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Complete Histology of the Digestive System

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PART 1: ORAL CAVITY - OVERVIEW

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PART 2: THE LIPS (LABIA)

1. Internal Mucous Surface (Oral Mucosa)

• Lined by thick, non-keratinized stratified squamous epithelium (lining mucosa)
• Lamina propria underlies the epithelium - loose connective tissue with blood vessels and nerves
• Submucosa is present and contains numerous minor labial salivary glands (mucous and mixed in character), which secrete continuously keeping the surface moist

2. Vermilion Zone (Red Zone)

• The prominent red/pink zone visible on the exterior of the lip
• Covered by very thin, keratinized stratified squamous epithelium - this is the transition from oral mucosa to skin
• No salivary glands, sweat glands, or hair follicles - this is clinically important
• The connective tissue papillae are extremely tall and vascular, with a rich capillary network that imparts the characteristic pink/red color visible through the thin epithelium
• Richly innervated with sensory endings (explains the exquisite sensitivity of the lips)
• Moistened by the tongue licking saliva onto it - since there are no glands, it dries rapidly in arid conditions

3. External Skin Surface

• Covered by thin, keratinized stratified squamous epithelium typical of skin
• Contains hair follicles, sebaceous glands, and sweat glands

Clinical Correlate - Cheilitis & Herpes Labialis

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PART 3: THE CHEEKS (BUCCAL MUCOSA)

Histological Structure

• Inner surface lined by non-keratinized stratified squamous epithelium (lining mucosa) - thick, resilient, and flexible
• Lamina propria: loose connective tissue with blood vessels, lymphatics, sensory nerve endings, scattered lymphocytes and plasma cells
• Submucosa: dense connective tissue containing minor buccal salivary glands (mixed serous and mucous), diffuse lymphoid tissue, and adipose tissue
• Deep to the submucosa: buccinator muscle (striated), responsible for keeping food between the teeth during chewing
• Parotid (Stensen) duct opens into the vestibule of the mouth at a small elevation on the mucosal surface opposite the upper second molar tooth
• Throughout the epithelium: scattered antigen-presenting Langerhans cells and rich sensory innervation

Clinical Correlate - Aphthous Ulcers / Leukoplakia

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PART 4: THE PALATE

Hard Palate

• Fatty zone (anterior): contains adipose tissue
• Glandular zone (posterior): contains numerous mucous palatine salivary glands (minor glands)
• Along the midline raphe: no submucosa; epithelium attaches directly to periosteum

Soft Palate

Clinical Correlate - Cleft Palate

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PART 5: THE TONGUE

Core - Striated Muscle

• The body consists of interlacing bundles of striated muscle running in three perpendicular planes: horizontal, vertical, and longitudinal
• This orthogonal arrangement provides the tongue's remarkable mobility and flexibility
• Between the muscle bundles: fibroadipose connective tissue, blood vessels, nerve bundles (including chorda tympani and lingual nerve)

Surfaces of the Tongue

Inferior Surface

• Thin non-keratinized stratified squamous epithelium (lining mucosa)
• Smooth, no papillae
• Thin lamina propria

Dorsal Surface

• Covered by specialized mucosa - the most complex oral mucosa
• Divided by the sulcus terminalis (V-shaped groove) into:
  • Anterior two-thirds (oral/body): studded with papillae
  • Posterior one-third (pharyngeal/root): no papillae, has lingual tonsil (aggregated lymphoid nodules)

Lingual Papillae (Anterior 2/3)

1. Filiform Papillae (most numerous)

• Small, conical/thread-like projections
• Covered by keratinized stratified squamous epithelium at the tip
• No taste buds
• Function: give the tongue its rough texture, aid in food manipulation (rasping)
• Arranged in rows parallel to the sulcus terminalis

2. Fungiform Papillae

• Mushroom-shaped (broad top, narrow stalk)
• Scattered among filiform papillae, more numerous at the tip and margins
• Covered by non-keratinized or lightly keratinized stratified squamous epithelium
• Contain taste buds on their superior surface (dorsal surface)
• Appear as red dots visible to the naked eye due to rich vascular connective tissue core (reflecting through thin epithelium)

3. Circumvallate (Vallate) Papillae

• 8-12 large papillae arranged in a V-shaped row just anterior to the sulcus terminalis
• Each papilla is wide and flat-topped, surrounded by a deep circular moat (furrow/trench)
• The walls of the furrow are densely packed with taste buds (hundreds per papilla) - the highest density of taste buds in the mouth
• Von Ebner's serous glands open into the bottom of the furrow via ducts - their watery secretion flushes taste substances into contact with taste buds and washes them away to allow detection of new substances
• Covered by non-keratinized stratified squamous epithelium

4. Foliate Papillae

• Leaf-like folds (5-8 parallel ridges) on the lateral margins of the tongue posteriorly
• Taste buds present in the epithelium lining the folds
• Well-developed in children; often rudimentary in adults
• Minor salivary glands drain into the furrows between the folds

