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Case 8: Histology 1, NOTE: GO OVER SEMESTER 2 GIT - Coggle Diagram
Case 8: Histology 1
The General Organization of the Gastrointestinal Tract (GIT)Describe the General Organization of the Gastrointestinal Tract (GIT)
- Organs of the Gastrointestinal Tract follow the same general tissue pattern:
- Concentric layers of tissue surround a space called a Lumen
- The wall of the GIT has two surfaces: Mucosal Surface and Serosal Surface
- Mucosal surface faces the Lumen and the Serosal surface faces the Blood
- Organs of the Gastrointestinal Tract contain 4 layers:
- Mucosa which is an inner epithelium
- Submucosa which is a layer of connective tissue
- Muscularis externa which is a layer of smooth muscle
- Adventitia (Serosa) which is an outer layer of connective tissue
Describe the features of the Mucosa
- Mucosa consists of three layers:
- Lining of Epithelium
- Lamina Propria
- Muscularis mucosa
- Lining of Epithelium
- Mucosal epithelium is Simple or Stratified depending on the Location, Function and Level of Stress
- Oral cavity, Pharynx, Esophagus and Anal canal are lined by the Stratified Squamous Epithelium
- This is because these regions undergo Mechanical Stress
- Stomach, Small Intestine and most of the Large Intestine are lined by the Simple Columnar Epithelium
- This is because these regions are responsible for Absorption
- Mucosal Epithelium has folding that increases the surface area for absorption
- Longitudinal folds disappear as the digestive tract fills
- Permanent Transverse Folds are called plicae circulares
- Lamina Propria
- Lamina Propria of the Mucosa is made up of Loose Areolar connective tissue and Reticula connective tissue
- Tissue contains:
- Sensory nerve endings
- Blood Vessels
- Lymphatic vessels
- Smooth muscle cells
- Scattered areas of Lymphoid Tissue
- Lamina Propria nourishes the Epithelium and absorbs the nutrients
- Lymph nodes (part of the GALT) are important in the defense against bacteria
- Muscularis Mucosa
- Muscularis Mucosa is a narrow band of Smooth Muscle and Elastic fibres
- Smooth muscle cells are arranged in two concentric layers:
- Inner layer encircles the Lumen: Circular Muscle
- Outer layer contains muscle cells arranged parallel to the Tract: Longitudinal layer
List the functions of the Mucosa
- The three main functions of the Mucosa:
- Secretion of mucous
- Absorption of the end-products of digestion
- Protection against infectious disease
Describe the features of the Submucosa
- Submucosa is a layer of dense, irregular connective tissue
- Submucosa surrounds the Muscularis mucosae
- Submucosa has large blood vessels and lymphatic vessels
- It contains a nerve network known as the Submucosal Plexus
- Submucosal plexus innervates the Mucosa and Submucosa
- It may contain Exocrine Glands that secrete buffers and enzymes into the digestive tract
Describe the features of the Muscularis Externa
- Muscularis externa is a smooth muscle
- In most areas the Muscularis externa consists of two layers:
- Circular layer
- Circular layer is the inner layer
- Contraction of the circular layer decreases diameter of lumen
- Longitudinal layer
- Longitudinal is the outer layer
- Contraction of the longitudinal layer shortens the length of the tube
- Contractile activity produces propulsive and mixing movements
- Myenteric plexus lies between the two muscle layers
Describe the features of the Serosa
- Serosa is a serous membrane covering Muscularis Externa
- Adventitia, is a dense sheath of collagen fibres, which firmly attaches the digestive tract to adjacent structures
- Retroperitoneal organs have both an Adventitia and Serosa
- Serosa is continuous with the Mesentery throughout much of the tract
- Serosa supports digestive organs in proper place while allowing for mixing and propulsive movements
- Serosa secretes serous fluid
- Serous fluid lubricates and prevents friction between the digestive organs and the viscera
