HISTOLOGY OF THE GIT

Oral Cavity

The oral cavity serves as the initial site for the mechanical breakdown of food into smaller, digestible particles. It comprises two main areas:

Oral Vestibule: Bordered by the inner surfaces of the lips and cheeks, lined with non-keratinized (NK) stratified squamous epithelium.
Oral Cavity Proper: Bounded by the teeth and gingiva, lined with NK stratified squamous epithelium and partially keratinized oral mucosa on the hard and soft palates. Taste buds are present on the soft palate.

Lips

The lips consist of:

Outer Surface: Composed of keratinized stratified squamous epithelium with hair follicles, sweat, and sebaceous glands.
Vascular Supply: Superficial blood vessels contribute to the characteristic pinkish-red color.
Core Structure: Contains skeletal muscle fibers from the orbicularis oris muscle and labial glands.
Receptors: Krause and Meissner’s receptors in the dermal layer are responsible for thermal and somatic perception.

Tongue

The tongue is primarily composed of skeletal muscle and connective tissue, featuring:

Dorsal Surface: Characterized by papillary folds and lymphatic tissue. The presence of papillary folds creates a rough surface.
Types of Papillae:
- Filiform Papillae: Narrow and conical, covering the entire dorsal surface.
- Fungiform Papillae: Larger and mushroom-shaped, located in the anterior region.
- Circumvallate Papillae: The largest, found on the posterior part of the tongue, surrounded by trenches.
- Foliate Papillae: Primitive and poorly developed in humans.
Taste Detection: Specialized neuroepithelial cells within taste buds allow for taste perception, located on the surfaces of papillae.
Lymphatic Aggregates: Lingual and palatine tonsils contain crypts for immune function.

Oropharynx and Laryngopharynx

The pharyngeal regions are lined with NK stratified squamous epithelium, providing protection during swallowing. Key features include:

Skeletal Muscle: Contains skeletal muscle fibers from the pharyngeal constrictors.
Epiglottis: Composed of elastic cartilage and covered by oral mucosa.

Esophagus

The esophagus is a tubular structure extending from the cricoid cartilage to the cardiac sphincter of the stomach. Its histological features include:

Epithelium: Lined with non-keratinized stratified squamous epithelium.
Glands: Contains esophageal cardiac glands and esophageal glands for lubrication and protection.
Muscle Composition: Transitions from skeletal to a mix of smooth and skeletal muscle.
Covering: Covered by connective tissue adventitia in the thorax and simple squamous mesothelium in the abdomen.

Stomach

Boundaries: Demarcated by the gastroesophageal junction and pyloroduodenal junction.
Epithelial Transition: Changes from non-keratinized stratified squamous epithelium to simple columnar epithelium.
Gastric Glands: Release acidic contents into gastric pits.
Muscular Layers: Three layers: inner oblique, middle circular, and outer longitudinal, mixing the bolus with gastric juices.
Histological Segments: Four segments (cardia, fundus, body, pylorus) but only three histologically distinct parts.
Cell Types in Gastric Glands: Mucous neck cells, parietal cells, and zymogenic cells.

Clinical Aspects

Gastroesophageal Reflux Disease (GERD): Affects the esophageal lining due to a weakened cardiac sphincter, leading to exposure to acidic contents.
Acute Gastritis: Inflammation of the stomach mucosa caused by NSAIDs, alcohol, H. pylori, or stress.

Small Intestine

The small intestine enhances absorption of water and nutrients. Its histological composition includes:

Mucosal Structure: Lined by simple columnar epithelium with villi increasing surface area.
Crypts of Lieberkühn: House various cell types including enteroendocrine, Paneth, absorptive, goblet, and stem cells.
Plicae Circulares: Permanent folds in the mucosa that increase surface area.

Cell Types in the Small Intestine

Common Cell Types: Absorptive cells, goblet cells, stem cells, Paneth cells, and enteroendocrine cells.
Duodenum Specifics: Contains Brunner’s glands that secrete alkaline mucus.
Ileum Specifics: Contains Peyer’s patches for immune response.

Large Intestine

The large intestine begins at the cecum and appendix, progressing through the colon to the rectum. Its primary function is to condense undigested residues into feces.

Histological Features: Absence of villi, simple columnar epithelium with deeper crypts and abundant goblet cells.
Vermiform Appendix: Similar histological appearance to the colon but lacks taeniae coli.

Rectum and Anus

Histological Transition: The epithelium of the colon continues into the rectum with longer crypts.
Muscularis Externa: Maintains its bilayer arrangement with taeniae coli.
Anorectal Junction: Transition from simple columnar to stratified squamous epithelium.
Sphincter Muscles: Internal anal sphincter formed by thickened inner circular muscle; external anal sphincter formed by skeletal muscle.

Clinical Aspects of the Colon

Diverticular Disease: Pseudo-diverticular outpouchings can form due to increased pressure in the sigmoid colon.
Angiodysplasia: A rare condition leading to chronic bleeding due to disfigured vasculature in the cecum or ascending colon.

Histological Overview of the Gastrointestinal Tract

Histological Overview of the Gastrointestinal Tract