Digestive Tract Anatomy and Functions

Digestive System Histology

• The digestive system's histology is studied to identify and differentiate histological structures in the digestive tube, parts of the small intestine, and small and large intestines.

Digestive Tube

• The digestive tube starts with the esophagus and ends with the anus.
• It comprises tubular and saccular organs.
• Its structure consists of four concentric tunics.
• Sometimes tunics may be partially or completely missing.
• Minor structural modifications can be present to meet specific local requirements.

Structural Plan

• The tunica mucosa is composed of epithelium, membrana propria, lamina propria mucosae, and lamina muscularis mucosae.
• The tunica submucosa is present.
• The tunica muscularis is present.
• The tunica adventitia (serosa) is present.

Esophagus

• The esophagus connects the pharynx and the stomach.
• It represents a transition.
• The muscularis mucosae replaces the elastic layer of the pharynx.
• The muscularis layer is more regular in the esophagus.

Stomach

• The stomach is a dilated, distorted, and capacious sac.
• It is divided into cardia, fundus, corpus (body), and pylorus.
• The mucosa lining is 0.3 mm thick at the cardia and 1.5 mm at the pylorus.
• The mucosa and submucosa form longitudinal folds called rugae.
• Distension changes the number and height of the rugae.
• Finer furrows form mound-shaped areas (foveolae) across the gastric areas, ranging in size from 1-5 mm.
• Foveolae vary in density by region (17/sq mm in fundus, 9/sq mm in pylorus).
• The stomach contains 3,400,000 foveolae.
• The surface epithelium is simple columnar and mucous-secreting.
• The mucigen is unique to the epithelium type.
• Gastric glands, of a branched tubular type, penetrate the full thickness of the mucosa.
• Foveolae serve as ducts for the glands.
• A human stomach has around 15,000,000 gastric glands.

Cardiac Glands

• Cardiac glands are a narrow band located at the cardio-esophageal junction.
• They measure 5-40 mm in width.
• These glands resemble superficial glands from the lower esophagus.
• They are small and sometimes compound tubular glands, and foveolae are about ½ the length of the gland.
• The secretory portion is tortuous, resembling the mucous cells.
• The lumen is clear and obvious.

Fundic Glands

• These glands are straight and tubular with minor branching.
• They are located in the fundus and corpus regions of the stomach.
• The gland regions are foveola (short duct), neck (constricted duct), body (long duct), and fundus (minor dilation & bend).
• The ducts and main gland have a 1:4 ratio.

Four Cell Types

• Mucous Neck Cells: Increase in number towards the pyloric end; nuclei are flattened and basal; mucus is watery.
• Chief Cells: Increase in number towards the cardiac end; comprise the gland below the neck; spherical nucleus with granular elements; basally striated chromidial substance; presecretion granules during rest; activity is reduction in granules.
• Parietal Cells: More abundant in the pyloric end; commonly wedged between mucous neck and chief cells, and position is peripheral; cell shape is wedge-like to conical; lack distinct secretory granules, but intercellular channels connect to the main lumen. Secrete hydrochloric acid.
• Argentaffin Cells: Enterochromaffin elements; located between chief cells and basement membrane; slender extension may reach the lumen; granules stain with chrome salts (precursors of serotonin—vasoconstrictive hormone).

Small Intestine

• The small intestine is a long, slender tube, longer than the peritoneal cavity, and coils within the cavity.
• The small intestine comprises the duodenum, jejunum, and ileum.
• The mucosa is lined with simple columnar epithelium covering villi and dipping into glands.
• Four cell types are present: simple columnar cells (absorptive), goblet cells, Paneth cells, and argentaffin cells. Cells are not evenly distributed.
• The lamina propria contains a delicate network of collagenous and reticular fibers, lymphocytes, and solitary lymph nodules ( Peyer's patches); especially in the ileum.
• The submucosa is loose areolar connective tissue with blood vessels and nerve plexuses. Ring-like folds (plicae circulares) are prominent. Brunner's glands are in the duodenum.
• The muscularis has inner circular and outer longitudinal layers.
• The small intestine serosa is continuous with mesenteries.

Large Intestine

• The large intestine is about 5 feet long and wider than the small intestine.
• It comprises the cecum, appendix, colon, and rectum.
• The lining lacks plicae circulares and villi.
• Intestinal glands are longer than in the small intestine and are arranged closer together.
• The epithelial cell types are identical to the small intestine.
• The ileocecal valve is an abrupt change from the ileum to the cecum, with opposing folds of mucosa and submucosa supported by a central smooth muscle plate.
• The cecum is a blind sac.
• The appendix is a finger-like process attached to the cecum. The gland count decreases with age. The wall is thick, and the lumen is relatively small, often filled with debris. It has deep pockets between mucosal folds.
• The colon forms haustra; taenia coli are longitudinal ribbons shorter than the colon's length. The serosa is incomplete. Appendices epiploicae are redundant serosa. The lamina propria is similar to that in the small intestine. Colon mucosa does not form folds but has regular rows of glands. Eosinophils are numerous in the lamina propria. Muscularis mucosae typically has two layers of smooth muscle.