OesophagusDescribe the characteristics and functions of the Oesophagus
- Oesophagus is a muscular tube that extends from the Pharynx to the Stomach
- Primary functions of oesophagus is propulsion and a small amount of secretion of mostly mucous
- During swallowing the skeletal muscle and smooth muscle of the Muscularis undergo peristalsis allowing for the bolus of food to be pushed inferiorly
- Thick Oesophageal epithelium protects oesophagus from abrasion by food
- And also prevents any significant absorption from taking place
- When the oesophagus is empty the Mucosa and Submucosa form longitudinal folds
- When the oesophagus has food in-transit the Mucosa and Submucosa flatten allowing for the expansion of the oesophageal diameter
- Oesophagus has an Adventitia instead of a Serosa
Oesophagus HistologyList the Histological Layers of the Oesophagus
- Oesophagus has the following layers:
- Mucosa
- Non-Keratinized Stratified Squamous Epithelium
- Lamina Propria
- Muscularis mucosa
- Submucosa
- Muscularis Externa
- Inner Circular Layer
- Outer Longitudinal Layer
- Adventitia
Describe the Histological Layers of the Oesophagus
- Mucosa
- Mucosa has a Non-Keratinized Squamous Epithelium
- Epithelium provides abrasion resistance to the internal surface of the Oesophagus
- Submucosa
- Submucosa contains Sero-mucous glands
- Sero-mucous glands are compressed by a bolus to secrete mucous
- Mucous lubricates the epithelium and aids int he passage of food
- Mucosal-mucous producing glands are part of the Oesophagus closest to the Stomach
- Mucosal-mucous producing glands protect the oesophagus mucosa from stomach acid reflux
- Muscularis externa
- Muscularis externa of the the oesophagus is made up of two layers of muscle:
- Superior Third is composed of Skeletal Muscle
- Middle Third is composed of a mixture of Skeletal and Smooth muscle
- Inferior Third is composed of Smooth muscle
- At the junction between the Pharynx and the Oesophagus, the Muscularis externa is modified into a Upper Oesophageal Sphincter
- Upper Oesophageal sphincter controls the passage of bolus into the oesophagus
- At the inferior end of the Oesophagus is a Gastroesophageal Sphincter also known as a Lower Oesophageal Sphincter
- Lower Oesophageal Sphincter regulates the passage of bolus into the stomach and prevents the contents of the stomach from re-entering the oesophagus
StomachDescribe the Gross Anatomy of the Stomach
- The anatomical regions of the Stomach:
- Cardia
- Cardiac is the regions where the oesophagus empties into the stomach
- Cardia receives the bolus of food when the Gastroesophageal Sphincter relaxes
- Fundus
- Fundus is the dome-shaped top of the stomach
- Body
- Body is the largest portion of the stomach
- Pyloric antrum
- Pyloric antrum is the inferior portion of the stomach
- Pylorus
- Pylorus is the terminal portion of the stomach, which abuts the Duodenum
- Pyloric Sphincter
- Pylorus contains a Pyloric Sphincter which controls the flow of ingested food between the Stomach and Small intestine
- Rugae
- Rugae are the Longitudinal Folds of the interior surface of the Mucosa and Submucosa of the Stomach
- Rugae allow the stomach to expand considerably
Function of the StomachDescribe the Function of the Stomach
- Mechanical Digestion and Chemical Digestion
Stomach is responsible for the formation and processing of ingesting food into a thick acid fluid known as Chyme
- Secretion
- Gastric juices are secreted by the cells of the Gastric Mucosa of the Stomach contributing to chemical digestion
- Four major components of the Gastric Juice: HCl, Pepsinogen, Intrinsic Factor and Mucous
- Storage
- Propulsion/ Motility
- Innervation of the Stomach is conducted by:
- Extrinsic innervation by the ANS
- Intrinsic Innervation from the Myenteric Plexus and Submucosal Plexus
- Limited Absorption
Stomach HistologyList the Histological Stomach Layers
