---

Histology of Organs Associated with the Digestive Tract

---

Overview

---

1. SALIVARY GLANDS

General Organization

---

Cell Types of Secretory Acini

1. Serous Cells

• Pyramidal protein-secreting cells
• Round, basally placed nuclei
• Well-developed basal RER (strongly basophilic cytoplasm)
• Apical secretory (zymogen) granules (eosinophilic/purple)
• Joined apically by tight and adherent junctions
• Arranged in spherical serous acini with a small central lumen
• Secrete enzymes (e.g., amylase, proline-rich proteins, lactoferrin)

2. Mucous Cells

• More columnar in shape
• Flattened, compressed basal nuclei with condensed chromatin
• Cytoplasm filled with hydrophilic mucin granules - appear pale/empty in routine H&E (because mucin washes out during processing)
• Arranged in elongated mucous tubules rather than spherical acini
• Secrete mucins (glycoproteins) for lubrication

3. Myoepithelial Cells

• Located inside the basal lamina surrounding acini and proximal ducts
• Flattened, stellate cells with contractile actin-containing processes
• Contract to expel secretion into and through the duct system

---

The Salivon (Basic Secretory Unit)

1. Acinus (secretory unit)
2. Intercalated duct
3. Striated duct
4. Excretory duct

Intercalated Ducts

• Drain acini directly
• Lined by simple cuboidal epithelium
• Short, narrow, and difficult to identify in mucous glands
• Most prominent in serous glands (parotid) where they are long and narrow
• Contain occasional myoepithelial cells at their bases
• Absorb Cl⁻ from the acinar product

Striated Ducts (Intralobular)

• Lined by simple cuboidal-to-columnar epithelium
• Characteristic basal striations under light microscopy = longitudinal infoldings of the basal plasma membrane enclosing elongated mitochondria
• These folds are a morphologic adaptation for active electrolyte transport
• Function: reabsorb Na⁺ from the primary secretion; secrete K⁺ and HCO₃⁻ into the lumen
• Net result: saliva becomes hypotonic
• Diameter often exceeds that of the secretory acinus
• Most prominent in the parotid gland

Excretory Ducts (Interlobular/Interlobar)

• Travel in the interlobular connective tissue septa
• Small excretory ducts: simple cuboidal epithelium
• Progressively transition to pseudostratified columnar → stratified cuboidal → stratified columnar
• At the oral cavity opening: stratified squamous epithelium
• Parotid duct = Stensen's duct (opens opposite upper 2nd molar)
• Submandibular duct = Wharton's duct

---

Three Types of Acini and Serous Demilunes

• Serous acini: spherical, only serous cells, small central lumen
• Mucous acini: tubular, only mucous cells
• Mixed acini: contain both - in routine H&E preparations, mucous cells swell during fixation and push serous cells to the periphery, creating crescent-shaped "serous demilunes." This is now recognized as a fixation artifact - rapid-freeze electron microscopy shows both cell types actually line the same row around the lumen.

---

The Three Major Salivary Glands

---

Saliva: Composition and Function

• Water (99%)
• Enzymes: salivary amylase (ptyalin), lingual lipase
• Antibacterial proteins: lactoferrin, lysozyme, peroxidase
• Immunoglobulins: secretory IgA (produced by plasma cells in lamina propria, transported through ductal epithelium)
• Mucins: for lubrication
• Electrolytes: Na⁺, K⁺, Cl⁻, HCO₃⁻ (hypotonic relative to plasma)
• Buffering capacity: helps maintain near-neutral pH in the oral cavity

---

Clinical Correlates: Salivary Glands

Xerostomia (Dry Mouth)

• Mumps (parotitis): paramyxovirus infects parotid gland; bilateral parotid swelling
• Radiation therapy: destruction of acinar cells (particularly serous) from head/neck radiation
• Sjögren syndrome: autoimmune lymphocytic infiltration destroys acinar and ductal cells; bilateral parotid/lacrimal gland enlargement, dry mouth and dry eyes (keratoconjunctivitis sicca)
• Drugs: anticholinergics, antihistamines, tricyclic antidepressants

Sialorrhea (Excessive Salivation)

