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• The Digestive Tract, also known as the Gastrointestinal Tract or Alimentary Canal, is composed of the Oral Cavity, Esophagus, Stomach, Small and Large Intestines, Anus, and associated Glands such as Salivary Glands, Liver, and Pancreas.
• The function of the Digestive Tract is to obtain molecules from the ingested food that are necessary for the maintenance, growth, and energy needs of the body.
• During digestion, complex molecules are broken to smaller pieces for easy absorption in the Small Intestine.
• Water Absorption in the Large Intestine is a mechanism in the Digestive Tract.
• The General Structure of the Digestive Tract includes the Oral Cavity, Tongue, Teeth, Dentin, Esophagus, Stomach, Small Intestine, Mucosa, Submucosa, Muscularis, and Serosa.
• The Mucosa, also called as mucous membrane, consists of the Epithelial Lining, Lamina Propria, Loose connective tissue rich in blood vessels, lymphatics, lymphocytes, smooth muscle cells, and small glands.
• Mucosal glands in the large intestine are less closely packed than in the large intestine.
• The distal end of the GI tract, the anal canal, is 3-4cm long and is lined with mucosa and submucosa which form several folds known as Anal Columns of Morgagni.
• Circumanal Glands, branched tubular glands that open at the recto-anal junction into small pits at the distal ends of the Columns of Morgagni, are also seen in the anal canal.
• The rectal mucosa is the same as the rest of the large bowel except that it has even more numerous goblet cells.
• A transition from skin containing sebaceous glands to large apocrine sweat glands, known as the peri-anal glands of Gay, is also seen at the recto-anal junction.
• The external anal sphincter is comprised of voluntary muscles involved in defecation.
• At the junction of the rectum and anus, there is an abrupt transition to stratified squamous epithelium.
• The internal anal sphincter is a circular layer of the rectum’s muscularis.
• The Submucosa contains Denser connective tissue with larger blood and lymph vessels, Submucosal (Meissner) Plexus of Autonomic Neves, Glands, and significant lymphoid tissues.
• The Muscularis is composed of smooth muscle cells organized as two or more sublayers: Inner Circular, Outer Longitudinal, and Blood and Lymph Vessels on the connective tissues as well as the myenteric (Auerbach Plexus).
• Root canal is a narrowed area of the pulp cavity that extends to the apical foramen at the tip of the root for the blood vessels, lymphatics, and nerves of the pulp.
• Amelogenin is the main structural protein of developing enamel that guides the growth of elongating enamel rods.
• Odontoblast process extends from the odontoblast within the dentinal tubules, penetrates the full thickness of the dentin, and is important in the maintenance of the dentin matrix.
• Enamel rods are uniform, interlocking columns of the enamel that form the enamel’s hardness and resistance to greater pressure.
• Ameloblast secretes the matrix of the enamel rods.
• Periodontal ligament is a fibrous connective tissue bundles of collagen fibers inserted into both the cementum and alveolar bone.
• Dentin is a calcified tissue harder than the bone made up of 70% hydroxyapatite and an organic matrix consisting of type 1 collagen and proteoglycans secreted by odontoblasts.
• Loose, mesenchymal connective tissues with much ground substance, collagen fibers, fibroblast, and mesenchymal stem cells make up teeth.
• Enamel is the hardest component of the human body made up of 96% hydroxyapatite and 2% - 3% organic materials (few proteins, no collagen).
• Odontoblast is a tall, polarized cell from the cranial neural crest that lines the tooth’s pulp cavity, secretes the predentin matrix, and stimulates to repair dentin if the tooth is damaged.
• The Serosa is a thin sheet of loose connective tissue rich in blood vessels, lymphatics, and adipose tissue, covered with simple squamous covering epithelium or Mesothelium.
• The Serosa of the Small and Large Intestines is continuous with the mesentery.
• Paneth Cells have an important role in innate immunity and in regulating the microenvironment of the intestinal crypts.
• The Small Intestine has other layers such as the Lamina Propria, which has extensive blood and lymph vasculature, nerve fibers, and smooth muscle cells, and diffuse lymphoid tissue.
• The Submucosa contains larger blood and lymph vessels and contains interconnected neurons of the submucosal (Meissner) Nerve Plexus.
• Smooth Muscle Fibers in the Muscularis Mucosae are producing a rhythmic movement of the villi that increases absorption efficiency and produces local movement of plicae circularis to propel the lymph from the lacteals into submucosal and mesenteric lymphatics.
• Defensins bind and break down membranes of microorganisms and bacterial cell walls.
• The Muscularis is well-developed in the small intestine and has inner circular and outer longitudinal layers.
• The Distinctly Alkaline pH of the Duodenal (Brunner) Glands (pH 8.1 - 9.3) Neutralizes chyme entering the duodenum from the pylorus, protecting the mucous membrane and bringing intestinal contents to optimal pH for pancreatic enzyme action.
• In the Ileum, both the lamina propria and the submucosa contain well-developed mucosa-associated lymphoid tissue (MALT) known as Peyer’s Patches.
• Release lysozyme, phospholipase A2 and defensins.
• M (Microfold) Cells are specialized for transepithelial transport of particles and microorganisms and are located mainly in the ileum’s mucosa overlying the lymphoid follicles of Peyer Patches.
• Enteroendocrine Cells are present in varying numbers throughout the length of the intestine and are secreting various peptide hormones.
• Many of these peptide hormones are “open” type and have a constricted apical end of the cell that contacts the intestinal lumen and has chemo receptors similar to taste buds.