- Stomach has 4 tissue layers:
- Mucosa
- Gastric pits
- Lining of Epithelium
- Lamina Propria
- Muscularis mucosa
- Submucosa
- Muscularis externa
- Serosa
- Submucosa and Muscularis externa are modified to better suit the stomach's function
- Muscularis externa of the Stomach has 3 layers of smooth muscle in the body of the Stomach:
- Longitudinal smooth muscle layer
- Circular smooth muscle layers
- Oblique smooth muscle layer
- Oblique smooth muscle layer allows the stomach to perform a churning motion that pummels food into a liquid called Chyme
Describe the Histology of the Stomach
- Mucosa: Epithelium
- Mucosa of the Stomach is lined by a Simple Columnar Epithelium
- Epithelium is a secretory sheet that produces a mucous that covers the interior surface of the stomach
- Epithelium of the stomach has Gastric Pits
- Gastric Pits are shallow depressions that open into the gastric surface
- Epithelium ahs Mucous Cells
- Mucous cells of the epithelium are located at the base or neck of each gastric pit
- These Mucous cells are actively divide to replace superficial cells
Gastric PitsDescribe features of Gastric Pits
- The Mucosa of the stomach is heavily indented to form deep structures called Gastric Pits
- Columnar cells and Goblet cells are found within these Gastric Pits
- Both Columnar Cells and Goblet Cells secrete a thick mucous that lines and protects cells of the Stomach from its own secretions
- Gastric Glands are found at the base of the Gastric Pits located in the Fundus and Body of the Stomach
- Gastric Glands are multiple branched glands that contain both Endocrine cells and secrete Hormones into the bloodstream and Exocrine cells that secrete an acidic, enzyme-containing fluid called Gastric Juice into the lumen of the stomach
Gastric GlandsDescribe the types of cells in a Gastric Gland
- There are 4 main types of cells in Gastric Glands, each of which secrete a different product
- From Deep to Superficial, these cell types are as follows:
- Enteroendocrine cells
- Enteroendocrine cells are located at the bottom of the glands
- Enteroendocrine cells secrete hormones that influence digestion
- Enteroendocrine cells called G Cells secrete the hormone Gastrin
- Gastrin stimulates secretions from other cells known as Parietal cells
- Chief cells
- Chief cells secrete an inactive precursor enzyme Pepsinogen
- When Pepsinogen encounters an acidic pH, it becomes an active enzyme Pepsin, which begins protein digestion in the stomach.
- Chief cells also secrete Gastric Lipase
- Parietal Cells
- Parietal cells secrete Hydrochloric acid (HCl), which is responsible for the acidic pH of Gastric Juice
- Acid is an important component of Gastric juice because:
- It activates Pepsinogen
- Destroys many disease causing organisms
- Stimulates Parietal Cell production of chemical Intrinsic Factor which is required for intestinal absorption of Vitamin B12
- Mucous Neck cells
- Mucous Neck Cells are located near the top or 'neck' of the Gastric Gland
- Mucous Neck cells secrete acidic mucous that prevents the neutralization of acid produced by Parietal cells
Stomach GlandsDescribe the Types of Stomach Glands
- There are three types of stomach glands:
- Cardiac Glands
- Cardiac Glands are located at the Gastro-esophageal Junction
- Cardiac glands are lined by a Simple Columnar Epithelium
- Cardiac Glands are Simple Tubular Glands with a coiled end
- Cardiac Glands open or empty into the Gastric pits
- Epithelium of Cardiac Glands is mainly a mucous-secreting epithelium
- Mucous provides protection against Gastric Juice
- Cardiac Glands secrete Lysozyme, HCO3-
- Corpus-Fundic Glands
- Corpus-Fundic Glands are located in the Body or Corpus of the stomach
- Corpus-Fundic Glands are 2-7 glands that open into a single gastric pit
- Corpus-Fundic glands are a major contributor to Gastric Juice
- Corpus-Fundic glands secrete H+, Cl-, Intrinsic Factor, Pepsinogen, Lipase and HCO3-
- Pyloric Glands