Taste Buds

• Oval-shaped neuroepithelial sensory organs - about 50-100 micrometers tall
• Found in: circumvallate papillae (walls of furrow), fungiform papillae (dorsal surface), foliate papillae, soft palate, epiglottis
• Total number in humans: ~10,000 taste buds
• Open to the surface via a taste pore - a small apical opening filled with microvilli
• Cell types within a taste bud:
  • Type I (dark/supporting cells): Most numerous; elongated cells with dark nuclei; wrap around other cells; secrete a glycoprotein matrix
  • Type II (light/receptor cells): Express taste receptor proteins (GPCRs) on microvilli for sweet, bitter, and umami detection; do NOT form synapses directly
  • Type III (intermediate cells): Spindle-shaped; form conventional synapses with afferent nerve fibers; detect sour (acidic) stimuli via proton channels
  • Type IV (basal cells): Small, round; located at base; are stem cells that give rise to the other types; taste bud cells turn over every 10-14 days
• Salty and sour detected via ion channels; sweet, bitter, umami detected via GPCRs (T1R and T2R receptor families)
• Innervation: Anterior 2/3 by chorda tympani (VII); posterior 1/3 by glossopharyngeal (IX); epiglottis by vagus (X)

Clinical Correlate - Taste Disorders & Genetic Basis

Posterior 1/3 of Tongue (Lingual Tonsil)

• No papillae
• Covered by non-keratinized stratified squamous epithelium
• Mucosa has irregular crypts (invaginations into the connective tissue) surrounded by lymphoid nodules (forming the lingual tonsil)
• Mucous glands (posterior lingual glands) in the submucosa open into the crypts

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PART 6: TEETH

1. ENAMEL

• The hardest substance in the body - 96-98% inorganic (hydroxyapatite crystals), 1-2% organic (mostly enamelins and amelogenins)
• Produced by ameloblasts (from oral ectoderm) during tooth development; once the tooth erupts, ameloblasts are lost - enamel cannot be regenerated
• Structure: Enamel is organized into enamel rods (prisms) - tightly packed, keyhole-shaped cylindrical structures running from the dentinoenamel junction to the surface
• Each enamel rod is a column of tightly packed hydroxyapatite crystals (30 nm in diameter) with their long axes parallel to the rod
• Incremental lines of Retzius: Brown striae visible in ground sections - represent periodic rhythmic appositional growth of enamel during development
• Neonatal line: A pronounced line of Retzius marking the stress of birth (present in primary and some permanent teeth)
• Hunter-Schreger bands: Alternating light and dark bands in longitudinal sections; represent alternating directions of enamel rod orientation - provides resistance to shear
• Enamel spindles: Short, club-shaped processes of odontoblasts that cross the dentinoenamel junction into enamel
• Enamel tufts: Defective, hypocalcified groups of enamel rods at the dentinoenamel junction - appear as grass-like tufts on cross section
• Enamel lamellae: Thin sheet-like faults running from the surface inward, partially filled with organic material - potential pathways for bacterial invasion

2. DENTIN

• Forms the bulk of the tooth - 70% inorganic (hydroxyapatite), 20% organic (mostly type I collagen), 10% water
• Produced by odontoblasts (from neural crest-derived mesenchyme) - these cells persist throughout life at the inner surface of dentin adjacent to the pulp
• Dentin is traversed by dentinal tubules running from the pulp cavity to the periphery (dentinoenamel junction at crown, dentinocemental junction at root)
• Each dentinal tubule contains an odontoblastic process (cytoplasmic extension of an odontoblast) - these processes are the basis of dentinal sensitivity
• Peritubular dentin: Highly mineralized dentin immediately surrounding each tubule - forms a cuff
• Intertubular dentin: Less mineralized dentin between the tubules; contains type I collagen fibers
• Incremental lines (von Ebner lines): Represent periodic growth cycles - visible in ground sections as fine lines perpendicular to tubules
• Owen contour lines: Accentuated lines of von Ebner, correspond to disturbance lines in dentin matrix formation
• Interglobular dentin: Areas of incompletely mineralized dentin between calcification globules - seen near dentinoenamel junction; prominent in vitamin D deficiency (rickets)
• Tomes granular layer: A thin zone of granular appearance at the periphery of root dentin

3. CEMENTUM

• Calcified connective tissue covering the root of the tooth (analogous to bone)
• ~65% inorganic (hydroxyapatite), ~35% organic (mostly type I collagen)
• Acellular (primary) cementum: Located in the coronal portion of the root; formed first; no cells embedded in matrix; Sharpey's fibers (periodontal ligament fiber ends) are anchored in it
• Cellular (secondary) cementum: Located at the apical portion of the root; contains cementocytes in lacunae within the matrix; formed throughout life; can repair root surface damage
• Produced by cementoblasts (from periodontal ligament)
• Unlike bone, cementum has no blood vessels (avascular) and undergoes minimal remodeling

4. DENTAL PULP

• Occupies the central pulp cavity (pulp chamber in crown + root canals)
• Loose connective tissue - gelatinous matrix rich in ground substance
• Contains: odontoblasts (at periphery, lining the walls), fibroblasts (most abundant cell), undifferentiated mesenchymal stem cells, macrophages, lymphocytes
• Nerve fibers: Myelinated and unmyelinated fibers enter via the apical foramen; unmyelinated fibers accompany odontoblastic processes into dentinal tubules (basis of dentin hypersensitivity)
• Blood vessels: Arterioles, venules, and capillaries enter via the apical foramen
• Ground substance: Rich in glycosaminoglycans (particularly hyaluronic acid and chondroitin sulfate)
• Pulp stones (denticles): Calcified deposits within the pulp - may be free-floating or attached to the wall

5. SUPPORTING TISSUES OF THE TEETH

Periodontal Ligament (PDL)