Salivary Gland Tumors

• Most common = pleomorphic adenoma (benign mixed tumor) - most frequent in the parotid gland; shows both epithelial and myxochondroid stromal components; locally recurrent if not completely excised
• Warthin tumor (papillary cystadenoma lymphomatosum): benign, almost exclusively parotid, bilateral in 10%; consists of cystic spaces lined by double-layered oncocytic epithelium within a lymphoid stroma
• Mucoepidermoid carcinoma: most common malignant salivary tumor
• Adenoid cystic carcinoma: characterized by perineural invasion; cribriform ("Swiss cheese") pattern on histology

Salivary Gland Aplasia

---

---

2. PANCREAS

General Organization

• Exocrine portion: ~99% of the gland; secretes digestive enzymes into the duodenum
• Endocrine portion: ~1%; the islets of Langerhans, secreting hormones into the blood

---

Exocrine Pancreas

Acinar Cells

• Pyramidal cells with round basal nuclei
• Strongly basophilic basal cytoplasm (abundant RER) - basophilia is most intense at the base
• Numerous apical zymogen granules (eosinophilic, acidophilic) containing inactive enzyme precursors (zymogens)
• Arranged around a very small central lumen
• No myoepithelial cells (unlike salivary glands)
• Secretes a wide array of enzymes:
  • Proteases (trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase) - secreted as zymogens activated in the duodenum by enterokinase
  • Lipase, colipase, phospholipase A2
  • Amylase (active as secreted)
  • Ribonuclease, deoxyribonuclease

Centroacinar Cells

• Unique to the pancreas - not found in salivary glands
• Small, pale, flattened cells that represent the initial cells of the intercalated duct system inserted back into the lumen of the acinus
• Light-staining cytoplasm and oval/flat nuclei
• Under the influence of secretin (from S cells of duodenum), centroacinar and intercalated duct cells produce a copious HCO₃⁻-rich fluid that:
  • Hydrates the enzymatic secretion
  • Alkalinizes it (neutralizes gastric acid entering duodenum)
  • Flushes enzymes into the duodenum

Duct System

---

Endocrine Pancreas - Islets of Langerhans

Cell Types in Islets

Regulation of Secretion

• CCK (cholecystokinin) - from I cells of duodenum/jejunum - stimulates enzyme secretion from acinar cells
• Secretin - from S cells of duodenum (triggered by acid) - stimulates HCO₃⁻-rich fluid from centroacinar/duct cells
• Autonomic (parasympathetic) fibers also stimulate secretion from both acinar and duct cells

---

Clinical Correlates: Pancreas

Acute Pancreatitis

• Premature activation of trypsinogen within the pancreas (rather than duodenum) leads to autodigestion
• Caused by: gallstones, alcohol, hyperlipidemia, medications, viral infections (mumps)
• Histology: acinar cell necrosis, fat necrosis, inflammatory infiltrate, hemorrhage (in severe cases)
• Clinical: severe epigastric pain, elevated serum lipase and amylase

Chronic Pancreatitis

• Repeated episodes of inflammation cause progressive replacement of acinar tissue by fibrosis (connective tissue)
• Islets may initially survive but eventually are lost
• Clinical: malabsorption (exocrine insufficiency), diabetes (endocrine insufficiency), steatorrhea

Type 1 Diabetes Mellitus

• Autoimmune insulitis: T-lymphocyte infiltration and destruction of β cells
• Remaining islets show selective loss of β cells while α, δ, and PP cells persist
• Islets may appear small and fibrosed

Type 2 Diabetes Mellitus

• Islet amyloid deposition (composed of islet amyloid polypeptide / amylin, co-secreted with insulin)
• Progressive β cell loss, with reduction in islet size and number
• Congo red staining and apple-green birefringence under polarized light

Pancreatic Adenocarcinoma

• 85% arise from ductal epithelium
• Histology: infiltrating glands with dense desmoplastic stroma
• Very aggressive; usually presents late
• Associated with KRAS mutation in >90% of cases

Insulinoma

• Most common islet cell tumor; benign in 90%
• β cell tumor producing excess insulin → hypoglycemia
• Whipple's triad: hypoglycemia symptoms, low blood glucose, relief with glucose

