Trachea: histology

Layers, from lumen outward

1. Mucosa
   • Epithelium: pseudostratified ciliated columnar epithelium
   • Major cells:
     • Ciliated columnar cells: move mucus and trapped particles upward toward the pharynx.
     • Goblet cells: secrete mucus.
     • Basal cells: stem/progenitor cells that replace epithelial cells.
     • Less conspicuous cells include brush cells and small granule neuroendocrine cells.
   • Basement membrane: conspicuously thick.
   • Lamina propria: loose, vascular connective tissue with elastic fibers and immune cells.
2. Submucosa
   • Dense connective tissue containing numerous seromucous glands.
   • Their secretion moistens the airway and contributes to mucociliary clearance.
3. Fibrocartilaginous layer
   • Composed of C-shaped rings of hyaline cartilage that prevent collapse of the airway.
   • The posterior gap in the rings is bridged by fibroelastic tissue and the trachealis muscle, a smooth muscle band adjacent to the esophagus.
4. Adventitia
   • Outer connective tissue layer attaching the trachea to neighboring structures.

Key identification points on an H&E slide

• Wide lumen lined by pseudostratified ciliated epithelium
• Many pale goblet cells
• Thick basement membrane
• Submucosal seromucous glands
• Large C-shaped hyaline cartilage
• Posterior trachealis smooth muscle

Oesophagus: histology

Layers from lumen outward

1. Mucosa
   • Epithelium: thick non-keratinized stratified squamous epithelium, protective against abrasion.
   • Lamina propria: loose connective tissue; contains mucous oesophageal cardiac glands especially near the upper and lower ends.
   • Muscularis mucosae: longitudinal smooth muscle, relatively thick in the lower oesophagus.
2. Submucosa
   • Dense irregular connective tissue with blood vessels, lymphatics, and the submucosal (Meissner) plexus.
   • Contains oesophageal glands proper, which are mucous/seromucous glands that lubricate and protect the lumen.
3. Muscularis externa
   • Inner circular and outer longitudinal muscle layers, with the myenteric (Auerbach) plexus between them.
   • Muscle type varies by level:
     • Upper one-third: skeletal muscle
     • Middle one-third: mixed skeletal and smooth muscle
     • Lower one-third: smooth muscle
4. Outer covering
   • Mostly adventitia, which anchors the oesophagus to surrounding mediastinal structures.
   • Only the short abdominal part is covered by serosa.

Gastro-oesophageal junction

• Non-keratinized stratified squamous epithelium of oesophagus
  to
• Simple columnar mucous-secreting epithelium of stomach.

Identification points in an H&E slide

• Thick, multilayered squamous non-keratinized lining
• Submucosal mucous glands
• No villi or gastric pits
• Two muscle layers with changing muscle type along its length
• Predominantly adventitial outer coat

Duodenum: histology

1. Mucosa

• Lined by simple columnar epithelium with:
  • Enterocytes with a brush border of microvilli for absorption
  • Goblet cells that secrete mucus
• Has broad, leaf-shaped intestinal villi that increase absorptive surface area.
• Crypts of Lieberkühn lie between villi and contain stem cells, enteroendocrine cells, and Paneth cells.
• Lamina propria contains blood capillaries, immune cells, and a central lymphatic capillary called a lacteal.
• Muscularis mucosae forms the deepest part of mucosa.

2. Submucosa

• The distinguishing feature is Brunner glands.
• These are branched tubular mucous glands that secrete alkaline, bicarbonate-rich mucus.
• Their secretion neutralizes acidic chyme arriving from the stomach and protects the duodenal mucosa.
• Contains vessels, lymphatics, and the submucosal (Meissner) plexus.

3. Muscularis externa

• Inner circular smooth-muscle layer
• Outer longitudinal smooth-muscle layer
• Myenteric (Auerbach) plexus lies between them.

4. Outer layer

• First part of duodenum: partly covered by serosa
• Remaining retroperitoneal parts: covered mainly by adventitia

Identification points on H&E slide

• Mucosal villi and crypts
• Simple columnar epithelium with goblet cells
• Brunner glands in the submucosa: the key feature distinguishing duodenum from jejunum and ileum
• No Peyer patches as seen prominently in ileum