• Dense fibrous connective tissue occupying the periodontal space between the root cementum and alveolar bone
• Width: ~0.2 mm (narrows with age and disuse)
• Composed of principal fiber bundles (type I collagen, with some type III):
  • Alveolar crest fibers: Most coronal; prevent tooth from being pulled out
  • Horizontal fibers: Resist lateral forces
  • Oblique fibers: Most numerous; run obliquely from cementum (apical) to alveolar bone (coronal); resist apical forces (main weight-bearing fibers)
  • Apical fibers: Around apex; prevent tipping
  • Interradicular fibers: Between roots of multirooted teeth
• Sharpey's fibers: Collagen bundle ends embedded in both cementum and alveolar bone
• Cells: Fibroblasts (predominant), cementoblasts, osteoblasts, osteoclasts, undifferentiated mesenchymal cells, rests of Malassez (epithelial remnants of Hertwig root sheath - may give rise to cysts), macrophages, mast cells
• Oxytalan fibers: Thin, elastic-like fibers running parallel to long axis of root; associated with blood vessel adventitia
• Blood vessels and nerve endings: Rich neurovascular supply - Meissner corpuscles and Ruffini endings for proprioception (important for control of bite force)

Alveolar Bone (Alveolar Process)

• Specialized bone forming the sockets (alveoli) that house the teeth
• Bundle bone (alveolar bone proper): The inner socket wall into which Sharpey's fibers insert; highly compact; lamellated; lines the socket
• Supporting alveolar bone: Cancellous and cortical bone surrounding the bundle bone

Clinical Correlate - Dental Caries

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PART 7: GUMS (GINGIVA)

Structure

• Keratinized or parakeratinized stratified squamous epithelium
• Lamina propria: thick papillary layer of dense connective tissue with collagen fiber bundles (gingival fibers) that radiate from the alveolar crest and cementum to maintain gingival attachment
• No submucosa - the lamina propria attaches directly to the periosteum of alveolar bone = mucoperiosteum
• Dentogingivial fibers (transseptal and alveologingival fibers): anchor the free gingiva to the tooth and alveolar bone

• Thin, non-keratinized epithelium that adheres directly to the enamel or cementum surface
• Secretes a basal lamina-like material on its inner surface (facing the tooth)
• Cells attach to this material via hemidesmosomes - this is called the epithelial attachment
• In young individuals: attachment is to enamel; in older individuals with gingival recession: attachment migrates onto cementum
• Extremely permeable - allows passage of gingival fluid (crevicular fluid) into the sulcus

• A shallow groove (normally ≤2 mm deep) between the free gingiva and the tooth
• Lined by crevicular epithelium (non-keratinized stratified squamous)
• The crevicular fluid contains immunoglobulins, complement, and neutrophils - first line of defense

Clinical Correlate - Periodontitis & Stem Cell Therapy

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PART 8: OROPHARYNX

Epithelium

• Non-keratinized stratified squamous epithelium (lining mucosa) - continuous with buccal and soft palate mucosa
• Reflects the functional need for flexibility and resistance to abrasion as food passes through

Lamina Propria

• Loose connective tissue with numerous lymphoid nodules and diffuse lymphoid tissue
• IgA-secreting plasma cells are abundant - the epithelial cells above them express the polymeric immunoglobulin receptor (pIgR), which binds IgA dimers and performs transcytosis into the lumen, releasing secretory IgA (sIgA) - the major immunological defense of mucosal surfaces

Submucosa

• Dense connective tissue with minor mucous salivary glands that continuously secrete to keep the surface moist
• Rich lymphoid tissue

Waldeyer's Tonsillar Ring

• Palatine tonsils: Located at the entrance to the oropharynx between the palatopharyngeal and palatoglossal arches
• Pharyngeal tonsil (adenoid): On the roof of the nasopharynx
• Tubal tonsils: Lateral nasopharynx near the auditory tube opening
• Lingual tonsil: Base of the tongue

Palatine Tonsils - Detailed Histology

• Surface epithelium: Non-keratinized stratified squamous
• Deep invaginations called crypts penetrate into the lymphoid tissue - provide huge surface area for antigen sampling
• Lymphoid follicles: Secondary follicles with germinal centers (B-cell zones) in the lamina propria and filling the core of the tonsil
• T lymphocytes in the interfollicular zones
• Reticular epithelium: Specialized epithelium lining the crypts with M-cell-like properties for antigen uptake
• Capsule of dense fibrous connective tissue on the external aspect
• No afferent lymphatics - antigen sampling occurs directly across the crypt epithelium

Clinical Correlate - Tonsillitis, Peritonsillar Abscess, Adenoid Hypertrophy

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PART 9: GENERAL STRUCTURE PLAN OF THE ALIMENTARY CANAL

Layer 1: MUCOSA (Innermost)

• Type varies by region (discussed per organ below)
• Rests on a basement membrane
• Functions: protection, secretion, absorption

• Loose connective tissue immediately beneath the epithelium
• Contains: capillaries, lymphatics (lacteals in small intestine), nerve fibers, plasma cells (especially IgA-secreting), lymphocytes, eosinophils, mast cells
• May contain mucosal glands (crypts of Lieberkühn in intestines, gastric glands in stomach)
• Contains MALT (mucosa-associated lymphoid tissue) - lymphoid nodules and diffuse lymphoid cells
• Secretory IgA is the dominant immunoglobulin of the gut lumen

• A thin layer of smooth muscle (usually 2 sub-layers: inner circular + outer longitudinal)
• Separates mucosa from submucosa
• Permits local independent movements of the mucosa (independent of outer muscularis externa) - agitates the mucosal surface to enhance mixing and absorption

