Gastrointestinal Tract Histology - Lecture 1

Gastrointestinal Tract Histology - Lecture 1

• The digestive system comprises the digestive tract and its associated glands.
• The digestive tract includes the oral cavity, esophagus, stomach, small intestine, large intestine, rectum, and anus.
• Associated glands include the salivary glands, liver, and pancreas.
• Digestive system functions include obtaining molecules for body maintenance, growth, and energy needs from ingested food.
• Breaking down large molecules (proteins, fats, carbohydrates, nucleic acids) into smaller, absorbable molecules, primarily in the small intestine.
• Absorbing water, vitamins, and minerals from ingested food.
• Creating a protective barrier between the digestive tract lumen contents and the internal body environment.

Contents of Foregut

• Histological structure of the oral cavity and its components (e.g., lips, tongue, teeth, salivary glands).
• Pharynx structure and function.
• Esophagus structure and function.

Mucosa Layer

• Composed of an epithelial lining.
• The lamina propria, a loose connective tissue rich in blood and lymph vessels, containing smooth muscle fibres, glands, and lymphoid tissue.
• The muscularis mucosae, a thin layer of smooth muscle dividing the mucosa from the submucosa. (inner circular, and outer longitudinal layer).

Submucosa Layer

• Composed of dense connective tissue with numerous blood and lymph vessels, and a submucosal (Meissner's) nerve plexus.
• May contain glands and lymphoid tissue.

Muscularis Layer

• Contains smooth muscle cells spirally oriented into two sublayers:
  • Inner circular layer (close to the lumen)
  • Outer longitudinal layer (external)
• Also contains the myenteric (Auerbach's) nerve plexus and blood and lymph vessels.

Serosa Layer

• A thin layer of loose connective tissue, rich in blood and lymph vessels, adipose tissue, and a simple squamous epithelium (mesothelium).
• In the abdominal cavity, continuous with the mesenteries and peritoneum.
• Replaced by adventitia in areas where the digestive organ is connected to other organs/structures.

Diseases

• Hirschsprung disease: Absence or injury to the enteric nervous system in the digestive tract leading to disrupted motility and dilation.
• Chagas disease (trypanosomiasis): Caused by Trypanosoma Cruzi, leading to gastrointestinal complications.

Main Functions of Epithelial Lining

• Provides a selectively permeable barrier between the digestive tract's contents and the body's tissues.
• Facilitates transport and digestion of food.
• Promotes the absorption of digested food products.
• Produces hormones affecting the digestive system activity. Produces mucus for lubrication and protection.

Additional Protective Features

• Abundant lymphoid nodules in the lamina propria and submucosa protect the organism from bacterial invasion.
• The lamina propria contains macrophages and lymphoid cells producing antibodies (mainly IgA) to protect against viral and bacterial invasion.
• The muscularis mucosae enhances contact between the food and digestive tract mucosa by promoting movement independent of other movements of the digestive tract.

Oral Cavity

• The oral cavity includes the mouth, lips, tongue, teeth, salivary glands.
• Lining: stratified squamous epithelium, with keratinized (hard palate, gingiva) and non-keratinized (soft palate, cheeks, floor of the mouth) zones.
• Lips: stratified squamous epithelium.
• Tongue: striated muscle, multiple types of papillae (filiform, fungiform, foliate, vallate) with taste buds.

Teeth and Associated Structures

• Adult humans typically have 32 permanent teeth.
• Each tooth has a crown (above gingiva), root (below gingiva), and the attachment to the bone via alveoli.
  • Enamel: hard outer covering of the crown.
  • Dentin: another calcified material surrounding the pulp cavity.
  • Pulp cavity: soft connective tissue filled space in the tooth.
• Periodontal ligament: fibrous connective tissue connecting cementum to periodontal ligament, alveolar bone, and gingiva.
  • Alveolar bone: the bony socket called alveolus in which the tooth is lodged.
• Cementum: mineralized tissue covering the tooth root.

Pharynx

• A transitional space between oral and respiratory/digestive systems, linking oral region, nasal region, and larynx.
• Lined with stratified non-keratinized squamous epithelium (esophagus connection) or pseudostratified columnar epithelium (near nasal area).
• Includes the tonsils.

Esophagus

• Muscular tube connecting the pharynx to the stomach.
• Outermost layer is adventitia, continuous with the trachea tissue.
• The mucosa is lined with stratified squamous non-keratinized epithelium, containing mucous glands (cardiac glands in the lower part) to aid in transport and protect the mucosa.