The Gastrointestinal TractPete Takizawa

What we’ll talk about...

• Structural and functional layers of the GI tract

• Structure and function of gastric glands

• Regulation of acid secretion

• Structure and function of small intestine

• Innervation and movement of chyme

The GI tract is a long tube that digests food, absorbs nutrients and processes waste.

• Stomach

• Small Intestine

• Large Intestine (Colon)

The GI tract is a long tube with multiple distinct functional and structural layers.

Epithelium

Basement Membrane

LaminaPropria

Lumen

MuscularisMucosa

Submucosa

Inner Circular Muscle

Outer Longitudinal

Muscle

Muscularis Externa

Mucosa

A cross-section of the esophagus reveals the histological layers of the GI tract.

Mucosa

Epithelium

Lamina Propria

Muscularis Mucosa

Submucosa

Muscularis Externa

The GI tract contains four different type of mucosa.

• Protective

• Secretory

• Absorptive - Nutrients

• Absorptive - Water and electrolytes

Junctions separate segments of the GI tract.

• Gastro-esophageal

• Gastro-duodenal

• Ileo-cecal

• Recto-anal

Mucosa surface area is amplified through several different structures.

• Glands

• Microvilli

• Villi

• Plicae circularis

Segments of the GI Tract

A stratified squamous epithelium lines the esophagus and skeletal muscle surrounds the upper portion.

Epithelium Submucosa Muscularis Externa

The epithelium transition from stratified squamous to simple columnar at the esophagus-stomach junction.

Stratified Squamous

Simple Columnar

Transition

Esophagus Stomach

The stomach contains several functionally and structurally distinct domains.

Duodenum

Lower esophageal sphincter

Pyloric sphincterBody

• Mucus • Bicarbonate • HCl • Pepsin

Antrum • Mucus • Bicarbonate • Pepsin

Cardia • Mucus • Bicarbonate

Fundus • Mucus • Bicarbonate • HCl • Pepsin

The stomach contains gastric glands arranged into large folds called rugae.

Muscularis Externa

Submucosa

Rugae

Gastic Pits and Glands

Muscularis Mucosa

The gastric gland is the main functional unit of the stomach and produces mucus, acid and pepsin.

Muscularis Mucosa

Base Neck Isthmus

Gastic Pit

Cells of Gastric Glands

Mucus-secreting cells predominantly reside at the tips of the gastric pits.

Mucous CellsLumen

Parietal cells reside in the isthmus region and secrete acid.

Parietal Cell

Parietal cells in the stomach epithelium secrete acid via hydrogen ion pumps in their apical membranes.

H2O

Cl-H+

H+

K+

H2O

Parietal Cells

ATP ADP

Mucus

Lumen

HCO3-HCO3-

Canaliculus

CO2H2O

CAHCO3-

Cl-

Cl-HCO3-

H-K pump

Acetylcholine, histamine and gastrin increase acid secretion by parietal cells.

HistamineH2 receptor

Acetylcholine

M3 receptor

From vagus nerve

H+H+

Gastrin

CCK2 receptor

Parietal CellECL Cell

Activation of parietal cells increases their apical surface area through formation of canaliculi.

Resting Activated

Tubules and vesicles with H-K pumps

Histamine Acetylcholine

Gastrin

Ca2+Ca2+

PKA

PKAPKC

PKC

D cells release somatostatin which inhibits acid secretion by parietal cells.

HistamineH2 receptor

H+H+

SST receptor

Parietal Cell D Cell

SomatostatinGαi

Chief cells reside at the base of glands and secrete the pro-enzyme pepsinogen.

Parietal Cell

Chief CellMuscularis Mucosa

Lamina Propria

Submucosa

Gastric glands contains different endocrine cells that primarily regulate the activity of parietal cells.

• Enterochromaffin-like cells (ECL) - produce histamine

• G cells - produce gastrin

• D cells - produce somatostatin

Regions of the stomach

The cardia contains highly convoluted glands that primarily produce mucus.

Submucosa

Muscularis Mucosa

Gastric Gland

Gastric Pit

The body contains long, straight glands that produce acid and pepsinogen.

Submucosa

Muscularis Mucosa

Gastric Glands

Gastric Pit

The antrum contains mucus-secreting cells and endocrine cells but lacks parietal cells.

Gastric Pit

Muscularis Mucosa

Gastric Glands

Submucosa

Small Intestine

Microvilli, villi and plicae circulares amplify the surface area of the small intestine.

Lam

ina

Prop

ria

Subm

ucos

aEnterocyte Villus Plicae Circularis

Microvilli

Villi are outfoldings of lamina propria and epithelium that contain mostly enterocytes and goblet cells.

Lamina Propria

Lumen

Goblet Cell

EnterocytesBrush Border

Crypt of Lieberkuhn

Crypts of Lieberkuhn are the base of villi and contain stem cells, endocrine cells and immune cells.

LumenGoblet Cell

Paneth Cell

Crypt

Enteroendocrine Cell

Renewal of intestinal epithelium involves stem cells and transit amplifying cells.

Crypt of Lieberkuhn

Differentiated Cells

To Lumen of Intestine

Transit Amplifying CellTransit Amplifying Cell

Segment of the small intestine

The duodenum is the initial segment of the small intestine and has Bruner’s glands in its submucosa.

Muscularis Mucosa

EnterocytesGoblet Cell

Brunner’s Glands

Lumen

Submucosa

The jejunum has extensive plicae circulares to increase its surface area.

Submucosa

Plicae Circularis

Muscularis Externa

Villi

The ileum lack plicae circulares but contains Peter’s patches in its submucosa.

Peyer’s Patch

Muscularis Externa

Villus

Large intestine

The colon concentrates waste and structurally lacks villi but contains glands.

Mucosa

Submucosa

Muscularis Mucosa

Muscularis External: Circular

Muscularis External: Longitudinal

Glands

The base of colonic glands contain stem cells and transit-amplifying cells and lymphocytes in the submucosa.

LymphocytesSubmucosa

Muscularis Mucosa

Colonic Gland

The epithelium transitions from simple columnar to stratified squamous at the junction between the rectum and anus.

Stratified Squamous

AnusRectum

Simple Columnar

Transition

Mixing and propulsion of chyme

Coordinated contraction of muscularis external layers mix and propel chyme.

Contraction RelaxationRelaxation

Relaxation ContractionContraction

Contraction Relaxation

Contraction Relaxation

Segmented Contraction Peristaltic Contraction

Orad Caudad Orad Caudad

Chyme

Chyme

Chyme

Chyme

Chyme

Ch yme

Enervation

Auerbach’s plexi are found between smooth muscle layers throughout the GI tract.

Muscularis External: Circular

Muscularis External: Longitudinal

Auerbach’s Plexus

Meissner’s plexi localize to the submucosa in the small and large intestine.

Mucosa

Meissner’s Plexus

Take home messages…

• The epithelium determines the primary functions of different segments of the GI tract.

• Acid production by parietal cells is regulated by several different molecules.

• The structure and secretions of gastric glands shows regional difference in the stomach.

• Microvilli, villi and plicae circulares increase the surface area in the small intestine.

• The base of villi in the small intestine and glands in the colon are the site of stem cells and renewal of the epithelium.