Layer 2: SUBMUCOSA

• Dense irregular connective tissue
• Contains: larger blood vessels (submucosal arterial plexus), lymphatics, adipose cells
• The submucosal (Meissner's) plexus of autonomic nerves resides here (see ENS below)
• Submucosal glands: Present only in the esophagus (esophageal glands proper) and duodenum (Brunner's glands) - nowhere else
• Lymphoid nodules (especially in ileum - Peyer's patches extend from mucosa into submucosa)

Layer 3: MUSCULARIS EXTERNA (Muscularis Propria)

• Thick layer of smooth muscle in two distinct orientations:
  • Inner circular layer: Muscle fibers run circumferentially - when it contracts, it narrows the lumen
  • Outer longitudinal layer: Muscle fibers run along the length - when it contracts, it shortens the tube
• These two layers work together to generate peristalsis (coordinated propulsive waves) and segmentation (mixing contractions)
• Exceptions to smooth muscle rule:
  • Esophagus (upper 1/3): striated (voluntary) muscle
  • Esophagus (middle 1/3): mixed striated + smooth
  • Esophagus (lower 1/3): smooth muscle only
  • External anal sphincter: striated (voluntary)
• Stomach has an additional oblique (inner) layer - giving it 3 muscle layers
• The myenteric (Auerbach's) plexus lies between the inner circular and outer longitudinal layers

Layer 4: SEROSA / ADVENTITIA (Outermost)

• In regions suspended within the peritoneal cavity (stomach, small intestine, most of large intestine)
• Thin layer of loose connective tissue + simple squamous mesothelium (visceral peritoneum)
• Rich in blood vessels, lymphatics, adipose cells
• The serosa is continuous with the mesentery, which suspends the intestines in the abdominal cavity and transmits nerves and vessels

• In regions attached to surrounding structures (esophagus, retroperitoneal portions of GI tract)
• Dense connective tissue without a mesothelial covering
• Blends with surrounding tissues

Regional Specializations Summary Table

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PART 10: ENTERIC NERVOUS SYSTEM (ENS)

Submucosal (Meissner's) Plexus

• Located in the submucosa, between the muscularis mucosae and muscularis externa
• Functions: Regulates secretion by mucosal glands, controls local blood flow, coordinates absorptive activities
• Neurons are smaller and less densely packed than in the myenteric plexus
• Contains sensory, interneurons, and secretomotor neurons
• Postganglionic parasympathetic and local enteric neurons

Myenteric (Auerbach's) Plexus

• Located between the inner circular and outer longitudinal smooth muscle layers of the muscularis externa
• Functions: Generates and coordinates contractile activity of the muscularis (peristalsis + segmentation); determines the direction and speed of peristaltic waves
• Contains ganglia (aggregations of neuron cell bodies) interconnected by nerve fiber bundles running in all directions
• Contains all three neuron types: sensory (IPAN - intrinsic primary afferent neurons), interneurons, and motor neurons

Neurotransmitters of the ENS

• Acetylcholine (ACh): Stimulates muscle contraction (excitatory motor neurons), stimulates secretion
• Vasoactive intestinal peptide (VIP): Inhibits smooth muscle - key inhibitory neurotransmitter of the gut
• Substance P: Excitatory; stimulates contraction
• Nitric oxide (NO): Key inhibitory neurotransmitter; mediates relaxation of circular muscle ahead of a peristaltic wave
• Serotonin (5-HT): Initiates peristaltic reflex; stimulates secretion - produced by enterochromaffin cells in the mucosa
• CGRP, enkephalins, somatostatin, neuropeptide Y: Various regulatory functions

Peristaltic Reflex (Law of the Intestine)

• Distension of a segment activates IPANs
• Above the bolus: ACh/Substance P released → circular muscle contracts, longitudinal muscle relaxes → propulsive wave moves aborally
• Below the bolus: VIP/NO released → circular muscle relaxes, longitudinal muscle contracts → accommodation

Clinical Correlates

• Failure of neural crest cell migration to distal colon results in an aganglionic segment (absent myenteric and submucosal plexuses)
• The affected segment cannot relax → functionally obstructed, causing massive proximal dilation (megacolon)
• Most commonly affects the rectosigmoid region
• Diagnosis: rectal suction biopsy showing absence of ganglion cells + increased acetylcholinesterase staining in nerve fibers
• Treatment: surgical resection of the aganglionic segment

• Trypanosoma cruzi infection destroys the ganglion cells of the ENS
• Results in chagasic megaesophagus (achalasia-like) and chagasic megacolon
• Pathology: chronic inflammation with lymphocytic infiltration → ganglion cell destruction → denervation dilation

• Alterations in ENS serotonin signaling (altered 5-HT3 and 5-HT4 receptor activity)
• Abnormal visceral sensation and gut motility without structural damage
• Junqueira, p.734 | Wheater's, p.1520

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PART 11: ESOPHAGUS

Overview

Layer-by-Layer Histology

Mucosa

• Upper esophagus (near pharynx)
• Lower esophagus (near stomach/cardia) These are simple tubular or branched glands producing neutral mucus to protect the epithelium.

