Liver and Kidney: Histology, Pathology, and Special Stains – Exam-Ready Study Notes

Liver: Normal Histology & Pathology

Overview of the Liver

The liver is the largest organ and the largest mass of glandular tissue in the body, with critical roles in metabolism, detoxification, and synthesis of essential proteins.

• Dual Blood Supply: Receives ~75% of its blood from the hepatic portal vein (nutrient-rich, low oxygen) and ~25% from the hepatic artery (oxygen-rich, low nutrients).

• Exocrine Function: Produces bile for fat digestion and waste removal.

• Endocrine-like Functions: Synthesizes albumin, coagulation factors, and insulin-like growth factor (IGF).

• Metabolic Functions: Stores iron, converts vitamins, degrades drugs/toxins, and regulates nutrient distribution.

• Regeneration: Can regenerate from as little as 51% of its mass, which is clinically significant for surgical procedures.

Example: The liver's dual blood supply is essential for its metabolic and detoxification functions, and its regenerative capacity is crucial in liver transplantation.

Structural Components of the Liver

• Parenchyma: Plates of hepatocytes (about 80% of liver cells) radiate from portal triads toward the central vein.

• Connective Tissue (Stroma): Collagen surrounds portal areas; expands in fibrosis/cirrhosis.

• Hepatic Sinusoids: Irregular vascular channels with discontinuous (fenestrated) endothelium, allowing fluid and protein passage but not blood cells.

• Space of Disse: Perisinusoidal space between hepatocytes and endothelium; site of exchange, contains stellate cells and hepatocyte microvilli.

Three Models of the Liver Unit

Three anatomical models describe the organization of the liver, each with distinct clinical and pathological relevance.

Model	Shape	Centre	Blood Flow Direction	Key Use
Classic Lobule	Hexagonal	Central vein	Portal triads → Central vein	Anatomical/histological
Portal Lobule	Triangular	Portal triad (bile duct)	Inward from portal triad	Bile secretion model
Liver Acinus	Diamond	Portal triad branches	Zone 1 → 2 → 3 → central vein	Perfusion, metabolism, pathology

• Acinus Zones: Zone 1 (closest to portal supply, most oxygen/nutrients, first to receive toxins), Zone 3 (furthest, most susceptible to hypoxic and toxic injury, e.g., paracetamol/alcohol), Zone 2 (intermediate).

• Centrilobular necrosis: Occurs in Zone 3.

Example: Paracetamol overdose causes necrosis in Zone 3 (centrilobular).

The Portal Triad

Located at the corners of the classic lobule, the portal triad contains four key structures:

• Portal Vein (PV): Nutrient-rich, low oxygen blood from the GI tract; thin wall, wide lumen.

• Hepatic Artery (HA): Oxygen-rich blood; thick wall with visible smooth muscle.

• Bile Duct (BD): Lined by cholangiocytes; carries bile to the duodenum.

• Lymphatic Vessel: Difficult to see in light microscopy; visible in electron microscopy.

Hepatocytes

• Large, polygonal cells with spherical (often binucleated) nuclei; acidophilic cytoplasm due to abundant organelles.

• Arranged in plates radiating toward the central vein.

• Basal surface: Faces sinusoids (space of Disse) for protein exchange.

• Apical surface: Forms bile canaliculi between adjacent hepatocytes.

• Bile Flow Pathway: Canaliculi → Canals of Hering → Intrahepatic ductules → Interlobular bile ducts → R/L hepatic ducts → Common hepatic duct → Gallbladder → Bile duct → Duodenum.

• Canals of Hering: Contain hepatic stem cells.

Non-Parenchymal Cells

• Kupffer Cells: Sinusoidal macrophage-like cells; phagocytose senile RBCs, recycle iron, monitor for pathogens.

• Hepatic Stellate Cells (Ito): Store vitamin A when quiescent; upon activation, become myofibroblasts and produce collagen type I, leading to fibrosis. Marker: alpha-smooth muscle actin (α-SMA).

• Sinusoidal Endothelial Cells: Discontinuous, highly fenestrated; allow passage of fluid and proteins, not blood cells.

Bile Formation & Flow

• Bile: Composed of waste products and bile salts for fat emulsification/digestion.

• Released into the duodenum under the influence of cholecystokinin (CCK).

• Bile flow is opposite to blood flow in sinusoids.

Liver Diseases & Pathology

• Hepatitis: Inflammation of the liver (e.g., viral, autoimmune, alcohol-induced). Histology shows mononuclear cell infiltrate, especially around portal veins.

