digestive system

• Oral cavity


  Internal part of the mouth
• Structures in the oral cavity

  • Lips
  • Cheeks
  • Tongue
  • Teeth
  • Gingiva
  • Palates (hard and soft)
  • Salivary glands
  • Tonsils
• Oral mucosa


  Includes an overlying epithelium and underlying connective tissue that lines the structures in the oral cavity
• Lining mucosa


  Covers the inner oral surfaces of the lips, cheeks, soft palate, the inferior surface of the tongue and the floor of the mouth
• Lining mucosa

  • Covered by non-keratinized stratified squamous epithelium with 2 distinct layers: stratum basale and stratum spinosum
  • Epithelium is similar to the epidermis of the skin except that it has neither a stratum corneum nor a stratum lucidum and the stratum granulosum is often absent
  • The non-keratinized stratified epithelium is moistened by saliva
• Connective tissues of the lining mucosa

  Can be divided into the lamina propria and the submucosa
• Specialized mucosa


  Covers the inferior surface of the tongue
• Specialized mucosa of the tongue

  • The muscles produce voluntary movement of the tongue and are innervated by the hypoglossal nerve
  • The mucosa of the tongue is divided into two parts by a V-shaped groove called the sulcus terminalis. The anterior 2/3 of the tongue is referred to as the body of the tongue. The posterior third of the tongue is the base of the tongue
• Specialized mucosa of the anterior 2/3 of the tongue

  • Consists of keratinized and non-keratinized squamous epithelium and numerous papillae
  • 4 types of Papillae: Filiform, Fungiform, Circumvallate, Foliate
  • The lamina propria is attached to the underlying skeletal muscle
• Lining mucosa

  • Provides a barrier against the invasion of pathogens and toxic chemicals
  • Contains receptors for sensations and serves immunological functions
  • Provides lubrication and buffering by minor glands in the submucosal layer
• Masticatory mucosa


  Covers the oral surfaces of the gingiva and the hard palate
• Masticatory mucosa

  • Covered by keratinized stratified squamous epithelium exposed to significant abrasion due to high compression and friction during chewing
  • Epithelium is composed of stratum basale, stratum spinosum, stratum granulosum, and stratum corneum
  • Has a thick lamina propria that has a dense network of collagen fibers and few elastic fibers
  • The layer has no submucosa and is directly attached to the underlying bone
• Common characteristics of the digestive tract

  • Organs are hollow
  • They are composed of 4 general tunic layers: mucosa, submucosa, muscularis externa and adventitia or serosa
  • They are innervated by enteric portion of the autonomic nervous system
  • They include epithelium, connective tissue, muscle, blood, lymphatic vessels, lymphatic nodules and nerve fibers
  • They contain glands in the lamina propria or submucosa
• Parts of the digestive tract

  • Esophagus
  • Stomach
  • Small Intestine
  • Large Intestine
• Upper esophagus

  • Connects the oropharynx to the middle esophagus
  • Contains numerous esophageal glands in the submucosa which secrete mucus to lubricate the esophageal walls so that food will pass through easily
  • Contains only skeletal muscle fibers in the muscularis externa. These are voluntary muscle fibers and are innervated by the glossopharyngeal nerve (cranial nerve)
• Lower esophagus

  • Connects the esophagus to the cardia of the stomach
  • Contains large numbers of mucous glands in the lamina propria and submucosa called esophageal cardiac glands
  • Produce mucous secretions to protect the lower esophagus from being damaged by reflux of acidic gastric juices from the stomach
  • Contains only smooth muscle fibers in the muscularis externa. These are controlled by the enteric branches of the vagus nerve
• Esophagus

  • The luminal surface is lined by non-keratinized stratified squamous epithelium
  • Mucous glands called esophageal glands are located in the submucosa of the esophagus
  • The muscularis externa consists of 2 layers of muscle: inner circular and outer longitudinal layers. Both skeletal and smooth muscle fibers are found here
  • The proportion of skeletal and smooth muscle fibers are different in different regions of the esophagus
  • 3 REGIONS: Upper esophagus, Middle Esophagus, and Lower esophagus
• Middle esophagus

  • Has mucosa similar to that of the upper esophagus
  • Esophageal glands are less numerous than in the upper esophagus
  • The muscularis externa contains both skeletal and smooth muscles
• Stomach

  • Divided into the Cardia, Fundus, Body and Pylorus
  • The inner surface is lined by simple columnar epithelium composed mainly of surface mucous cells
  • The surface epithelium is invaginated into the lamina propria to form gastric pits. These pits serve as ducts for the glands in the lamina propria
• Cardiac region of the stomach

  • Connects to the lower esophagus at the esophagogastric junction characterized by a change from nonkeratinized stratified squamous epithelium to the simple columnar epithelium
  • A thickened smooth muscle ring called the gastroesophageal sphincter surrounds the opening at the junction of the lower esophagus and cardiac region of the stomach. This smooth muscle contracts to prevent the acidic stomach contents from entering the esophagus
  • The glands in the lamina propria of the cardia are called cardiac glands and are branched tubular glands with coiled secretory portions
• Cardiac glands