Submucosa

• Dense connective tissue
• Contains esophageal glands proper (the TRUE submucosal glands of the esophagus) - compound tubuloacinar mucous glands that open via long ducts through the mucosa into the lumen. These glands secrete acidic mucus for lubrication.
• Submucosal plexus (Meissner's)
• Lymphoid nodules clustered near gland ducts

Muscularis Externa

• Upper 1/3 (proximal): Skeletal (striated) muscle - under voluntary control; involved in the initiation of swallowing
• Middle 1/3: Mix of skeletal and smooth muscle
• Lower 1/3 (distal): Smooth muscle only - controlled by the ENS and ANS

• Inner circular layer (prominent)
• Outer longitudinal layer
• Myenteric (Auerbach's) plexus between the two layers

Adventitia (NOT Serosa)

• The esophagus does NOT have a serosa because it is not suspended in the peritoneal cavity
• Has only an adventitia - loose connective tissue blending with surrounding structures

Esophagogastric Junction (Z-line/Ora Serrata)

• Abrupt transition from stratified squamous epithelium of esophagus to simple columnar mucous epithelium of stomach
• Visible endoscopically as a distinct Z-line (zigzag line) located at the level of the LES
• This transition is normally at or below the diaphragmatic hiatus

Clinical Correlates

• Chronic reflux of acidic gastric contents damages the squamous epithelium of the lower esophagus
• The squamous epithelium undergoes metaplasia - replaced by intestinal-type columnar epithelium (with goblet cells) = Barrett's esophagus
• Barrett's is a premalignant condition - risk of adenocarcinoma is 30-125x higher than in the general population
• Histologically: intestinal metaplasia with goblet cells (PAS/Alcian blue positive); biopsy graded for dysplasia
• Treatment: proton pump inhibitors, surveillance endoscopy, ablative therapy

• Degeneration of inhibitory neurons (VIP/NO-containing) of the myenteric plexus in the lower esophagus
• Results in failure of LES relaxation and loss of peristalsis
• Manometry shows high LES pressure and aperistalsis
• Endoscopy: bird-beak appearance; biopsy shows reduced/absent myenteric ganglion cells

• Portal hypertension causes dilation of the submucosal venous plexus of the lower esophagus
• Thin-walled varices are prone to catastrophic hemorrhage
• Wheater's, p.1527

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PART 12: STOMACH

General Features of the Gastric Mucosa

• Lined by simple columnar mucous epithelium (surface mucous cells)
• No villi in the stomach (unlike the small intestine)
• Surface studded with small depressions called gastric pits (foveolae) - openings of the gastric glands
• The ratio of pit depth to gland depth varies by region (short pits in fundus/body, deep pits in pylorus)
• The entire mucosal surface is covered by an alkaline mucus layer (produced by surface mucous cells) that protects against HCl

Gastric Mucosal Zones

A. Cardia

• Narrow ring (0.5-3 cm wide) at the esophagogastric junction
• Gastric pits: Short
• Glands: Branched tubular mucous glands (cardiac glands) - primarily mucus-secreting; simple branched tubular
• Contains scattered enteroendocrine cells (G cells, ECL cells)
• The lamina propria shows mucous glands similar to the cardiac glands of the lower esophagus
• Transition zone - may contain glands of mixed esophageal and gastric type

B. Fundus and Body (Corpus) - Oxyntic Mucosa

• Neck (upper, isthmus): mitotically active zone containing mucous neck cells and stem cells
• Body/middle: Contains the main secretory cells
• Base: Richest in chief cells

• Line the pit (foveolae) and surface
• Tall columnar cells with pale, foamy supranuclear cytoplasm (due to mucus-filled vesicles)
• Secrete neutral mucopolysaccharide (mucin) - forms the protective mucus layer
• Nuclei: basal, flattened
• High rate of turnover: replaced every 3-5 days from stem cells in the neck/isthmus

• Located in the neck of the glands
• Short, irregular cells with irregular nuclei
• Secrete acidic (sulfomucin) mucus - different from surface mucous cells
• Contains few small granules
• Represent progenitor/transit amplifying cells - they divide and migrate up (to become surface mucous cells) or down (to become chief or parietal cells)

• Large, pyramidal cells - most characteristic cell of the gastric body
• Nuclei: central, round, prominent (binucleate in some)
• Cytoplasm: intensely eosinophilic (PAS-negative) due to enormous numbers of mitochondria (which provide energy for active proton pumping)
• Ultrastructure: Deep intracellular canaliculi (branched invaginations of apical plasma membrane) lined with microvilli greatly increase secretory surface area; tubulovesicular system (cytoplasmic membrane vesicles carrying H+/K+-ATPase in the resting state)
• H+/K+-ATPase (the proton pump): Pumps H+ into the canaliculus in exchange for K+; coupled with Cl- channels to produce HCl
• Also secrete intrinsic factor (IF) - a glycoprotein essential for vitamin B12 absorption in the ileum
• Located in the neck and body of the gland; bulge toward the lumen and toward the basement membrane

• Predominate in the base of fundic glands
• Basophilic cytoplasm (abundant rER in the basal cytoplasm for protein synthesis)
• Apical cytoplasm full of zymogen granules - eosinophilic in well-fixed tissue, but often appear empty in routine sections due to extraction of loosely bound pepsinogen
• Secrete pepsinogen I and II (precursors of pepsin) - activated by HCl in the lumen
• Also secrete gastric lipase and small amounts of rennin (chymosin - curdles milk)

