may recur after removal
Overall orderly appearance but thicker lining than papilloma
Scattered hyperchromatic nuclei
Mitotic figures confined to basal half of urothelium

Marked cytologic and architectural atypia
Cells in disarray; loss of polarity
Variability in nuclear size and shape
Hyperchromatic, enlarged nuclei
Mitotic figures at all levels of urothelium
High incidence of invasion and metastases
Invades: lamina propria → muscular layer → vessels

Carcinoma In Situ (CIS) — Special Mention

CIS — Flat Carcinoma

Overtly malignant cells within a flat (non-papillary) urothelium
Tumor cells lack cohesiveness → shed into urine → detected by urine cytology
Commonly multifocal; may involve most of bladder, ureters, urethra
If untreated: 50–75% will progress to muscle-invasive cancer

Clinical Features of Bladder Tumors ▼

Symptoms

Painless hematuria — most common presenting symptom
Frequency, urgency, dysuria
Pyelonephritis (if ureter obstructed)
Hydronephrosis (if ureter obstructed)

Important Principles

Recurrence: Requires adequate follow-up cystoscopy
Prognosis: Determined by grade AND stage
Early detection: Cystoscopy and biopsy

7 Wilms Tumor (Nephroblastoma)

Overview & Genetics ▼

Basics

Most common primary renal neoplasm of childhood
Peak age: 2–5 years
All bilateral + ~1/3 of unilateral cases are hereditary
Genetic loci: WT1 and WT2
WT1 gene is critical for normal renal and gonadal development

Associated Congenital Syndromes (WT1/WT2 aberrations)

Syndrome	Genetic Change	Features
WAGR syndrome	WT1 deletion	Wilms tumor, Aniridia, Genital anomalies, Retardation
Denys-Drash syndrome (DDS)	WT1 mutation	Gonadal dysgenesis, nephropathy, Wilms tumor
Beckwith-Wiedemann syndrome (BWS)	WT2 imprinting abnormalities	Organomegaly, macroglossia, hemihypertrophy, Wilms tumor

Morphology of Nephroblastoma ▼

Gross

Usually large, solitary, well-circumscribed mass
10% are bilateral or multicentric at diagnosis
Cut section: soft, homogeneous, tan to gray
Occasional hemorrhage, cystic degeneration, necrosis
May protrude into calyces as polypoid masses

Classic Triphasic Histology

🧠 TRIPHASIC = Blastema + Epithelium + Stroma (BES)

1. Blastemal Component

Highly cellular
Undifferentiated small round-to-oval cells
Hyperchromatic nuclei
Scant cytoplasm

2. Epithelial Component

Immature tubules or glomeruli
Resembles developing embryonic kidney

3. Stromal Component

Spindle cells
Fibroblastic and myxoid stroma
Less cellular areas

Anaplasia — Important Adverse Feature

Anaplasia

Cells with enlarged, hyperchromatic, pleomorphic nuclei and abnormal mitoses
Correlates with p53 mutation
Associated with resistance to chemotherapy
Detected by p53 immunohistochemistry (positive stain)

Heterologous Elements (Rare)

Squamous/mucinous epithelium, smooth muscle, adipose tissue, cartilage, osteoid, neurogenic tissue

Clinical Features & Key Teaching Points ▼

Presentation

Most common: large abdominal mass (may cross midline, extend to pelvis)
Hematuria, abdominal pain
Intestinal obstruction
Hypertension

Prognosis — EXCELLENT

Generally very good prognosis
Excellent results with nephrectomy + chemotherapy

Key Teaching Concepts Illustrated by Nephroblastoma

1. Developmental Link

Congenital malformations → increased tumor risk (WAGR, DDS, BWS)

2. Tumor Resembles Organ

Histologic similarity between tumor and developing kidney — tumor recapitulates embryonic renal development

3. Childhood Tumor Success

Remarkable success of childhood tumor treatment — even advanced tumors respond well to combined therapy

📋 Master Revision Summary

One-Page Quick Revision ▼

Disease	Who	Key Morphology	Clinical	Remember
Post-strep GN	Children	Diffuse proliferative; subepithelial humps on EM; granular IF	Nephritic; ↓C3; ↑ASO Ab	"Humps" = pathognomonic
RPGN	Any age	Crescents; GBM disruption; Linear IF (anti-GBM type)	Rapidly progressive nephritic; Goodpasture if lung involved	Prognosis ∝ number of crescents
Membranous Nephropathy	Adults 30–50y	GBM thickening; spike & dome (PASM/EM); granular IF	Nephrotic; no steroid response	"Spike and dome" key pattern
Minimal Change Disease	Children	Normal LM; foot process effacement EM; no IF	Nephrotic; selective proteinuria; responds to steroids	Only EM abnormal
Chronic GN	Adults	Symmetrically shrunken, granular; hyalinization	CKD → ESRD	Symmetric = GN; Asymmetric = Pyelonephritis
Acute Pyelonephritis	Females	Yellowish abscesses; WBC casts; glomeruli SPARED	Sudden; fever; loin pain; pyuria	E. coli most common
Chronic Pyelonephritis	Any	Asymmetric scarring; thyroidization; calyceal deformity	Gradual CKD; HTN; polyuria	Calyceal deformity distinguishes from chronic GN
Clear Cell RCC	Adults; M>F	Yellow-orange; clear cells; highly vascular; renal vein invasion	Hematuria, mass, flank pain	VHL gene; most common RCC
Papillary RCC	Adults	Bilateral/multiple; papillae + foam cells	Hematuria; less yellow	c-MET oncogene
Chromophobe RCC	Adults	Gray-brown; plant-cell appearance	Best prognosis	Collecting duct intercalated cells
Bladder Ca (HGPUC)	M>F; >50y; White	Marked atypia; all-level mitoses; invades muscle	Painless hematuria	Smoking #1 risk factor; p53 mutations
Wilms Tumor	Children 2–5y	Triphasic (blastema + epithelium + stroma); anaplasia = p53	Abdominal mass; HTN; hematuria	WT1/WT2 genes; excellent prognosis

Structure of the Glomerulus

Glomerular Filtration Barrier (3 layers)

Mesangium

Two Basic Forms of Antibody-Associated Injury

4 Histologic Alterations

Important Descriptive Terms

Other Clinical Presentations

Morphology

Clinical Features

What is a Crescent?

Morphology

Goodpasture Syndrome

Clinical Course

Morphology

Clinical Features

Morphology

Clinical Features

Gross Morphology

Microscopic Morphology

Clinical Features

Morphology

Clinical Features

Morphology

Clinical Features

Risk Factors

Gross Morphology

Microscopy

Key Features

Histology

Clinical Presentation

Other Features

Summary Comparison Table

Risk Factors

Genetics

Carcinoma In Situ (CIS) — Special Mention

Symptoms

Important Principles

Associated Congenital Syndromes (WT1/WT2 aberrations)

Gross

Classic Triphasic Histology

Anaplasia — Important Adverse Feature

Presentation

Key Teaching Concepts Illustrated by Nephroblastoma