MED-202 GI System Digestive I II Fall 2023 PDF

Gastrointestinal (GI) System: Digestive System I and II MED-202 Histology/Embryology I Fall 2023 Annita Achilleos, PhD  Reading Material Wheater’s Functional Histology: Chapter 14 Lecture Material  Learning Objectives...

Gastrointestinal (GI) System: Digestive System I and II MED-202 Histology/Embryology I Fall 2023 Annita Achilleos, PhD  Reading Material Wheater’s Functional Histology: Chapter 14 Lecture Material  Learning Objectives Describe the general histologic features of the tongue, and the salivary glands. Outline the different histological features of the upper GI tract (eosophagous and stomach) and describe the function of the accessory cells. Outline the different histological features of the lower GI tract (small and large intestine, rectum) and describe the function of the accessory cells.  Gastrointestinal System The function of the gastrointestinal system is to break down food for absorption into the body. This process occurs in five main phases: 1. Ingestion 2. Fragmentation 3. Digestion 4. Absorption 5. Elimination of waste products Gastrointestinal system 5  Overview of the Digestive System The digestive system consists of: a. alimentary canal (of GI tract) b. associated organs − Tongue − Teeth − Salivary glands − Pancreas − Liver − Gallbladder As it passes through the alimentary canal, food is broken down physically and chemically and the degraded products can be absorbed into the body. Ø the various parts of the alimentary canal are morphologically specialized for digestion and absorption. Undigested food are excreted as faeces. Histology of the Alimentary Canal  Oral Cavity: Tongue Tongue − a muscular organ − projects into the oral cavity Lingual muscles (tongue muscles) − voluntary muscles − function in human speech, digestion and swallowing The dorsal surface of the tongue is covered in numerous lingual papillae Ø mucosal elevations responsible for the sensation of touch (also give the tongue its texture) Four types of papillae − circumvallate (or vallate) − Fungiform − Filiform − Foliate − All except the filiform papillae are associated with taste buds  Oral Cavity: Tongue Types of papillae: Oral Cavity: Tongue  Oral Cavity: Taste buds Taste buds are present in fungiform, foliate and circumvallate papillae. They contain three cell types: Sensory cells (neuroepithelial), supporting cells and basal cells  Oral Cavity: Salivary Glands MAJOR: – Parotid gland: − Located outside the oral cavity – Submandibular gland: – Located under the floor of the mouth – Mixed glands mostly serous. – Sublingual gland: – Located in the floor of the mouth anterior to the submandibular – Mixed glands mostly mucous. MINOR: – Lingual, Labial, Buccal, Molar, Palatine Parotid Submandibular Sublingual gland gland gland  Oral Cavity: Salivary Glands The structural unit of the salivary gland is the salivon, consisting of: 1. Acinar cells - secrete the initial fluid and mucin 2. Intercalated ducts - form a short connection to the striated ducts 3. Striated ducts - composed of epithelial cells 4. Myoepithelial cells - contract to expel salivary fluid into the oropharynx  Saliva Saliva includes the combined secretions of all the major and minor salivary glands. Functions: – Moistening of the mucosa and food – Dissolve food material – Buffering of the oral cavity – Digestion of carbohydrates – Immunological control. Regulating bacterial flora of the oral cavity.  Oesophagus Oesophagus − a fibromuscular tube − about 25 cm long in adults Ø starts from the pharynx Ø travels behind the trachea and heart Ø passes through the diaphragm Ø empties into the uppermost region of the stomach  Oesophagus The wall of the esophagus consists of: 1. Mucosa - stratified squamous epithelium 2. Submucosa - connective tissue 3. Muscularis Externa - layers of muscle fibers between layers of fibrous tissue − mostly smooth muscle − striated muscle in its upper third − two muscular rings or sphincters in its wall 4. Subserosa/adventitia − outer layer of connective tissue − a serous membrane consisting of: − simple squamous epithelium (mesothelium) − thin connective tissue  Oesophagus Food is transported through the oesophagus by peristalsis Auerbach’s plexus Submucosa − myenteric plexus − between the outer and inner muscle layers Muscularis propria − creates peristaltic activity − the major nerve supply to the gastrointestinal tract Longitudinal − controls GI tract motility muscular layer Adventitia