---

---

3. LIVER

General Organization

---

Structural Models of Liver Organization

1. Classic Hepatic Lobule (Hexagonal)

• The traditional structural unit
• Roughly hexagonal prism of liver tissue centered on a central vein (terminal hepatic venule)
• Corners of the hexagon are occupied by portal triads (portal areas), each containing:
  • Portal vein branch (thin-walled, large lumen)
  • Hepatic artery branch (thick-walled, smaller lumen)
  • Bile duct (lined by cholangiocytes - cuboidal/columnar epithelium)
  • Lymphatics
• Blood flows centripetally: from portal triads → hepatic sinusoids → central vein
• Bile flows centrifugally: from bile canaliculi → bile ductules → bile ducts in portal triad

2. Portal Lobule

• Triangular unit centered on a bile duct at its center
• Corners are three adjacent central veins
• Emphasizes the exocrine (biliary) function of the liver

3. Liver Acinus (of Rappaport)

• Oval/diamond-shaped functional unit
• Centered on a terminal portal venule and its associated hepatic arteriole
• Divided into three zones based on proximity to blood supply:
  • Zone 1 (periportal): Closest to portal triad; receives most oxygenated blood; most active in protein synthesis, gluconeogenesis, β-oxidation; first to be damaged by toxins, first to regenerate
  • Zone 2 (mid-zonal): Intermediate
  • Zone 3 (centrilobular / perivenular): Adjacent to central vein; lowest O₂; most active in glycolysis, lipogenesis, detoxification by CYP450 enzymes; most susceptible to ischemic damage and toxic injury (e.g., alcohol, acetaminophen)

---

Hepatocytes

Light Microscopy

• Large polygonal cells (20-30 μm diameter)
• Round, large nucleus with prominent nucleolus; ~25% are binucleate (reflects polyploidy and active metabolism)
• Eosinophilic cytoplasm due to abundant mitochondria
• Cytoplasm may contain glycogen (clear spaces, PAS-positive) and lipid droplets
• Arranged in hepatic plates (cords) - one to two cells thick - radiating from the central vein and anastomosing with each other

Ultrastructure

• Abundant RER (protein synthesis - plasma proteins)
• Abundant SER (lipid metabolism, drug detoxification by CYP450 enzymes)
• Numerous mitochondria (~800 per cell)
• Golgi apparatus (bile component processing)
• Lysosomes and peroxisomes (oxidative reactions, H₂O₂ metabolism)
• Glycogen granules (rosettes)
• Lipid droplets (storage)

Dual Polarity of Hepatocytes

1. Sinusoidal (basolateral) domain: faces the perisinusoidal space of Disse; exchanges substances with blood (nutrient uptake, protein secretion into blood)
2. Canalicular (apical) domain: two adjacent hepatocytes form the bile canaliculus by apposing their grooved apical surfaces, sealed by tight junctions; microvilli project into the canalicular lumen

---

The Sinusoidal System

Hepatic Sinusoids

• Wide, irregularly shaped vascular channels between hepatic plates
• Lined by fenestrated endothelial cells (no basement membrane - "discontinuous" endothelium)
• Fenestrations allow direct exchange of proteins, lipoproteins, and other substances between blood and hepatocytes
• Between the sinusoidal endothelium and hepatocytes lies the perisinusoidal space of Disse - a narrow fluid-filled space containing reticular fibers and stellate cells

Kupffer Cells

• Specialized resident macrophages embedded in the sinusoidal endothelium
• Largest population of fixed macrophages in the body
• Stellate, with phagocytic processes projecting into the sinusoidal lumen
• Functions:
  • Phagocytose effete (aged) erythrocytes, cellular debris, bacteria
  • Process antigens
  • Secrete cytokines (TNF, IL-1, IL-6)
• On India ink injection (experimental): appear as black stellate cells in sinusoids

Hepatic Stellate Cells (Ito Cells / Perisinusoidal Lipocytes)

• Located in the space of Disse
• Normally quiescent; store vitamin A (fat-soluble vitamin) in lipid droplets
• Small and difficult to identify in routine H&E
• When activated by liver injury → transform into myofibroblasts → secrete Type I collagen = the basis of hepatic fibrosis and cirrhosis

