DIGESTIVE SYSTEM

Functions of the alimentary mucosa: 1. Barrier function: The mucosa serves as a barrier to the entry of noxious substances, antigens, and pathogenic organisms. 2. Immunologic function: Lymphatic tissue in the mucosa serves as a first line of defense of the body by the immune system. 3. Secretory function: The lining of the alimentary canal secretes, at specific sites, digestive enzymes, hydrochloric acid, mucin, and antibodies. 4. Absorptive function: The epithelium of the mucosa absorbs metabolic substrates, i.e., the breakdown products of digestion, as well as vitamins, water, electrolytes, and other substances.

Four types of papillae of the tongue: Filiform papillae are the most numerous in humans and the smallest. They are conical, elongated projections of connective tissue that are covered with partly keratinized stratified squamous epithelium. They are distributed over entire anterior dorsal surface of the tongue. Fungiform papillae are mushroom shaped projections located on the dorsal surface of the tongue. They are more numerous near the tip of the tongue.

Foliate papillae occur on the lateral edge of the tongue Foliate papillae occur on the lateral edge of the tongue. In aged humans, the foliate papillae may not be recognized. In younger individuals, they are easily found. Circumvallate papillae are the large, dome-shaped structures that reside in the mucosa just anterior to the sulcus terminalis. Each papilla is surrounded by a moat-like invagination lined with stratified squamous epithelium.

Gingiva The gingiva is firmly attached to the teeth and underlying bony tissue. It is composed of stratified squamous epithelium and numerous connective tissue papillae. This epithelium is bound to the tooth enamel by means of a cuticle that resembles a thick basal lamina and forms the epithelial attachment of Gottlieb. Between the enamel and the epithelium is the gingival crevice a small deepening surrounding the crown.

Classification of the salivary glands The major salivary glands parotid submandibular sublingual The minor salivary glands lingual labial buccal molar palatine

Salivary ducts Intercalated ducts are located between a secretory acinus and a larger duct and are lined by low cuboidal epithelial cells. Several of these ducts join to form an intralobular duct, the striated duct. Striated duct cells have numerous infoldings of the basal plasma membrane with numerous elongated mitochondria. Striated ducts are lined by a simple cuboidal epithelium that gradually becomes columnar. The infoldings of the basal plasma membrane are seen in histologic sections as “striations”. The striated ducts of each lobule converge and drain into the connective tissue septae separating the lobules, where they become interlobular or excretory. Excretory ducts travel in the interlobular and inter lobar connective tissue. Excretory ducts constitute the principal ducts of each of the major glands. They connect with oral cavity. The epithelium of small excretory ducts is simple cuboidal. It gradually changes to stratified cuboidal or pseudostratified columnar.

Diagram comparing the components of the salivon in the three major salivary glands The salivon is the salivary secretion unit, consisting of the terminal acini, the intercalated duct, the striated duct, and the excretory duct

Submandibular gland The submandibular glands are branched tubuloacinar gland. Its secretory portion contains mainly serous and some mucous cells. The paired, large, mixed submandibular glands are located under either side of the floor of the mouth, close to the mandible. A duct from each of the two glands runs toward and medially to a papilla located on the floor of the mouth just lateral to the frenulum of the tongue. Intercalated ducts are less extensive than in the parotid gland

Alimentary canal structure Mucosa consists of a lining epithelium, an underlying connective tissue called lamina propria which contains glands, vessels and elements of the immune system, and also of muscularis mucosae, composed of smooth muscle cells arranged as an inner circular and an outer longitudinal layer. Submucosa consists of dense irregular connective tissue which contains the larger blood vessels and the nerve network

Diagram of general organization of the alimantary canal

Esophagus The esophagus is lined with a nonkeratinized stratified squamous epithelium. The underlying lamina propria and the muscularis mucosae are not unique. The submucosa along with the muscularis mucosae forms a number of longitudinal folds and creates a highly irregular luminal profile. Upper one-third of the muscularis externa is striated muscle, the middle third is striated and smooth muscle and the distal third consists of smooth muscle. The outer layer of esophagus in the thoracic cavity is composed of adventitia. After entering the abdominal cavity it is covered by serosa.