  • Contains mainly mucus-secreting cells and some stem cells, enteroendocrine cells and occasionally parietal cells
  • The mucus secreting cells mainly produce mucus and lysozymes. The mucus protects the stomach wall from acidic gastric juices; lysozymes destroy bacterial membranes, preventing bacterial infections
• Fundus and body regions of the stomach

  • Form the largest portions of the stomach
  • The mucosa has similar histological characteristics including short gastric pits and long branched tubular glands in the lamina propria
  • The glands are called fundic or gastric glands in both the fundus and the body regions
• Gastric glands


  • Contain mainly parietal cells and chief cells along with some stem cells, mucous neck cells and enteroendocrine cells
  • Parietal cells are more numerous in the superior regions of the glands and produce large amounts of HCl. They also secrete intrinsic factor required for the absorption of Vitamin B12
• Pyloric region of the stomach

  • Mucosa is similar to that of the cardia with long gastric pits and short coiled secretory portions
  • A circular smooth muscle ring called the pylorus sphincter (pyloric valve) surrounds the end of the pylorus region. This valve controls the entry of stomach contents into the duodenum
  • The glands in the lamina propria of the pylorus are called pyloric glands and contain primarily mucus secreting cells and 2 special types of enteroendocrine cells: G-cells (gastrin-secreting cells) and D-cells (somatostatin secreting cells). These enteroendocrine cells regulate HCl secretion
• Duodenum

  • The shortest segment about 20-25 cm long has small openings called duodenal papillae (minor and major) which allow pancreatic juice and bile to enter the digestive tract
  • The Brunner glands (mucus secreting glands) in the submucosa are a unique feature
• Small intestine

  • Important features are villi and microvilli which increase surface area for absorption
  • Intestinal glands called glands (crypts) of Lieberkuhn are located in the lamina propria
  • Villi project into the lumen of the intestine
  • The glands of Lieberkuhn open into the mucosa at the base of the villi
  • Divided into 3 parts: Duodenum, Jejunum, ileum
• Jejunum

  • Much longer than the duodenum about 2.5m long (2/5 of the rest of the intestine)
  • It has long villi and increased number of goblet cells
  • It has neither Brunner glands nor Peyer patches
• Ileum


  • The longest segment about 4 m long (3/5 of the rest of the small intestine)
  • Has short villi with significantly increased numbers of goblet cells on the surface of the mucosa
  • There are clusters of lymphatic nodules in the lamina propria which sometimes extend to the submucosal layer
  • These clusters of lymphatic nodules are called Peyer's patches and are unique to the ileum
• Cells in the surface epithelium of small intestine

  • Enterocytes
  • Goblet cells
  • Enteroendocrine cells
  • M-cells
• Cells in the intestinal glands (crypts or glands of Lieberkuhn)

  • Stem cells
  • Enterocytes
  • Goblet cells
  • Paneth cells
  • Enteroendocrine cells
• Large intestine

  • Most have tunics similar to small intestine but there are no villi in the mucosa
  • There are large number of goblet cells in the large intestine which produce mucus that help in the formation of feces and protects and lubricates the surface of the intestinal wall
  • Includes the cecum, appendix, colon, rectum and anal canal
• Cecum


  • A sphincter muscle, a thickening of the muscularis mucosae is called the ileocecal valve located at the junction of the ileum and cecum
  • Ileocecal valve prevents the contents of the large intestine from backing up into the small intestine
• Peyer's patches

  Clusters of lymphatic nodules unique to the ileum
• Cells in the surface epithelium of small intestine

  • Enterocytes
  • Goblet cells
  • Enteroendocrine cells
  • M-cells
• Cells in the intestinal glands (crypts or glands of Leiberkuhn)

  • Stem cells
  • Enterocytes
  • Goblet cells
  • Paneth cells
  • Enteroendocrine cells
• Large intestine

  • Most have tunics similar to small intestine but there are no villi in the mucosa
  • Large number of goblet cells which produce mucus that help in the formation of feces and protects and lubricates the surface of the intestinal wall
• Ileocecal valve


  A sphincter muscle, a thickening of the muscularis mucosae located at the junction of the ileum and cecum, prevents the contents of the large intestine from backing up into the small intestine
• Colon

  • The large intestinal glands, the glands of Leiberkuhn contain primarily goblet cells, columnar cells, enteroendocrine cells and stem cells
  • Lymphatic nodules may also be found in the lamina propria
  • The muscularis externa consists of inner circular layers of muscles; the outer longitudinal smooth muscle layer becomes three tenia coli
• Colon


  • Longest segment of the large intestine
  • Proximal half is responsible for the majority of the absorption of water and salt
  • Distal half has only a small absorptive function and is predominantly for processing and storing of feces
  • Does not have a villi and has smoother surface than the small intestine
  • Columnar absorptive cells and goblet cells line the mucosa
• Appendix


  • A small blind tube that attaches to the postero-medial wall of the cecum
  • Has the general tunic structure of the intestine and a small irregular lumen
  • There are many lymphatic nodules in the lamina propria