Digestive Tract: Oral Cavity

Oral Cavity Structure

• Lined primarily by mucosa with nonkeratinized stratified squamous epithelium
• Hard palate and gingiva have keratinized stratified squamous epithelium

Lingual Papillae

• Four types of papillae on the dorsal surface of the tongue: filiform, foliate, fungiform, and large vallate papillae
• Filiform papillae have keratinized epithelium, while others have nonkeratinized epithelium
• All lingual papillae, except filiform, have epithelial taste buds on their sides

Taste Buds

• Contain chemosensory gustatory cells with synapses to basal sensory innervation
• Support cells and an apical taste pore are present in taste buds

Tooth Structure

• Each tooth has enamel covering its crown and neck
• Enamel is made up of parallel enamel rods, guided by the protein amelogenin
• Enamel rods are secreted by columnar epithelial cells called ameloblasts in the enamel organ of the embryonic tooth bud

Dentin and Pulp Cavity

• Dentin makes up the roots and extends into the neck of the tooth
• The pulp cavity is vascularized and innervated, with a central cavity within the dentin
• Predentin is secreted as elongated d entinal tubules from tall odontoblasts that line the pulp cavity
• Apical odontoblast processes extend between the tubules

Periodontium

• Consists of a thin layer of bonelike cementum surrounding dentin of the roots
• The periodontal ligament binds the cementum to alveolar bone on the jaw socket

Layers of the Digestive Tract

• The digestive tract from the esophagus to the rectum has four major layers: mucosa, submucosa, muscularis, and adventitia or mesothelium-covered serosa.

Mucosa Layer

• The mucosa layer varies regionally along the tract, consisting of a lining epithelium on a lamina propria of loose connective tissue and smooth muscle fibers extending from the muscularis mucosae layer.

Esophagus

• The esophagus has nonkeratinized stratified squamous epithelium in its mucosa layer.
• The muscularis layer of the esophagus is striated at its superior end and smooth at its inferior end, with mixed fiber types in the middle.
• Most of the outer layer of the esophagus is adventitia, which merges with other tissues of the mediastinum.
• At the esophagogastric junction, the stratified squamous epithelium changes abruptly to simple columnar epithelium that invaginates into the lamina propria as many branched tubular glands.

Stomach Regions

• The stomach has four major regions: superior cardia, inferior pylorus, fundus, and body.
• The fundus and body are histologically similar, as are the superior cardia and inferior pylorus.

Mucosa of the Stomach Fundus and Body

• The mucosa is penetrated by numerous gastric pits.
• Gastric pits are lined with surface mucous cells and lead into branching gastric glands.
• Surface mucous cells secrete a thick layer of viscous mucus with bicarbonate ions, protecting the cells and underlying lamina propria.

Gastric Glands

• Gastric glands are lined by epithelium with four major cell types and pluripotent stem cells.
• The four major cell types are:
  • Mucous neck cells, which produce less alkaline mucus and are immature precursors of surface mucous cells.
  • Parietal cells, which produce HCl and intrinsic factor for vitamin B12 uptake.
  • Chief (zymogenic) cells, which secrete pepsinogen activated by low pH in the lumen to form pepsin.
  • Enteroendocrine cells, which release peptide hormones to regulate neighboring tissues during food digestion.

Mucosa of the Stomach Cardiac and Pyloric Regions

• The mucosa has branching cardial and pyloric glands.
• These glands consist almost entirely of columnar mucous cells, lacking parietal and chief cells.

Small Intestine Structure

• The small intestine has three regions: duodenum, jejunum, and ileum.
• The duodenum has large mucous glands in the submucosa called duodenal glands.
• The ileum has large mucosal and submucosal Peyer patches.

Mucosa Composition

• The mucosa has millions of projecting villi.
• Villi are covered with simple columnar epithelium over cores of lamina propria.
• Intervening simple tubular intestinal glands (or crypts) are found in the mucosa.

Cellular Composition

• Stem cells in the glands produce columnar epithelial cells of villi.
• Main cell types produced by stem cells: goblet cells, enterocytes for nutrient absorption, and Paneth cells (defensin-producing cells).
• Paneth cells are located deep in the glands.

Nutrient Absorption

• Sugars and amino acids produced by carbohydrate and polypeptide digestion in the glycocalyx undergo transcytosis through enterocytes for uptake by capillaries.
• Lipid digestion products associate with bile salts, are taken up by enterocytes, and are converted to triglycerides and lipoproteins.
• Chylomicrons are released from enterocytes for uptake by lacteals in the core of each villus.

Muscle Function

• Smooth muscle in the lamina propria and muscularis mucosae helps move villi and propel lymph through lacteals.
• Smooth muscle in the muscularis (inner circular and outer longitudinal layers) produces strong peristalsis under the control of the autonomic myenteric (Auerbach) plexus.

Large Intestine Structure

• The large intestine has three main regions: cecum, colon, and rectum.
• The colon has four distinct portions: ascending, transverse, descending, and sigmoid.

Large Intestine Function

• Millions of short simple tubular intestinal glands line the mucosa, producing lubricant and absorptive cells.
• These glands facilitate the uptake of water and electrolytes.

Muscularis Function

• The outer longitudinal layer of the muscularis is divided into three bands of smooth muscle called teniae coli.
• Teniae coli aid in the peristaltic movement of feces towards the rectum.

Anal Canal Structure

• The anal canal marks a sudden shift from simple columnar epithelium lining the rectum to stratified squamous epithelium of the skin at the anus.

Anal Sphincter Control

• The circular layer of the rectum's muscularis forms the internal anal sphincter.
• The external anal sphincter, composed of striated muscle, provides additional control.

Large Intestine Structure

• The large intestine has three main regions: cecum, colon, and rectum.
• The colon has four distinct portions: ascending, transverse, descending, and sigmoid.

Large Intestine Function

• Millions of short simple tubular intestinal glands line the mucosa, producing lubricant and absorptive cells.
• These glands facilitate the uptake of water and electrolytes.

Muscularis Function

• The outer longitudinal layer of the muscularis is divided into three bands of smooth muscle called teniae coli.
• Teniae coli aid in the peristaltic movement of feces towards the rectum.

Anal Canal Structure

• The anal canal marks a sudden shift from simple columnar epithelium lining the rectum to stratified squamous epithelium of the skin at the anus.

Anal Sphincter Control

• The circular layer of the rectum's muscularis forms the internal anal sphincter.
• The external anal sphincter, composed of striated muscle, provides additional control.