General Histology: Digestive Tract Structure

General Structure of the Digestive Tract

• The digestive tract is a hollow tube with a lumen of variable diameter and a wall made up of four main layers: mucosa, submucosa, muscularis, and serosa.
• The mucosa consists of:
  • Epithelial lining
  • Lamina propria of loose connective tissue rich in blood vessels, lymphatics, lymphocytes, smooth muscle cells, and often containing small glands
  • Muscularis mucosa (a thin layer of smooth muscle)
• The submucosa contains:
  • Denser connective tissue with larger blood and lymph vessels
  • Submucosal (Meissner) plexus of autonomic nerves
  • Glands and significant lymphoid tissue
• The muscularis (or muscularis externa) is composed of:
  • Smooth muscle cells organized as two or more sublayers
  • Internal sublayer with circular fiber orientation
  • External sublayer with longitudinal fiber orientation
  • Connective tissue between the muscle sublayers containing blood and lymph vessels, and the myenteric (Auerbach) nerve plexus
• The serosa is a thin layer of:
  • Loose connective tissue rich in blood vessels, lymphatics, and adipose tissue
  • Simple squamous covering epithelium or mesothelium
  • In places where the digestive tract is not suspended in a cavity, the serosa is replaced by a thick adventitia (a connective tissue layer that merges with the surrounding tissues and lacks mesothelium)

Tongue

• The tongue is a mobile mass of striated muscle and mucous membrane that functions in taste, chewing, swallowing, and speech.
• The tongue is divided into:
  • Anterior two-thirds
  • Posterior one-third
  • The demarcation between these regions is the V-shaped groove at the back of the tongue called the sulcus terminalis
• The tongue is covered by:
  • Stratified squamous epithelium
  • Papillae (numerous minute projections) with varying shape and number
  • Filiform papillae (small, slender, conical, pointed) covering the entire dorsal surface
  • Fungiform papillae (larger, mushroom-shaped) scattered among the filiform papillae
  • Circumvallate papillae (large, circular, dome-shaped) with many taste buds

Small Intestine

• The small intestine extends from the stomach to the large intestine (a distance of about 6-8 m) and has 4 layers resembling the pattern of the GI tract.
• The small intestine is divided into:
  • Duodenum
  • Jejunum
  • Ileum
• The small intestine has three principal functions:
  • Completing the digestion of food by the action of enzymes
  • Absorbing the finished product of digestion into the blood and lymph vessels
  • Synthesizing and releasing hormones
• The mucosa of the small intestine undergoes several specializations to increase the surface area, including:
  • Plicae circularis (permanent spiral or circular mucosal folds)
  • Villi (long, finger-like or leaf-like projections of the mucosa)
  • Intestinal glands (crypts of Lieberkühn)
  • Microvilli (striated border of the lining epithelial cells)

Large Intestine

• The large intestine extends from the small intestine to the anus and has 4 layers resembling the pattern of the GI tract.
• The large intestine is divided into:
  • Cecum
  • Colon
  • Rectum
  • Anus
• The mucosa of the large intestine is similar to that in the small intestine, except:
  • Solitary lymph nodules are larger and more numerous
  • The muscularis has the typical 2 layers of smooth muscles
• The submucosa of the large intestine contains:
  • No glands
  • Meissner's plexus
  • Peyer's patches
• The serosa of the large intestine is incomplete, present only on the anterior surface of the ascending and descending colon.

Cecum and Appendix

• The cecum is the beginning of the large intestine and is a large sac similar to the transverse colon, including serosa.
• The appendix is a small, worm-shaped tube projection from the end of the cecum and has lymphatic nodules in the lamina propria.