• Scattered throughout the glands; most numerous at the base
• Small, clear cells with basal granules - "clear cells" or "D cells," "G cells" etc.
• Part of the diffuse neuroendocrine system (DNES) - also called APUD (Amine Precursor Uptake and Decarboxylation) cells
• Types in the stomach:
  • ECL cells (Enterochromaffin-Like): In body/fundus; secrete histamine in response to gastrin and vagal stimulation; histamine binds H2 receptors on parietal cells to stimulate HCl secretion
  • G cells: In the antrum (and duodenum); secrete gastrin in response to protein digestion products, distension, and vagal stimulation → stimulates ECL cells and parietal cells
  • D cells: In fundus and antrum; secrete somatostatin → inhibits gastrin release and acid secretion (paracrine feedback)
  • A cells (X/A cells): Secrete ghrelin ("hunger hormone") - stimulates appetite and GH release
  • Enterochromaffin (EC) cells: Secrete serotonin (5-HT) - stimulates peristalsis

• Located in the isthmus/neck region
• Source of all other gastric mucosal cells

C. Pyloric Antrum / Pylorus

• Gastric pits: Deep (occupy top 50-70% of mucosal thickness)
• Glands: Pyloric glands - branched, coiled tubular glands; shorter than fundic glands; mucous-secreting (acidic mucin); some contain G cells
• No parietal cells, no chief cells in the pyloric glands
• The pyloric sphincter: Thickened inner circular smooth muscle layer separating stomach from duodenum
• Pyloric mucosa transitions into duodenal mucosa at the pyloric valve

Submucosa, Muscularis, and Serosa

• Inner oblique layer (innermost) - aids in churning
• Middle circular layer (forms the pyloric sphincter)
• Outer longitudinal layer

Clinical Correlates

• Autoimmune destruction of parietal cells → loss of HCl + loss of intrinsic factor
• Without IF, vitamin B12 cannot be absorbed in the ileum → megaloblastic anemia
• Histology: atrophic gastritis with intestinal metaplasia of the gastric body; absence of parietal cells; lymphocytic infiltrate; compensatory hypergastrinemia
• Risk factor for gastric carcinoid (from ECL cell hyperplasia due to high gastrin)

• Mucosal defense fails (mucus layer, bicarbonate secretion, mucosal blood flow)
• Major cause: H. pylori infection - gram-negative spiral bacterium that colonizes the gastric antrum, induces chronic gastritis, increases gastrin production and HCl secretion
• Antral (H. pylori) ulcers: Associated with hypergastrinemia and hypersecretion; duodenal ulcers more common
• Gastric ulcers: Mucosal defense failure; often associated with NSAID use (inhibit COX → ↓ prostaglandins → ↓ mucus + bicarbonate secretion)
• Histology: ulcer base shows necrosis, granulation tissue, fibrosis; edges may show intestinal metaplasia

• Gastrinoma - neuroendocrine tumor (usually in pancreas/duodenum) secreting massive amounts of gastrin
• → Extreme parietal cell hyperplasia and gastric acid hypersecretion
• → Multiple refractory peptic ulcers, often in unusual locations
• Histology: fundic mucosa shows parietal cell hyperplasia; no H. pylori

• Intestinal type (associated with H. pylori): chronic gastritis → atrophy → intestinal metaplasia → dysplasia → carcinoma; gland-forming
• Diffuse type: Poorly cohesive cells (loss of E-cadherin); "signet ring cells" (mucin-filled cells pushing nucleus to periphery); linitis plastica pattern with desmoplasia
• Wheater's, p.1576 | Junqueira, p.755

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PART 13: SMALL INTESTINE

Surface Area Amplification - Three Levels

Mucosa - Unique Features

Plicae Circulares (Circular Folds / Valves of Kerckring)

• Permanent, circular or helical folds of both mucosa AND submucosa
• They do NOT disappear when the intestine is distended (unlike gastric rugae)
• Most prominent in the jejunum and proximal ileum; sparse in the duodenum proximally and ileum distally

Villi

• Finger-like or leaf-like projections of mucosa (epithelium + lamina propria core) extending 0.5-1.5 mm into the lumen
• Each villus has a central lacteal (blind-ending lymphatic capillary) surrounded by a network of capillaries and a supporting core of smooth muscle fibers from the muscularis mucosae (causes villi to contract and pump lymph)
• Shape varies by region:
  • Duodenum: Broad, leaf-shaped
  • Jejunum: Finger-shaped (tallest)
  • Ileum: Shorter, more finger-like

Crypts of Lieberkühn (Intestinal Glands)

• Straight tubular glands that extend from the base of villi down to the muscularis mucosae
• Open at the base of each villus
• Site of cell proliferation and renewal
• Ratio villus:crypt = approximately 5:1 in the jejunum

Cell Types of the Small Intestinal Epithelium

• Tall columnar cells with oval basal nuclei
• Apical brush border: Dense layer of microvilli (1-2 µm tall) coated with glycocalyx (heavily glycosylated integral membrane proteins)
• The glycocalyx contains key brush border enzymes anchored into the microvillus membrane:
  • Peptidases (enterokinase/enteropeptidase, aminopeptidases, dipeptidases)
  • Disaccharidases (lactase, sucrase-isomaltase, maltase)
  • Alkaline phosphatase
• Lateral membranes connected by tight junctions (zonula occludens), adherens junctions, and desmosomes - the junctional complex
• Intracellular digestion: Fatty acids and glycerol absorbed into SER → reassembled into triglycerides → packaged with apoprotein → chylomicrons → exocytosed into the lacteal
• Rapid turnover: replaced every 3-5 days (fastest cell renewal in the body)

• Scattered among enterocytes; increase in number distally (most in ileum > jejunum > duodenum)
• Classic goblet shape: narrow base, distended apical portion filled with mucus granules
• Secrete mucin (MUC2 predominantly) forming a protective gel layer over the epithelium
• In H&E sections: apical mucus appears pale/clear

