Lecture 16 (H) Digestive Tract and Associated Glands - PDF

Unified Lecture 16 Digestive Tract and Associated Glands HIHD-211 TERM-3 Basic Science Department COSHP, KSAU-HS, KSA LEARNING OUTCOMES By the end of this session, you should be able to: 1. Describe the histological characteristics of the layers comprising each segment of the 2. 3. 4. 5. 6. 7. gastr...

Unified Lecture 16 Digestive Tract and Associated Glands HIHD-211 TERM-3 Basic Science Department COSHP, KSAU-HS, KSA LEARNING OUTCOMES By the end of this session, you should be able to: 1. Describe the histological characteristics of the layers comprising each segment of the 2. 3. 4. 5. 6. 7. gastrointestinal tract and describe how they relate to their function Name and describe four transitional junctions in the GI tract Describe the histological features of esophagus Describe the topography of the gastric gland, its component cells, and architectural differences between glands in the three regions of the stomach Describe the structure of the small intestine, how its surface area is maximized, and the cells that comprise its epithelium Contrast the histological appearance of the large intestine from that of the small intestine Define the concept of the classic liver lobule and recognize it in histological tissue sections 8. Recognize the cells of the liver tissue: hepatocytes, Kupffer cells,portal tract,liver stroma 9. Discuss the histological features of the gall bladder 10.Discuss the histological features of pancreas Digestive Tract ▪ The digestive tract is a muscular tube lined by mucous membrane extends from oral cavity to anus ▪ Also known as Gastrointestinal Tract (GIT) or Alimentary Canal ▪ It consist of the following parts:  Mouth  Pharynx  Esophagus  Stomach  Small intestine  Large intestine  Anus LO-1 Histological Organization of GIT Wall of the GIT consists of four distinct layers: 1. ▪ ▪ Mucosa, 2. Submucosa, 3. Muscularis, 4. Serosa Mucosa: An inner lining of gut, consisting of: ▪ Epithelium ▪ Lamina propria: supporting tissue ▪ Muscularis mucosae: thin layer of smooth muscles for local movements and folding of mucosa In the GIT mucosa undergoes abrupt transition from one form to other at four points. Theses sites are: ▪ Gastroesophageal junction ▪ Gastroduodenal junction ▪ Ileocecal junction ▪ Recto-anal junction LO-1 Histological Organization of GIT ▪ Submucosa: A loose collagenous connective tissue layer containing large blood vessels and nerves (Submucosal plexus / Plexus of Meissner) and lymphatics Muscularis externa (propria): Smooth muscles made of an inner, circularly arranged muscles layer and an outer longitudinally arranged muscles layer. The Myenteric plexus (Auerbach plexus) of nerves lies in between these two layer s of the muscles. They produce peristaltic movements Serosa / adventitia: An outer fibrous connective tissue layer LO-2 Esophagus ▪ A hollow muscular tube extending from pharynx to the stomach ▪ Conveys solid masticated food and liquids to stomach Histologically, esophagus has 3 layers: Mucosa: a deeply folded layer Lined by stratified squamous epithelium Lamina propria contains lymphoid aggregates LO-4 Esophagus Submucosa loose elastic tissue (for distension) Contain sero-mucous glands Muscularis propria Upper one third has skeletal muscles and lower part has smooth muscles LO-4 Gastro-oesophageal Junction ▪ At gastroesophageal junction there is abrupt transition of stratified squamous epithelium of esophagus to tightly packed glandular epithelium of the stomach LO-3 The Stomach Anatomical regions of the stomach ▪ Cardia: small area of mucous secreting glands surrounding entrance of esophagus ▪ Fundus: a dome like upward convexity on the left side of cardia ▪ Gastric body (Corps): main part of the stomach between cardia and pyloric antrum. Glands of these two regions secrete acid-pepsin gastric juices & mucous ▪ Pylorus: narrow part between antrum and pyloric sphincter. Contain mucous and neuroendocrine cells that secrete mucus and gastrin LO-,4 Structure of a Gastric Gland ▪ The mucosa of the fundus and body of the stomach consists of straight tubular glands that synthesize and secrete gastric juice Surface mucous cells (Mu) cover the luminal surface of the stomach and partly line the gastric pits Neck mucous cells are squeezed between the parietal cells in the neck and base of the gastric glands Parietal or oxyntic cells (P) are distributed along the length of the glands. These large rounded cells have an extensive eosinophilic (oxyntic) cytoplasm and a centrally located nucleus. Parietal cells secrete gastric acid as well as intrinsic factor Chief cells: secretes pepsinogen LO-4 The cardia contains highly convoluted glands that primarily produce mucus. The cardia is a circumferential ring about 3 centimeters deep distal to the gastro-esophageal junction. Its glands tend to be convoluted and are lined primarily by mucus-secreting cells that lubricate the incoming food and protect the lining of the stomach near the gastroesophageal junction. Below the glands of the mucosal layer are the lamina propria and muscularis mucosa, which are responsible for support and folding, respectively. 