• Cirrhosis: End-stage of chronic liver diseases. Hallmarks: (1) Thick bands of mature fibrous tissue (collagen), (2) Nodules of regenerative hepatocytes. Macroscopically: shrunken, nodular, stiff liver. Can progress to hepatocellular carcinoma (HCC).

• Hepatocellular Carcinoma (HCC): Most common primary liver tumor, usually in cirrhotic livers. The liver is also the most common site for secondary (metastatic) tumors, especially from the GI tract.

• Obstructive Jaundice (Icterus): Caused by blocked bile ducts (e.g., tumor, gallstones), leading to bile accumulation and hepatocyte necrosis in portal areas. Clinical signs include yellow sclera and right upper quadrant pain.

• Genetic Disorders: Haemochromatosis (genetic iron overload, HFE mutation) vs Haemosiderosis (secondary/acquired iron overload, e.g., transfusions).

Laboratory Investigations — Liver

• Liver Function Tests (LFTs): ALT/AST (hepatocyte damage), bilirubin (jaundice/hemolysis), ALP (biliary obstruction), albumin (synthetic function).

Stain	What It Shows	Clinical Use
H&E	Nuclei = blue/purple; cytoplasm = pink	General architecture, inflammation, necrosis
Perls' Prussian Blue	Iron (haemosiderin) = blue	Haemochromatosis, haemosiderosis
Reticulin (Silver)	Reticular fibres = black	Cirrhosis/fibrosis, lobular architecture
Van Gieson / Masson Trichrome	Collagen = blue/green	Cirrhosis, fibrosis, bile accumulation
PAS	Glycogen = magenta/purple	Glycogen storage diseases
PASD	PAS + diastase; non-glycogen material	Alpha-1 antitrypsin deficiency
Oil Red O / Sudan Black	Lipid/fat = red/black (frozen section)	Fatty liver (steatosis/NAFLD)

Example: Perls' Prussian Blue stains iron blue, useful for diagnosing haemochromatosis.

Kidney: Normal Histology & Pathology

Overview of the Kidney & Urinary System

The kidneys are retroperitoneal organs responsible for urine production, waste elimination, and homeostatic regulation.

• Components: 2 kidneys, 2 ureters, bladder, urethra.

• Urine Production: 180 L filtrate/day reduced to ~1.5 L urine via reabsorption/secretion.

• Functions: Waste elimination, acid-base balance, fluid/electrolyte conservation, blood pressure regulation.

• Endocrine Functions: Erythropoietin (EPO) for RBC production, renin for blood pressure (RAAS).

Gross Structure of the Kidney

• Capsule: Dense irregular collagenous connective tissue.

• Cortex: Contains glomeruli, PCT, DCT, cortical collecting tubules, medullary rays.

• Medulla: Contains renal pyramids (loop of Henle, collecting ducts); tip = renal papilla.

• Renal Pelvis & Calyces: Minor calyx (7–13) → major calyx (2–4) → renal pelvis → ureter.

• Arcuate arteries/veins: Landmark at corticomedullary junction.

The Nephron — Functional Unit of the Kidney

Each kidney contains about 1 million nephrons, which filter blood and form urine.

Segment	Epithelium	Key Features	Function
Proximal Convoluted Tubule (PCT)	Simple cuboidal	Intensely eosinophilic, brush border, narrow lumen	Active reabsorption (glucose, amino acids, Na+, water)
Thin Loop of Henle	Simple squamous	Nuclei bulge into lumen, thin walls	Passive water/ion movement
Distal Convoluted Tubule (DCT)	Simple cuboidal	Pale, wide lumen, low brush border	Active ion transport, macula densa present
Collecting Tubules/Ducts	Simple cuboidal	Pale cytoplasm, prominent cell borders	Urine transport, ADH-regulated water reabsorption

• Juxtamedullary nephrons: Long loops, concentrate urine.

• Cortical nephrons: Shorter loops, less significant in concentration.

Renal Corpuscle — Glomerulus & Bowman's Capsule

• Glomerulus: Ball of fenestrated capillaries; afferent arteriole enters, efferent arteriole exits.

• Bowman's Capsule: Parietal layer (simple squamous), visceral layer (podocytes with foot processes and filtration slits).

• Bowman's/Urinary Space: Collects filtrate, which enters the PCT.