Auerbach’s plexus  Gastroesophageal junction The gastroesophageal junction (GEJ), − the point where the distal esophagus joins the proximal stomach − commonly exposed to the injurious effects of GERD (gastroesophageal reflux disease) and/or Helicobacter pylori infection − Barrett's esophagus is a condition in which the epithelium of the esophagus at the GEJ becomes damaged by acid reflux, which causes the lining to thicken and become red − associated with an increased risk of developing esophageal cancer Gastroesophageal junction  Stomach Stomach − lies beneath the diaphragm − it receives food from the esophagus − partially digested food mixed with gastric secretions leaves the stomach and is called chyme Histologically is divided into three regions based on the types of glands present: – Cardiac region (cardia): − the part near the esophagous − contains cardiac glands – Fundic region (fundus): – the part situated between the cardia and the pylorous – contains fundic and cardiac glands – Pyloric region (pylorous): – the part proximal to the pyloric sphincter – contains pyloric glands  Gastric mucosa − a number of longitudinal folds called rugae Ø poorly developed in the uppers portion Ø Allow the organ to distend  Gastric mucosa Same basic structure as oesophagus. Gastric pits  Gastric Pits Gastric pits − openings in the mucosal surface − denote entrances to the tubular shaped gastric glands − they are deeper in the pylorus than they are in other parts of the stomach − there are several millions of these pits in the human stomach Gastric mucosa  Gastric mucosa Gastric mucosa Surface − the epithelium that lines the gastric pits mucus − simple columnar cells − contains 4 types of terminally differentiated cells a. Surface mucous foveolar (pit) cells b. Oxyntic (parietal) cells - secrete acid (HCL) c. Zymogenic (chief) cells - secrete Mucus cells pepsinogen - not activated until pH is low Parietal cell d. Enteroendocrine cells – hormone-secreting Surface mucous cells − have mucous granules on their apical surface Entero- − the mucous secretion is rich in bicarbonate endocrine ions which protect the epithelium from the cell acidic content of the gastric juice.  Gastric mucosal barrier Safe containment of gastric acid required for digestion. The barrier consists of three protective components: 1. Compact epithelial cells bound by tight junctions. Ø repel harsh fluids that may injure the stomach lining. 2. Surface mucous foveolar (pit) cells − insoluble mucus forms a protective gel-like coating over the entire surface of the gastric mucosa Ø The mucus protects the gastric mucosa from autodigestion 3. Bicarbonate ions, secreted by the surface epithelial cells. Ø act to neutralize harsh acids Ø If the barrier is broken acid diffuses back into the mucosa where it can cause damage to the stomach itself. Gastric mucosal barrier  Gastric Glands Gastric glands − located in different regions of the stomach – in the stomach lining (mucosa) − fundic glands, the cardiac glands, and the pyloric glands − secrete mucus, pepsinogen, hydrochloric acid, intrinsic factor, gastrin, and bicarbonate Gastric glands mucosa  Gastric Glands 1. Cardiac glands − found in the cardia of the stomach − primarily secrete mucus − they are fewer in number than the other gastric glands and are more shallowly positioned in the mucosa 2. Fundic glands (or oxyntic glands) − found in the fundus and body of the stomach − almost straight tubes − secrete hydrochloric acid (HCl) Cardiac glands 3. Pyloric glands − located in the antrum of the pylorus − secrete gastrin Fundic glands Pyloric glands Gastric Glands 30  Small Intestine Small intestine − the longest component of the digestive tract (>6m) − the principal site for the digestion and absorption of food − enterocytes (small intestine cells) secrete enzymes that contribute to the digestive process Divided anatomically into three regions: 1. Duodenum: 25 cm long 2. Jejunum: 2.5m long 3. Ileum: 3.5m long  Small Intestine Small intestine − absorptive surface due to specialization of the submucosa and mucosa: Plicae circularis (circular folds): − Permanent transverse folds of the intestinal surface Villi: − Unique, finger-like projections of the mucosa − Completely cover the surface of the small intestine. Absent in stomach. Microvilli: − Each enterocyte posses several thousand microvilli in the apical surface − Major amplification structure of the luminal surface  Small Intestine - Villi Villi consists of: − a core of