11 LO-,4 Body Of The Stomach ▪ Mucosa (M) of the stomach is thrown into prominent longitudinal folds, the Rugae ▪ Epithelium: Simple columnar epithelium with gastric glands that extend from muscularis mucosae (MM) and open in lumen via gastric pits or foveoli ▪ Submucosa: loose and distensible containing blood vessels and nerves ▪ Three layers of smooth muscles  Inner oblique layer  Middle circular  Outer longitudinal LO-4 The body contains long, straight glands that produce acid and pepsinogen. Antrum The antrum is the distal third of the stomach. It possesses glands with deeper pits and large amounts of coiling and branching. These glands contain many mucus-secreting cells, D-cells, and G-cells but lack parietal cells. The antrum is connected to the initial segment of small intestine by short section of stomach called the pylorus. 13 LO-,4 Gastro-duodenal Junction The pyloric sphincter (PS) consists of a marked thickening of the circular layer of the muscularis at the gastroduodinal junction The pyloric sphincter PS marks a sharp transition from the glandular mucosa of the stomach S to the villous mucosa of the duodenum D LO-3 D Small Intestine ▪ LO-3 Comprise of duodenum, jejunum and Ileum ▪ It is a principal site of absorption Enormous surface area of small intestine is due to:  Small intestine is extremely long (4-6 meter)  Mucosa and submucosa is thrown into circular folds, the plicae circulares  Presence of a series of fingerlike projections villi, lined by simple columnar epithelium in mucosa of small intestine  The cells called enterocytes are covered with thousands of microvilli LO-5 Small Intestine: Duodenum The duodenum represents the first part of the small intestine and receives partly digested food in the form of acidic chyme The main function of the duodenum is to neutralize gastric acid and pepsin and to initiate further digestive processes The mucosa M has the characteristic villous form with short glands known as crypts of Lieberkühn extending down to the muscularis mucosae MM The feature unique to the duodenum is coiled branched tubular Brunner's glands B found mainly in the submucosa SM The muscularis propria of the duodenum consists of an inner circular layer CM and an outer longitudinal layer LM LO-5 Small Intestine (Ileum) ▪ Ileum ▪ The ileum has the shortest villi and is characterized by abundant Peyer’s patches in the submucosa. Peyer’s patches are diffuse lymphoid tissue that play an important immunological role in sampling the contents of the GI tract. The ileum lack plicae circulares but contains Peter’s patches in its submucosa. LO-5 Small Intestine Villi And Crypts ▪ The intestinal villi are lined by a simple columnar epithelium , Villi are out foldings of lamina propria and epithelium that contain mostly enterocytes and goblet cells. ▪ which is continuous with that of the crypts The base of a villus that is closest to the submucosa is know as the crypt of Lieberkuhn ▪ Cell types in the small intestine epithelium include: ▪ Enterocytes: Most numerous tall columnar cells with surface microvilli (brush border). These cells are the main absorptive cells LO-5 Goblet cells: are scattered among the enterocytes; produce mucin for lubrication and protection Paneth cells are found at the base of the crypts and (eosinophilic). These cells have a defensive functions Neuroendocrine cells produce locally acting hormones that regulate gastrointestinal motility and secretion Stem cells are found at the base of the crypts 19 Distinguishing Features Of Different Parts Of The Small Intestine ▪ Duodenum: Presence of the submucosal, Brunner’s glands & longer villi ▪ Jejunum and proximal ileum: Presence of the plicae circulares (submucosal folds) Ilium: Lymphoid cells and peyer’s patches are prominent in ileum Goblet cells increase distally ▪ ▪ LO-5 Ileo-cecal junction at the transition from small intestine to large intestine There is an abrupt transition in the lining of the valve from the small intestinal villiform pattern S to the glandular form in the large intestine L The ileocaecal valve consists of a thickened extension of the muscularis propria M that provides robust support for the mucosa Lymphoid tissues Ly in the form of large Peyer's patches are found in the mucosa 21 LO-3 Large Intestine Extends from the terminal ileum to the anus Main function is water absorption (recovery) from faeces and propulsion of solid faeces to rectum The mucosa is the same from caecum to rectum It does not exhibit distinct plicae circulares like those of the small intestine Consistent with its functions of water absorption and faecal lubrication, the mucosa consists of cells of two types: absorptive cells mucus-secreting goblet cells LO-6 Colon The epithelial (mucosal) cells are arranged in closely packed straight tubular glands or crypts, which extend to the muscularis mucosae MM cells Goblet cells predominate in the base of the glands, whereas the luminal surface is almost entirely lined by columnar absorptive cells Lamina propria & Sub mucosa contain scattered lymphocytes and small aggregates LO-6 Appendix ▪ Also called Vermiform Appendix is a blind ended tubular sac from caecum ▪ Suspensory mesentery M continues with the outer serosal layer S. ▪ The most characteristic feature is the presence of masses of lymphoid tissue in the mucosa and submucosa ▪ The lamina propria LP and upper submucosa SM are diffusely infiltrated with lymphocytes ▪ The lymphoid tissue also forms follicles F LO-6 Recto-anal Junction Recto-anal junction at the transition from rectum to anus The rectum is characterized by the same columnar epithelium that lines the majority of the gastrointestinal tract’s secretory and absorptive areas. The anus, on the other hand, has a stratified squamous epithelium that provides a greater deal of protection to the underlying tissue. 