Photomicrograph of the esophagus

Stomach The stomach is an expanded part of the digestive tube that lies under the diaphragm. The stomach has mucosae, submucosa, muscularis externa and a serosa. The inner surface of the empty stomach has a number of longitudinal folds or ridges called rugae. In the mucosal surface is present numerous openings. These are the gastric pits or foveolae. The smaller regions of the mucosa are formed by grooves or shallow trenches that divide the stomach surface into bulging irregular areas called mamillated areas.

Photograph of a hemisected human stomach

Diagram of gastric glands

Fundic glands are composed of Undifferentiated cells that give rise to the mature cells. Mucous neck cells secrete soluble mucus compared with insoluble or cloudy mucus produced by the surface mucous cells. Chief cells secrete pepsin in an inactive precursor form designated pepsinogen and a weak lipase Parietal cells secrete HCL and intrinsic factor. Intrinsic factor, a glycoprotein that is essential for the absorption of vitamin B12. Enteroendocrine cells secrete gastrin, one of the gastrointestinal polypeptide hormones, is the principal effective agent for stimulating the secretion of HCL.

Diagram of chief cell Chief cells are located in the deepest part of the fundic glands. Chief cells are typical protein-secreting cells.

Diagram of enteroendocrine cell Enteroendocrine cells may be found at any level of the fundic gland but they tend to be more prevalent in the base. They are small and rest on the basal lamina and do not always reach the lumen. Enteroendocrine cells secrete their product into the lamina propria

Small intestine Mucosa consists of: Plicae circulares are permanent transverse folds that contain a core of submucosa. Villi are finger-like and leaf-like projections of the mucosa that extend into the intestinal lumen. Microvilli of the enterocytes give the apical region of the cell a striated appearance, the so-called striated border. The intestinal glands or crypts of Liberkühn are simple tubular structures. They open on to the luminal surface of the intestine of the base of the villi. The lamina propria contains numerous cells of the immune system and nodules of lymphatic tissue that represent a major component of the Gut Associated lymphoid tissue The muscularis mucosae consist of two thin layers of smooth muscle cells, an inner circular and an outer longitudinal layer.

Enterocytes Structure: They are tall columnar cells with a basally positioned nucleus. Microvilli of the enterocytes increase the apical surface area as much as 600 times. Functions: Enterocytes are specialized for the absorption and transport of substances from the lumen of the intestine to the circulatory system. Enterocytes are also secretory cells producing glycoprotein enzymes needed for terminal digestion and absorption.

Paneth cells Structure: The acidophilic secretory granules contain the antibacterial enzyme lysozyme, other glycoproteins, an arginine rich protein and zinc. Functions: Lysozyme digests the cell walls of certain groups of bacteria. This antibacterial action and the phagocytosis of certain bacteria and protozoa by Paneth cells suggest that they have a role in regulating the normal bacterial flora of the small intestine

Enteroendocrine cells Functions: Cholecystokinin, secretin and gastric inhibitory peptide are the most active regulators of gastrointestinal physiology that are released in this portion of the gut. These three hormones increase pancreatic and gallbladder activity and inhibit gastric secretory function and motility.

Intermediate cells Structure: . These cells have short, irregular microvilli and small mucin-like secretory droplets which form a column in the center of the supranuclear cytoplasm. Intermediate cells have characteristics of both immature absorptive cells and goblet cells. Function: These cells are still capable of cell division

Submucosa consists of: A dense connective tissue and localized sites that contain aggregates of adipose cells Submucosal branched tubuloalveolar glands (of Brunner) The secretion of these glands contains neutral and alkaline glycoproteins and bicarbonate ions. Function: Secretion of these glands serves to protect the proximal small intestine by neutralizing the acid-containing chime that is delivered to it Secretion of these glands serves to bring the Ph of the intestinal contents close to the optimal pH for the pancreatic enzymes that are also delivered to the duodenum.

Muscularis externa consists of: an inner layer of circularly arranged smooth muscle cells an outer layer of longitudinally arranged smooth muscle cells Serosa Serosa is not unique.