• Located exclusively at the base of the crypts
• Conspicuous large, brightly eosinophilic, pyramidal apical granules - the most dramatic secretory granules in the GI tract
• Secrete:
  • Defensins (cryptdins) - antimicrobial peptides active against bacteria, fungi, some viruses
  • Lysozyme - cleaves bacterial cell wall peptidoglycan
  • Phospholipase A2 - antimicrobial
  • Tumor necrosis factor-α (TNF-α)
• Function: regulate the composition of the intestinal microbiome; maintain the intestinal stem cell niche by producing Wnt3 and Notch ligands
• Long-lived cells (~20 days)

• Scattered throughout the crypts; part of the DNES
• Key types in the small intestine:
  • S cells (duodenum): Secrete secretin in response to acid → stimulates pancreatic bicarbonate secretion
  • I cells (duodenum/jejunum): Secrete cholecystokinin (CCK) in response to fats and proteins → stimulates pancreatic enzyme secretion + gallbladder contraction
  • K cells: Secrete GIP (glucose-dependent insulinotropic peptide/gastric inhibitory peptide) → stimulates insulin release
  • L cells (ileum): Secrete GLP-1 (glucagon-like peptide-1) - major incretin; PYY - inhibits appetite
  • EC cells: Secrete serotonin - promotes peristalsis
  • Mo cells: Secrete motilin - stimulates MMC (migrating motor complex)

• Located in the follicle-associated epithelium (FAE) overlying Peyer's patches
• Lack well-developed brush border; instead have irregular microfolds
• Specialized for transcytosis of antigens from the intestinal lumen to underlying lymphoid tissue
• Central to intestinal immune surveillance - present antigens to underlying dendritic cells, B and T cells in Peyer's patches
• After antigen exposure, B cells leave the Peyer's patches via lymphatics → travel to mesenteric lymph nodes → enter circulation → home back to the intestinal lamina propria as IgA-secreting plasma cells

• Located at the base of the crypts between Paneth cells (position 1-4 from crypt base = Lgr5+ crypt base columnar cells)
• Express Lgr5 (leucine-rich repeat-containing G protein-coupled receptor 5)
• Divide rapidly; generate transit amplifying cells that migrate upward
• Regulated by Wnt signaling (Paneth cells provide Wnt3), Notch signaling, and BMP signaling (from subepithelial myofibroblasts)

Regional Differences Along the Small Intestine

• Located in the submucosa of the duodenum (proximal more than distal)
• Branched tubuloacinar compound mucous glands
• Secrete: alkaline (bicarbonate-rich) viscous mucus that neutralizes gastric acid entering the duodenum; also secrete urogastrone (EGF) which promotes mucosal proliferation
• Ducts pass through the muscularis mucosae to open into the base of the crypts

• Aggregated lymphoid nodules in the lamina propria and submucosa of the ileum
• Each patch contains 10-200 follicles; follicles have germinal centers (B-cell zones) surrounded by T-cell zones
• The overlying epithelium is the follicle-associated epithelium with abundant M cells
• Major site of gut-associated immune responses; initiates mucosal IgA responses

Submucosa of the Small Intestine

• Dense connective tissue
• Brunner's glands (duodenum only)
• Meissner's plexus
• Blood vessels and lymphatics

Muscularis Externa

• Inner circular + outer longitudinal smooth muscle layers
• Auerbach's myenteric plexus between the layers
• Generates both peristalsis and segmentation (non-propulsive mixing contractions)

Clinical Correlates

• Autoimmune reaction to gliadin (component of gluten) → T-cell mediated injury to small intestinal mucosa
• Histological changes (Marsh classification):
  • Type 0: Normal
  • Type 1: Increased intraepithelial lymphocytes (>25 per 100 enterocytes)
  • Type 2: + crypt hyperplasia
  • Type 3: + villous atrophy (subtotal to total)
  • Type 3c: Total villous atrophy + crypt hyperplasia - mucosa appears flat
• Loss of brush border enzymes → malabsorption of fats, carbohydrates, proteins, vitamins A, D, E, K, B12, folate, iron, calcium
• Anti-tissue transglutaminase (anti-tTG) antibodies are the serological marker
• Risk of small intestinal lymphoma (EATL) if untreated

• Transmural granulomatous inflammation affecting any part of the GI tract but most commonly the terminal ileum and proximal colon
• Histology: non-caseating granulomas (Langerhans giant cells), transmural lymphoid aggregates, skip lesions, fissuring ulcers, submucosal fibrosis, neural hyperplasia
• Complications: strictures, fistulas, abscess, malabsorption

• Arise from enteroendocrine cells (EC cells); most common in ileum
• Histology: nests of uniform round cells with granular cytoplasm; chromogranin A positive; synaptophysin positive
• Carcinoid syndrome: serotonin hypersecretion causing flushing, diarrhea, bronchoconstriction, right heart valve disease (hepatic metastases required for systemic syndrome)

• Reduced expression of lactase (brush border disaccharidase) - most common enzyme deficiency worldwide (especially in East Asian, African, Indigenous populations)
• Undigested lactose passes to colon → bacterial fermentation → osmotic diarrhea, bloating, flatulence
• Wheater's, p.976 | Junqueira, p.776

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PART 14: LARGE INTESTINE

Key Distinguishing Features vs. Small Intestine

Mucosa of the Colon

• Long, straight, test tube-shaped crypts (longer than in the small intestine)
• Open directly onto the flat surface (no villi)
• Cells: Absorptive cells (surface), goblet cells (throughout), enteroendocrine cells, stem cells (base)
• No Paneth cells (or very few at base in the proximal colon only)
• Stem cells at crypt base express Lgr5+

