Histology of the GI Tract

• Walls of the Alimentary Canal

  from the oesophagus to the anus, the walls of the tract have the same 4 basic layers:

  mucosa

  submucosa

  muscularis externa

  serosa

  
• Mucosa - Structure


  innermost layer = mucosa

  = moist epithelial membrane that lines the tract lumen from mouth to anus

  
• Mucosa - Functions


  secrete mucus, digestive enzymes + hormones

  absorb the end products of digestion into the blood

  protect against infectious disease
• Complexity of the Mucosa of the GI Tract

  more complex than most other mucosae in the body, the digestive mucosa consists of 3 sublayers:

  lining epithelium

  lamina propria

  muscularis mucosae

  except for that of the mouth, oesophagus + anus, where it is stratified squamous, the epithelium of the mucosa = simple columnar epithelium rich in mucus-secreting cells

  
• Mucosa - Mucus


  the slippery mucus it produces protects certain digestive organs from being digested by enzymes working w in their cavities + eases food passage along the tract
• Mucosa - Enzymes & Hormones

  in the stomach + small intestine, the mucosa also contains enzyme-synthesising + hormone-secreting cells

  in such sites, the mucosa is a diffuse endocrine organ as well as part of the digestive organ
• Mucosa - Lamina Propria

  the lamina propria underlies the epithelium

  it is loose areolar connective tissue

  its capillaries nourish the epithelium + absorb digested nutrients

  its isolated lymphoid follicles help defend against bacteria + other pathogens, which have rather free acces to our digestive tract

  
• Mucosa - Muscularis Mucosae

  external to the lamina propria is the muscularis mucosae

  = layer of smooth muscle cells that produces local movements of the mucosa that can enhance absorption + secretion

  
• Submucosa


  the submucosa, just external to the mucosa, is aerolar connective tissue containing a rich supply of blood + lymphatic vessels, lympoid follicles + nerve fibres that supply the surrounding tissues of the GI tract wall

  its abundant elastic fibres enable the stomach to regain its normal shape after temporarily storing a large meal

  
• Muscularis Externa


  surrounding the submucosa is the muscularis externa

  responsible for segmentation + peristalss

  has an inner circular + an outer longitudinal layer of smooth muscle cells

  in several places along the tract, the circular layer thickens, forming sphinctersthat act as valves to control food passage from one organ to the next, preventing backflow

  
• Serosa


  serosa = outermost layer = visceral peritoneum

  in most GI tract organs, it is formed of areolar connective tissue covered w a single layer of squamous epithelial cells