Histology of the GI tract

What is histology?

The study of the microanantomy of tissues & organs

• Stains and dyes are used too make structures visible

• Used in understanding structure and function of tissues

• Used in diagnosis and staging of disease

What is epithelium and its function?

A type of tissue primarily involved in 3 functions:

1. Lining and protection of body surfaces and cavity

2. Secretion

3. Absorption

Basic structure of epithelium

• Epithelial cell layer

• Epithelial cells are anchored to the basement membrane

• Basement membrane is supported by the laminate propria

How can epithelium differ?

Epithelial cell layer can vary in:

• The number of epithelial cells layers

• The shape of the epithelial cells

• Some epithelial cell layers have surface specialisations

Epithelium types - simple squamous

Simple squamous epithelium

• Simple = one cell layer

• Squamous = flattened cell shape

This thin epithelial layer can be found in the lung alveoli → permits gas exchange

Epithelium types - simple cuboidal

Simple cuboidal epithelium

• Cuboidal = cube shaped cell

• Permits secretion and absorption e.g. kidney

Epithelium types - simple columnar

Simple columnar epithelium

• Columnar = column shaped

• Permits secretion and absorption e.g. small & large intestine

Epithelium types - stratified squamous

Stratified squamous epithelium

• Stratified = multiple layers

• Squamous = flattened cell shape

• Primarily responsible for protection from abrasion

How does the epithelium protect?

• Protects from friction e.g. epidermis of skin

• Some epithelium are specialised to protect against stretch e.g. epithelium of utters and bladder

• The mucous secreted by gastrointestinal and respiratory epithelium protects from toxic and pathogenic material

• Immune cells found in lamina propria

How does epithelium facilitate absorption?

• Selective absorption = allowing absorption selectively of only certain molecules

• Molecule absorption regulated based on molecule size, charge and molecular structure

• Tight junctions - leak-proof gaps between cells → provide cell polarity

Villi and microvilli - surface specialisations

How do epithelia secrete?

• They have glands which secrete enzymes into the lumen of the GI tract

Glands are specialised for:

• Synthesis

• Storage

• Release

Examples:

• Pancreatic enzymes

• Sebaceous glands (lipids)

Goblet cells

Mucous secreting cells found in the epithelium

Why does the epithelium have no blood vessels?

Blood vessels located in lamina propria - gas, nutrients & waste diffuse between epithelial cell layer and blood vessels → protects blood vessels from abrasion

Epithelia summary

• Tightly packed cell layer(s) anchored by intercellular junctions with a supporting basement membrane

• Epithelia provide a protective barrier between the external and internal environments

• By doing so they are effective at maintaining cell polarity and selective absorption and secretion

• The epithelia can be classified by: number of layers, cell shape, surface specialisations

• Glands are derived from epithelia and can be endocrine or exocrine

Layers of the GI tract

From innermost to outermost:

• Lumen

• Mucosa = epithelium + lamina propria + muscularis mucosae

• Submucosa = connective tissue and blood vessels (arterioles/venules)

• Muscularis externa = circular and longitudinal smooth muscle

• Adventitia/serosa = outer connective tissue/visceral peritoneum + major nerves/blood vessels

Epithelium of oesophagus

• Stratified squamous epithelium

• Protects the oesophagus from abrasion

• SS = stratified squamous

• LP = lamina propria

• MM = muscularis mucosae

• GL = glands

• D = duct

Stomach epithelium

• Stomach epithelium → produces acids and enzymes

• At gastrointestinal-oesophageal junction there is a transition from stratified squamous (SSE) to stratified columnar epithelium (SCE)

Stomach - gastric pits & gastric glands diagram

Gastric pits & glands - what are they and what type of cells do they contain?

• Gastric pits: opening to the gastric glands, contain mucous cells

• Gastric glands: secrete stomach acids, enzymes and local signals

• Simple columnar epithelium

• Chief cells (pepsinogen and gastric lipase)

• Parietal cells (HCl and GIF) HCl activates pepsinogen to pepsin

• Mucous cells (rich in HCO3-)

• Enters endocrine cells (secret gastrin to stimulate acid secretion)

Barrett oesophagus

• Heartburn

• Gastro-oesophageal reflux disease (GORD)

• GORD can lead to metaplastic changes i.e. oesophageal type epithelium becomes stomach type epithelium

• Barrett oesophagus is diagnosed histologically

• These changes can become cancerous

Normal gastro-oesophageal junction image

Barrett oesophagus diagram

Summary of oesophagus and stomach

• Oesophagus = stratified squamous epithelium

• Stomach = simple columnar epithelium

• Gastric pits are invaginations where gastric glands secrete HCl- for example:

  • Parietal cells = HCl-

  • Chief cells = enzymes

  • Enteroendocrine (APUD) cells = local hormone control (gastrin)

• Barrett oesophagus

Small intestine structure

• Villi (V)

• Lamina propria (P)

• Submucosa (SM)

• Muscularis externa (M)

• Serosa (S)

Small intestine

• Below the simple columnar epithelial cell layer, the lamina propria occupies the core of each villus

• Lamina propria contains blood capillaries and lacteal

• Lacteal: absorbs dietary fats and transmits them through the lymphatic system

Duodenum

Characteristic features:

• Brunner’s glands: collection of mucous secreting glands - help neutralise chyme from stomach

• Crypts:

  • Contain enzyme secreting cells (digestion)

  • Contain stem cells - replenish the cells of the epithelium

What do crypt stem cells do?

Stem cells of the crypts replenish cells of the villi

Jejunum

• Contains no Brunner’s glands

• Contain villi and crypts

• Highly folded (plicae circulares)

Ileum

• Contains the shortest villi of the small intestine

• Contains Peyer’s patches - aggregations of lymphoid tissue

Large intestine

• Lacks villi but has glands and crypts

• Crypts contain mucous secreting cells and stem cells

• Simple columnar epithelial cells

Liver histology overview

• Cells are organised into hexagonal shaped lobules

• Central vein located in the centre of lobule

Portal triad located at edges:

• Portal vein branches

• Hepatic artery branches

• Bile duct branches

Liver histology - blood flow, sinusoids and bile

• Blood from the portal vein branches and hepatic artery branches flow through sinusoids toward the central vein

• Sinusoids have a fenestrated endothelium which permits exchange between the blood and the adjacent hepatocytes

• Bile flows to bile duct through bile canaliculi

Pancreas histology

Has clusters of exocrine and endocrine cells

Exocrine:

• Pancreatic acini

• Secrete contents into duct system → duodenum

Endocrine:

• Islets of Langerhans

Pancreas histology table - cell type, hormones secreted & function

Pancreas histology - tissue diagram

Ac = acini

IL = Islets of Langerhans

D = exocrine duct

Exocrine portion of pancreas is larger