loose connective tissue covered by simple columnar epithelium − lamina propria (connective tissue under the epithelium) that contains a central lymphatic capillary called the lacteal − Lieberkuhn’s crypts: − intestinal simple tubular glands − originate from the muscularis mucosa − covered with simple columnar epithelium that is continuous with the villi epithelium intestinal villi lacteal  Small Intestine - Villi Lieberkuhn’s crypts: − found in between villi in the intestinal epithelium lining of the small intestine and large intestine − they contain − enterocytes (absorbing water and electrolytes), goblet cells (secreting mucus), enteroendocrine cells (secreting hormones), cup cells, tuft cells, and at the base of the gland, Paneth cells (secreting anti-microbial peptides) − stem cells Ø replenish the cells lost due to abrasion, as well as enteroendocrine cells to synthesize and secrete hormones lumen  Small Intestine Cells types found in the intestinal mucosa: Enterocytes: Secretory and absorptive functions Goblet cells: Mucous secreting cells Paneth cells: Secrete antimicrobial substances. Enteroendocrine cells: releasing various hormones. M cells (microfold cells): modified enterocytes that cover the lymphoid nodules.  Small Intestine Histologic differences between the duodenum, jejunum, ileum Duodenum − Brunner’s glands (or duodenum glands − found only in the duodenum − produce alkaline secretion, pH 8.8 to 9.3, Ø neutralises the acidic chime from the stomach − ovilli are short and broad (leaflike)  Small Intestine Histologic differences between the duodenum, jejunum, ileum Jejunum − long finger-like villi − well-developed lacteal Ø a lymphatic capillary that absorbs dietary fats and triglycerides − no Brunner’s glands in submucosa  Small Intestine Histologic differences between the duodenum, jejunum, ileum Ileum − Peyer’s patches: lymphoid follicles (nodules) in the mucosa and part of the submucosa (GALT: gut-associated lymphoid tissue) − Lack of Brunner’s glands − Shorter finger-like villi  Large Intestine The large intestine is composed of − cecum with its projection the appendix − colon − rectum − anal canal  Large Intestine Function Large intestine − completes absorption − retrieves water and sodium from the luminal contents which become fecal residue − secretes large amounts of mucus and some hormones − no digestive enzymes − thick mucosa − has deep crypts, but no villi − columnar absorptive epithelial cells, such as goblet cells, endocrine cells and basal stem cells, but no Paneth cells − the surface epithelial cells are shed into the lumen, and have to be replaced about every 6 days − the lamina propria and submucosa are similar to the small intestine − muscularis externa − smooth muscle arranged in three longitudinal bands called taenia coli − at the anus, the circular muscle forms the internal anal sphincter.  Large Intestine - Mucosa Mucosa − “smooth” surface − it has circular folds − villi are NOT present − contains numerous straight tubular glands that extent through the full thickness of the mucosa − covered by simple columnar epithelium Large Intestine - Mucosa Small Vs Large Intestine Stomach vs Small vs Large Intestine  Rectum Rectum (7-10 cm): – can be distinquished by the presence of transverse rectal folds (or Huston Valves). Anal canal is divided into three parts − The zona columnaris: − the upper half of the canal − lined by simple columnar epithelium Rectum − The lower half of the anal canal is divided into two zones: − zona hemorrhagica: stratified squamous non-keratinized − zona cutanea: stratified squamous keratinized epithelium − separated by Hilton's white line – Keratinized cells are specially structured to be waterproof and reduce evaporation from underlying tissues.  Anal Canal Anal canal (7-10 cm): − the terminal part of the large intestine − divided into three zones: − Colorectal zone: simple columnar epithelium − Anal Transitional Zone: Transition between simple columnar to stratified squamous epithelium − Squamous zone: Stratified squamous epithelium. Rectum and Anal Canal  Summary 1. Components of the digestive system. 2. Basic organisation of the alimentary canal. 3. Basic histology of the tongue. 4. Salivary gland histology. 5. Histology of the oesophagus 6. Stomach, divisions, glandular distribution, mucosal histology and function. 7. Small and large intestine mucosal organisation. 8. Key mucosal histological differences between stomach, small and large intestine. Thank you