---

Bile Canaliculi and Intrahepatic Bile Ducts

• Bile canaliculi: intercellular channels formed between adjacent hepatocytes; sealed by tight junctions; 0.5-1 μm diameter but total length >1 km; microvilli project into lumen; bile flows centrifugally toward portal tracts
• Canal of Hering (bile ductules): transition zone between canaliculi and bile ducts; lined by a mix of hepatocytes and cholangiocytes; thought to contain hepatic progenitor cells
• Interlobular bile ducts: simple cuboidal-to-columnar cholangiocyte epithelium; travel with portal vein and hepatic artery branches in portal triads
• Left and right hepatic ducts: columnar epithelium

---

Functions of the Liver

1. Exocrine: bile synthesis and secretion (bile acids, bilirubin, phospholipids, cholesterol)
2. Endocrine/protein synthesis: albumin, fibrinogen, prothrombin, apolipoproteins, transferrin, complement proteins
3. Carbohydrate metabolism: glycogen synthesis/glycogenolysis, gluconeogenesis
4. Lipid metabolism: fatty acid oxidation, lipoprotein synthesis, ketogenesis
5. Detoxification: CYP450 enzymes oxidize drugs and toxins; conjugation reactions
6. Bilirubin processing: Kupffer cells phagocytose RBCs → heme → bilirubin → hepatocytes conjugate bilirubin with glucuronic acid → excreted in bile
7. Iron storage: ferritin and hemosiderin in hepatocytes and Kupffer cells
8. Vitamin storage: vitamin A (stellate cells), vitamins D, K, B12

---

Clinical Correlates: Liver

Jaundice (Icterus)

• Prehepatic (hemolytic): excess unconjugated bilirubin from RBC destruction; normal liver function
• Hepatocellular: damaged hepatocytes fail to conjugate/excrete bilirubin; both fractions elevated
• Posthepatic (obstructive): bile duct obstruction causes backpressure, bile regurgitates into blood; conjugated hyperbilirubinemia; pale stools, dark urine

Liver Cirrhosis

• End-stage result of chronic liver injury (alcohol, viral hepatitis, NASH, primary biliary cholangitis)
• Activated hepatic stellate cells produce excessive Type I collagen, replacing normal liver parenchyma with fibrotic bands
• Normal lobular architecture destroyed; formation of regenerative nodules surrounded by fibrous septa
• Consequences: portal hypertension, esophageal varices, ascites, hepatic encephalopathy, coagulopathy

Viral Hepatitis

• Hepatitis A, B, C, D, E viruses
• Histology: hepatocyte necrosis (ballooning degeneration), lobular disarray, lymphocytic infiltration, Councilman bodies (eosinophilic apoptotic hepatocytes)
• Hepatitis B/C chronic infection → cirrhosis → hepatocellular carcinoma

Non-Alcoholic Fatty Liver Disease (NAFLD/NASH)

• Steatosis: triglyceride accumulation in hepatocytes (macro/microvesicular fat droplets)
• NASH: steatosis + lobular inflammation + hepatocyte ballooning (± fibrosis)
• Zone 3 (centrilobular) predilection

Drug-Induced Liver Injury

• Acetaminophen (paracetamol) toxicity: CYP2E1 (highest in zone 3) converts excess acetaminophen to NAPQI, depleting glutathione → centrilobular (zone 3) necrosis; this is the classic histologic pattern

Hepatocellular Carcinoma (HCC)

• Most common primary liver cancer; arises on background of cirrhosis (80%)
• Associated with HBV, HCV, aflatoxin B1 exposure
• Histology: trabecular, pseudoglandular (acinar), or solid patterns; hepatocyte-like cells with large nuclei, prominent nucleoli; bile production may be seen

Wilson's Disease

• Autosomal recessive defect in ATP7B (copper transporter)
• Copper accumulates in hepatocytes → steatosis, inflammation, fibrosis; Kayser-Fleischer rings (corneal copper deposits)
• Rhodanine stain or orcein stain demonstrates copper deposits in hepatocytes

---

---

4. GALLBLADDER

General Organization

---

Layers of the Gallbladder Wall

1. Mucosa

• Epithelium: simple tall columnar epithelium (cholangiocytes)
  • Cells have prominent microvilli on their apical surface (for water absorption)
  • Abundant mitochondria (energy for active transport)
  • Distinct lateral intercellular spaces (indicative of active water transport)
  • Scattered apical secretory granules containing mucus (for protection)
  • No goblet cells in the main mucosa
• Lamina propria: thin layer of loose connective tissue with capillaries
• The mucosa forms prominent folds/rugae when the gallbladder is empty; these folds flatten when the organ is distended with bile
• No muscularis mucosae