Layer	Structure	Function
Fenestrated Endothelium	Large pores	Allows fluid/small molecules; blocks blood cells
Glomerular Basement Membrane (GBM)	Negatively charged glycoprotein	Blocks large/negatively charged proteins
Podocyte Filtration Slits	Foot processes with slit diaphragms	Final molecular sieve; damage causes proteinuria

Juxtaglomerular Apparatus (JGA) — Blood Pressure Control

• Juxtaglomerular (JG) Cells: Modified smooth muscle cells in the afferent arteriole wall; secrete renin in response to low blood pressure or sodium.

• Macula Densa: Specialized DCT cells at the vascular pole; sense decreased salt concentration and signal JG cells to release renin.

• Renin-Angiotensin-Aldosterone System (RAAS): $\text{Renin} \rightarrow \text{Angiotensinogen} \rightarrow \text{Angiotensin I} \rightarrow \text{ACE (lungs)} \rightarrow \text{Angiotensin II} \rightarrow \text{Aldosterone}$

Identifying Cortex vs Medulla in Histology

• Cortex: Contains glomeruli, medullary rays, PCT, DCT, arcuate arteries.

• Medulla: No glomeruli; contains thin segments of loop of Henle, collecting ducts, vasa recta.

• Transitional Epithelium: Lines calyces, renal pelvis, ureters, bladder; allows stretching.

Blood Supply of the Kidney

• Arterial Pathway: Renal artery → Segmental → Lobar → Interlobar → Arcuate (corticomedullary junction) → Interlobular → Afferent arteriole → Glomerulus → Efferent arteriole.

• Venous Pathway (cortex): Stellate veins → Interlobular veins → Arcuate veins.

Kidney Diseases & Pathology

• Nephritis: General inflammation of the kidney.

• Acute Glomerulonephritis: Inflammation of glomeruli; presents with hematuria, proteinuria, edema.

• Congenital: Polycystic kidney disease.

• Diagnosis: Uses paraffin sections (special stains), immunofluorescence (IF), and electron microscopy (EM).

Stain/Technique	What It Shows	Clinical Use
H&E	General structure, thickening of glomerular BM	First-line assessment
PAS	Basement membrane, brush border of PCT	Glomerulonephritis
PA Silver	Spikes, serrations, deposits in BM	Membranous nephropathy
MSB	Fibrin, connective tissue, clots	Thrombosis, fibrin deposits
Sirius Red / Congo Red	Amyloid deposits (apple-green birefringence)	Amyloidosis
Immunofluorescence (IF)	IgG, IgA, IgM, C3, C4, fibrinogen deposits	Autoimmune diseases
Electron Microscopy (EM)	Dense deposits in/on GBM, podocyte changes	Membranous nephropathy, podocyte disease

Quick-Reference Summary

• Liver Key Numbers: 80% hepatocytes, 75% portal vein / 25% hepatic artery, 51% minimum for regeneration, 3 unit models, 4 portal triad components.

• Kidney Key Numbers: 180 L filtrate/24h → 1.5 L urine, 7–13 minor calyces, 2–4 major calyces, 3-layer glomerular filtration barrier, JGA at vascular pole, arcuate arteries at corticomedullary junction.

Sample MCQ Topics (Exam Focus)

• Liver blood supply: ~75% portal vein, ~25% hepatic artery

• Acinus zones: Zone 1 (first to receive toxins), Zone 3 (paracetamol/alcohol necrosis)

• Liver unit model: Acinus best correlates with pathology

• Portal triad: Portal vein, hepatic artery, bile duct, lymphatics

• Stellate cells: Quiescent (vitamin A storage), activated (myofibroblasts, fibrosis)

• Kupffer cells: Remove senile RBCs, recycle iron

• Cirrhosis: Fibrous bands + regenerative nodules

• HCC: Most common primary liver tumor; liver is most common site for secondary tumors

• Iron stain: Perls' Prussian Blue (blue)

• PASD stain: Alpha-1 antitrypsin deficiency

• PCT vs DCT: PCT = narrow, brush border; DCT = wide, pale

• Loop of Henle: Simple squamous epithelium

• JGA: JG cells secrete renin; macula densa in DCT

• Filtration barrier: Fenestrated endothelium → GBM → podocyte slits

• Kidney stains: Congo Red/Sirius Red for amyloid

• Immunofluorescence: Requires frozen section

• Arcuate arteries: At corticomedullary junction

Additional info: These notes integrate high-yield exam tips and MCQ predictions based on the lecture content, providing a comprehensive review for students preparing for cellular and transfusion science exams.