Lamina Propria of the Colon

• Loose connective tissue between the crypts
• Rich in plasma cells (predominantly IgA-secreting), lymphocytes, eosinophils, macrophages, mast cells
• Lymphoid nodules - scattered throughout but most prominent in the cecum and appendix
• The entire lamina propria forms the innate immune barrier between the microbiome and systemic circulation

Muscularis Mucosae

• Two thin layers of smooth muscle (inner circular + outer longitudinal) as elsewhere

Submucosa

• Dense connective tissue
• Meissner's plexus
• No submucosal glands
• Abundant lymphoid nodules (extending from lamina propria)
• Fat cells may be present

Muscularis Externa

Serosa / Appendices Epiploicae

• The colon has a serosa (mesothelium-covered) where it is suspended by mesentery
• Appendices epiploicae: Small fat-filled pendulous projections of the serosa; prominent in the transverse and sigmoid colon

Regional Differences Within the Large Intestine

Cecum

• Relatively wide caliber, blind-ending pouch
• Histology similar to the rest of the colon
• Contains many lymphoid follicles

Vermiform Appendix

• A narrow tube with a very small, irregular lumen (often partially or completely obliterated by fibrosis in adults)
• Mucosa: Similar to colon; crypts of Lieberkühn present but few; few goblet cells relative to colon
• Lamina propria and submucosa: Massively infiltrated with lymphoid nodules (often forms a near-complete ring of lymphoid tissue around the lumen) - analogous to a tonsil; this is the immunological role of the appendix
• Muscularis externa: Complete inner circular + outer longitudinal (unlike the taeniae of the colon)
• Serosa: Present (mesoappendix)

Rectum

• Lacks taeniae coli - the outer longitudinal muscle is again complete (like the appendix)
• Mucosa similar to sigmoid colon: no villi, crypts, abundant goblet cells
• Rectal columns of Morgagni: Longitudinal folds of mucosa in the upper rectum; at their bases are transverse folds = anal valves; below the valves are anal sinuses containing anal glands

Anal Canal - Zone Transitions

1. Upper zone (above pectinate/dentate line): Mucosa continuous with rectum; simple columnar epithelium; anal columns and sinuses here
2. Pectinate line (Dentate line): Key anatomical landmark at the level of the anal valves
3. Transitional zone (0.5-1 cm above dentate line): Stratified cuboidal or columnar epithelium - transitional epithelium
4. Below pectinate line: Non-keratinized stratified squamous epithelium (anoderm) - very sensitive to pain (important clinically)
5. Perianal skin: Keratinized stratified squamous epithelium with hair follicles and sweat glands

Clinical Correlates

• Most arise from adenomatous polyps (tubular, villous, or tubulovillous) - the adenoma-carcinoma sequence
• Adenoma histology: Dysplastic crypts with crowded, pseudo-stratified, hyperchromatic nuclei; mitoses; loss of goblet cell differentiation
• Molecular pathways:
  • Chromosomal instability pathway (most common, ~80%): APC → KRAS → SMAD4 → TP53 mutations (Vogelstein sequence)
  • Microsatellite instability (MSI) pathway (~15%): Lynch syndrome (germline MLH1/MSH2 mutations) or sporadic MLH1 methylation; right-sided tumors, mucinous histology, lymphocytic infiltrate; better prognosis; MSI-H tumors respond to immunotherapy
• Histology of CRC: Gland-forming adenocarcinoma; mucin production; desmoplastic stroma; lymphovascular invasion

• Diffuse, continuous inflammation limited to the mucosa and submucosa of the colon
• Begins in the rectum and extends proximally
• Histology: crypt architecture distortion (branching, dropout), crypt abscesses (neutrophils filling crypts), basal plasmacytosis, goblet cell depletion, surface ulceration
• No transmural inflammation, no granulomas (distinguishes from Crohn's)
• Pseudopolyps: Islands of regenerating mucosa surrounded by ulceration
• Risk: colorectal cancer after 10+ years of pancolitis

• Obstruction of the appendiceal lumen (usually by fecalith or lymphoid hyperplasia) → bacterial overgrowth → mucosal ulceration → transmural inflammation
• Histology: neutrophilic infiltration of the muscularis, mucosal ulceration, periappendiceal inflammation
• Peritonitis risk if perforated

• Internal hemorrhoids: Arise above the pectinate line from the superior hemorrhoidal venous plexus; covered by columnar mucosa; not painful (no somatic sensation above dentate line)
• External hemorrhoids: Arise below the pectinate line; covered by squamous anoderm; exquisitely painful (somatic innervation below dentate line)

• Herniations of mucosa and submucosa through gaps in the muscularis (where blood vessels penetrate)
• Most common in the sigmoid colon
• Risk: low-fiber diet reducing stool bulk, increased intraluminal pressure
• Diverticulitis: Inflammation of diverticula; microperforation possible; abscess, fistula, peritonitis

• Lymphatics in the colon do not extend into the mucosa above the muscularis mucosae; therefore, in situ carcinoma and intramucosal adenocarcinoma cannot metastasize
• Invasion through the muscularis mucosae into the submucosa is the threshold for metastatic potential (pT1 staging)
• Wheater's, p.346 | Junqueira, p.800

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SUMMARY TABLE - Key Staining and Identification Features

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