GIT

• Gastric glands are found within the mucosa and produce gastric juice containing hydrochloric acid, pepsinogen, and intrinsic factor.
• Generic Wall Plan of the gastrointestinal tract

  • Extends from the esophagus to the rectum
  • Consists of four standard layers with specific tissues and cell types within each layer
  • The generic plan is based on the jejunum, the middle part of the small intestine
• Absorption and diffusion pathway

  1. Absorption and diffusion occur through the epithelial lining

  2. Lipids, tissue fluids, and larger substances enter lymph capillaries (lacteals)

  3. Connective tissue cells target and destroy absorbed pathogens and antigens
• From lumen

  • CT only, no mesothelium
  • Tissues within GIT wall layers
• Adaptations for absorption

  • Macroscopically visible folds increase surface area (SA)
• Structural components of the GI tract

  • Consists of a flexible tube with essential structural components
  • Comprises a lumen facilitating the passage of contents and a flexible wall enabling movement in response to environmental conditions
• Functional considerations of the GI tract

  1. Passage through the tube involves modification, extraction, or addition to contents

  2. The GI tract features a multilayered wall structure with specific roles for each layer, tissue, and cell type
• Epithelial lining and general functions

  • Stomach to rectum: simple columnar epithelium; esophagus: stratified squamous
  • Small and large intestines lined by absorptive cells (enterocytes) and goblet cells
  • Mucus functions in lubrication, protection, and adhesion
• Mucosa - Muscularis Mucosae


  1. Thin layer of smooth muscle

  2. Allows local movement of mucosa, independent of the external muscle layers

  3. Muscle fibres extend from the muscularis mucosae into the core of the villi - contraction of muscle maximises the contact of contents in lumen with the epithelial lining
• Wall layers of the GI tract

  • Mucosa (M): epithelium, lamina propria, muscularis mucosae
  • Submucosa (SM): dense connective tissue
  • Muscularis externa (ME): inner circular sub-layer, outer longitudinal sub-layer
  • Serosa (S): connective tissue covered by mesothelium (or adventitia if no mesothelium)
• Mucosa - adaptations for absorption


  Vill
• Submucosa

  • Irregular dense CT, usually very few defence cells; connects mucosa to layers in outer wall, houses large vessels & nerves, forms core of submucosal folds
• Microvilli


  Apical surfaces of enterocytes have microfilament supported extensions of plasma membrane = microvilli; high density of microvilli forms brush border; microvillous height is approximately 1 micrometer
• Adaptations for absorption

  • Macroscopically visible folds increase surface area
  • Villi & crypts of mucosa increase surface area
  • Microvilli increase surface area of cells
• Myenteric (Auerbach’s) plexus (MP)


  • Innervates both circular and longitudinal sub-layers of smooth muscle
• Mucosa - muscularis mucosae


  • Thin layer of smooth muscle between mucosa and submucosa
  • Allows local movement of mucosa and maximises contact of contents with the epithelial lining
• Submucosa - components and general functions

  • Layer of dense (fibrous) connective tissue containing large blood and lymph vessels, glands, and lymphatic tissue; connects mucosa to outer muscle layers, conduit for vessels and nerves
• Villi & crypts of mucosa

  • Upward extensions of epithelial covered lamina propria = villi; downward extensions of epithelial lined lumen between bases of villi = crypts (glands) with specialised cells and secretory activity
• Serosa

  • CT covered by simple squamous epithelium (mesothelium); secretes slippery fluid for movement of organ independent of body wall; conduit for entry/exit of vasculature and nerves into/out of organ
• Muscularis externa


  • Two sub-layers of muscle - inner circular orientation of fibres, outer longitudinal orientation of fibres; smooth muscle from oesophagus to rectum, responsible for mixing, crushing, and propelling food along by peristalsis
• There is limited skeletal muscle in the mouth, pharynx, upper esophagus, and anus, allowing voluntary control over swallowing and defecation
• GIT is sectioned longitudinally
• Inner layer fibers


  Run circumferentially around the tube
• Sectional plane of the GIT

  • Transverse
  • Longitudinal
• Inner layer fibers are sectioned transversely
• The sectional plane of the GIT is determined by the orientation of muscle fibers in the outer-sub layer of the muscularis externa
• Outer layer fibers


  Run parallel to the long axis of the tube
• Gastrointestinal Tract: Longitudinal or Transverse Section
• Outer layer fibers are sectioned longitudinally
• INNERVATION PATHWAYS OF GIT WALL

  1. Two neural networks: submucosal plexus in the connective tissue of the submucosa, and myenteric plexus between external muscle sub-layers

  2. GIT linked to sympathetic and parasympathetic systems via extrinsic nerve fibers
• REGION-SPECIFIC NAMING OF MESENTERY

  • Mesentery: suspends the small intestine
  • Mesocolon: suspends the transverse colon
  • Lesser omentum: extends from the lesser curvature of the stomach
  • Greater omentum: extends from the greater curvature of the stomach
  • Duodenum, pancreas and parts of the large intestine are retroperitoneal organs, not suspended from a mesentery
• SEROSA-MESENTERY RELATIONSHIP

  1. Serosa is continuous with mesenteries

  2. Mesentery is a thin, double layer of peritoneum

  3. It suspends and connects the majority of the intestines to the posterior abdominal wall

  4. Links innervation, vascular supply and lymph drainage of GIT with body
• BLOOD & LYMPH VESSELS IN LAMINA PROPRIA

  Larger caliber blood vessels run through submucosal connective tissue and branch into smaller vessels that enter the lamina propria
• BLOOD AND LYMPH DRAINAGE PATHWAYS OF GIT

  1. Essential for nutrient dispersal from GIT to the body

  2. Blood drainage diverts to the liver via the hepatic portal venous system

  3. Lymph drainage occurs via the thoracic duct to the subclavian vein

  4. Extra visceral drainage connections run in mesenteries
• mesentery-peritoneum relationship

  1. Mesenteries consist of a double layer of peritoneum between organs and body wall

  2. They suspend organs and serve as a utility entry/exit pathway

  3. The peritoneal cavity contains serous fluid
• INNERVATION OF THE GIT

  1. Governed by the enteric (autonomic) nervous system

  2. Includes the submucosal (meissner’s) plexus and myenteric (auerbach’s) plexus

  3. Plexus comprises intricate networks of ganglia and connecting nerves

  4. Control functions such as peristalsis, segmentation and secretory activities (exocrine & endocrine)
• BLOOD SUPPLY AND LYMPH DRAINAGE OF GIT

  1. Vital for nutrient absorption, transport, and defense against pathogens

  2. Tissue fluid, large molecules, antigens, and lipids drain into lymphatic capillaries (lacteals)

  3. Most nutrients are absorbed into blood capillaries
• Role of muscle in swallowing peristalsis

  1. Swallowing is voluntarily initiated (requires skeletal muscle)

  2. Peristalsis is an involuntary process (utilises smooth muscle)
• Stomach mucosal regions

  • Cardia: near the junction with the oesophagus
  • Fundus and body: two thirds of the stomach with similar histological structure
  • Pylorus: distal third with deeper pits and shorter glands
• Oesophagus wall structure

  • Stratified squamous epithelial lining
  • Skeletal muscle in the proximal muscularis externa