2. Muscularis (Smooth Muscle Layer)

• Bundles of smooth muscle oriented in multiple directions (longitudinal, circular, oblique) - unlike the typical two-layer arrangement of the gut
• Functions to contract and expel bile into the cystic duct
• Relatively thin, not a clearly organized smooth muscle coat

3. Perimuscular Connective Tissue (Subserosal Layer)

• Loose connective tissue with vessels and nerves
• Contains Rokitansky-Aschoff sinuses: invaginations of the surface epithelium penetrating into the muscularis - these are considered features of chronic inflammation/cholecystitis but may be normal variants

4. Serosa/Adventitia

• The gallbladder has a serosa (visceral peritoneum) where it is exposed to the peritoneal cavity
• An adventitia where it contacts the liver

---

Bile Concentration Mechanism

1. Na⁺/K⁺-ATPase in basolateral membranes pumps Na⁺ into intercellular spaces
2. Cl⁻ and HCO₃⁻ follow (electroneutrality)
3. Water follows osmotically via transcellular and paracellular routes
4. Water is rapidly removed by subepithelial capillaries in the lamina propria
5. Bile becomes concentrated 5-10x, with cholesterol, bilirubin, bile acids, and other solutes retained

---

Bile Flow

• The biliary ducts are lined by cholangiocytes - cuboidal to low columnar cells - similar to gallbladder epithelium but without the prominent water-transporting specializations
• The sphincter of Oddi at the ampulla of Vater regulates bile flow into the duodenum
• CCK (from duodenal I cells, triggered by ingested fats) → contracts gallbladder muscularis + relaxes sphincter of Oddi → bile enters duodenum

---

No Submucosal Layer

---

Clinical Correlates: Gallbladder

Cholelithiasis (Gallstones)

1. Cholesterol stones (~80% in Western countries)
   • Supersaturation of cholesterol in bile (insufficient bile salts to keep it in solution) → nucleation and crystallization
   • Pure: white/yellow, often solitary, radiolucent
   • Mixed: contain bile pigment as well; more common
   • Risk factors: "5 Fs" - Female, Fat, Fertile (multiparous), Forty, Fair (Northern European)
2. Pigment stones (black and brown)
   • Black: chronic hemolysis (e.g., sickle cell disease, hereditary spherocytosis, thalassemia) → excess unconjugated bilirubin → calcium bilirubinate crystals; usually in gallbladder; radio-opaque
   • Brown: biliary infection/stasis → bacterial β-glucuronidase deconjugates bilirubin; often found in bile ducts (choledocholithiasis)

Cholecystitis

• Acute cholecystitis: gallstone impaction in the cystic duct → bile stasis → chemical/bacterial inflammation; histology: edema, neutrophilic infiltration, mucosal necrosis
• Chronic cholecystitis: repeated mild episodes → mucosal atrophy, fibrosis of muscularis, lymphocytic infiltration, Rokitansky-Aschoff sinuses (glandular herniations through the muscle wall - a hallmark feature)

Cholangitis (Primary Sclerosing / Primary Biliary)

• Primary sclerosing cholangitis (PSC): inflammation and fibrosis of bile ducts, often associated with ulcerative colitis; "onion skin" periductal fibrosis on histology; → biliary cirrhosis
• Primary biliary cholangitis (PBC): autoimmune destruction of small interlobular bile ducts; florid duct lesion - lymphocytic infiltration destroying bile duct epithelium; positive anti-mitochondrial antibodies (AMA)

Carcinoma of the Gallbladder

• Rare; most are adenocarcinomas arising from the columnar epithelium
• Strongly associated with chronic cholelithiasis and cholecystitis
• Carries a very poor prognosis (usually detected late)

Biliary Obstruction

• Causes: gallstones (choledocholithiasis), strictures, pancreatic carcinoma, cholangiocarcinoma
• Result: obstructive jaundice (conjugated hyperbilirubinemia), bile duct dilation (choledochectasis), biliary cirrhosis if chronic
• Bile duct proliferation (ductular reaction) is a histologic feature of biliary obstruction

---

Quick Comparison Summary

---