General Histology: Digestive Tract Structure

Study Notes

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)

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

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)

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.

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.