25 LO-3 GIT Epithelium Cell types Distinctive features Esophagus St. squamous Squamous cells Sub mucosal glands Stomach Body & fundus Glandular- straight tubular Mucous cells Parietal & chief cells Sparse lymphoid cells No aggregates Stomach Pylorus & cardia Glandular- branched tubular Mucous cells Occ parietal Sparse lymphoid cells No aggregates Duodenum Glandular- villi & crypts liberkuhn Enterocytes Paneth cells Goblet cells Brunner’s glands Plicae circulares Jejunum & ileum Glandular- villi & crypts liberkuhn Colon & rectum Glandular- straight & crypts Goblet cells Absorptive cells Teniae coli Appendix Glandular- straight & crypts Goblet cells Tall columnar cells Prominent lymphoid tissues Anus Stratified squamous Squamous cells Column of morgagni LO-2,3 Plicae circulares Peyer’s patches The main functional cell in the liver is hepatocyte These cells are arranged as thin plates separated by fine vascular sinusoids through which blood flows Blood flow into the liver sinusoids comes from terminal branches of both the hepatic portal vein and hepatic artery The hepatic portal vein brings blood that is rich in amino acids, simple sugars and other products of digestion After passing through the sinusoids, venous drainage of blood from the liver occurs via the hepatic vein into the vena cava The main blood vessels and ducts run through the liver within a branched collagenous framework termed the portal tracts which also contain the bile ducts LO-7 The outer surface of the liver is covered by a called Glisson's capsule (C) over which is a layer of mesothelial cells M from the peritoneum The sinusoids can just be seen as palestained spaces between the plates of liver cells The portal tracts P contain the main blood vessels running into the liver and branches of the bile ducts The centrilobular venules (hepatic venules) V take blood away from the liver Hepatocytes: Large polyhedral cells with round nuclei and prominent nucleoli. Some are bi-nucleated with irregularly stained cytoplasm The hepatocytes are arranged in one cell thick plates Sinusoid are lined by endothelium with scattered Kupffer cells (cells of monocyte – macrophage system) LO-7,8 P Hepatic Lobule The structural unit of the liver can be considered as a conceptually simple hepatic lobule The hepatic lobule is roughly hexagonal in shape and is centered on a terminal hepatic venule (centrilobular venule) V. The portal tracts T are positioned at the angles of the hexagon (lobule) The blood from the portal vein and hepatic artery branches flows away from the portal tract to the adjacent central veins LO-7 Portal Tract The portal tract contains three main structures. 1. The largest is a terminal branch of the hepatic portal vein PV (terminal portal venule) 2. Smaller diameter thick-walled vessels are terminal branches of the hepatic artery A 3. Bile ductules B usually located at the periphery of the tract. The bile ductules trabecular ducts which drain into the right and left hepatic ducts, the common hepatic duct Because these three structures are always found in the portal tracts, the tracts are often referred to as portal triads Surrounding the portal tract are anastomosing plates of hepatocytes H, between which are the hepatic sinusoids S LO-,8 Liver Stroma- Stromal Reticulin The structural integrity of the liver is maintained a fine meshwork of reticulin fibres (collagen type III) The reticulin meshwork supports both the hepatocytes and the sinusoidal lining cells (endothelial cells) The reticulin is present on both sides of liver cell plates The sinusoids are also bounded by the same reticulin framework The reticulin merges with the sparse collagenous supporting tissue of the portal tract T and terminal hepatic venule V LO-8 Gall Bladder  It is a muscular sac containing bile  Mucosa have folds, lined by Simple     columnar epithelium with microvilli Sub mucosa consist of loose elastic fiber Muscularis propria consists of three layers arranged in longitudinal, transverse and oblique layers Thick serosa Cystic duct Join common hepatic duct to form common bile duct. It combines with pancreatic duct to open at ampula of vater, guarded by sphincter of Oddi in the duodenum LO-9 Pancreas Pancreas is a lobulated gland covered by a thin capsule which sends septa, Sp between the lobules. It contains endocrine as well as exocrine cells. 1. Endocrine cells are scattered as islets of Langerhans in exocrine part of gland. They secrete Insulin and Glucagon 2. Exocrine cells have closely packed acini which secrete enzymes. The pancreatic enzymes are drained into pancreatic duct which join common bile duct and open into duodenum via ampulla of Vater LO-1,2 34