- Pyloric Glands are located in the Pylorus
- Gastric Pits of the Pyloric Glands are deeper, with a larger lumen and are highly branched
- Pyloric Glands are scattered endocrine cells:
- G cells which secrete Gastrin
- D cells which secrete Somatostatin which inhibits the secretion of Gastrin
- Pyloric glands secrete Mucous, Pepsinogen, gastrin, Somatostatin and HCO3-
The Structure of Stomach GlandsDescribe the structure of the Stomach Glands
- The Stomach Glands are made up of:
- Body
- Body is the End-portion of the gland
- Body contains Enteroendocrine Cells and Enterochromaffin-like Cells (ECL)
- Enteroendocrine cells that monitor the contents of the gland lumen and release their hormones based on their sensing response
- Enteroendocrine cells called G Cells secrete hormone Gastrin
- Enterochromaffin-like Cells (ECL) are a types of neuroendocrine and enteroendocrine cell
- Enterochromaffin-Like Cells (ECL) aid in the production of Gastric acid via the release of Histamine
- Chief cells are located in the lower portion of the Body
- Parietal cells are numerous in the upper portion of the Body
- Neck
- Mucous-neck cells are located in the narrow portion of the gland near the gastric pit.
- This narrow region is known as the Neck of the Gland.
- Parietal cells are also found the Neck
- Pit
- Gastric Pit is lined by a Simple Columnar Epithelium
- Lamina Propria contains Collagen and Reticular fibers
- Gastric Pit contains Surface Mucous Cells that line the surface of the stomach and the pits
- Surface-mucous cells differ from Goblet cells because their nucleus is oval-shaped and their mucous is stored in multiple small droplets
- Surface-mucous cells produce a thick mucin of Bicarbonate ions (HCO3-) than protects the internal surface of the stomach from the acidic Gastric Juice
- Pyloric Glands contain: Enteroendocrine cells, Gastrin-secreting G cells in particular which are abundant in the Antrum Pyloric Region
Histology of the Stomach: Parietal CellsDescribe the function and the mechanisms of the Parietal cells
- Parietal cells are responsible for the production of Hydrochloric acid (HCl) which is responsible for the acidic pH of Gastric Juice
- Parietal Cells of the stomach have a Proton Pump (H+/K+ ATPase) that is responsible for the secretion of HCl acid
- Proton Pump is located on the Apical membrane of the Parietal Cell
- Proton Pump of the Parietal cell uses ATP Hydrolysis to pump Hydrogen ions to lumen in exchange for Potassium
- When the Parietal cell is resting and is not stimulated the proton pump is located in the Tubulovesicles inside the cell
- When the Parietal cell is stimulated, the Tubuloesicles translocate and fuse with the plasma membrane
- Fusion increases the surface area of the membrane and the number of proton pumps in the membrane
Outline the three distinct features of Parietal cells
- Parietal cells have three distinct features:
- Abundant Mitochondria
- Mitochondria provide ATP to pump Hydrogen ions (H+) into the lumen of Secretory Canaliculus
- Secretory Canaliculus
- Secretory Canaliculus is an invagination of the Apical Surface and it is continuous with the Gastric Gland
- Secretory Canaliculus are lined by microvilli
- Tubulovesicles
- Tubulovesicles are enriched with H+, K+ dependent ATPase-rich (Proton Pumps)
Hydrochloric Acid Production in parietal cellsExplain the process of Hydrochloric Acid production in the Parietal Cells
- Carbon Dioxide produced from Aerobic metabolism combines with Water in the presence of the Carbonic Anhydrase enzyme inside the Parietal Cell
- This results in the formation of carbonic Acid, H2CO3
- Carbonic Acid (H2CO3) dissociates spontaneously in the parietal cells resulting in Hydrogen Ions (H+) and Carbonate Ions (HC03-)
- Carbonate ions (HCO3-) are transported through basolateral membrane out of the parietal cells into the Lamina propria of blood capillary via the Anion Exchanger (Transport Protein)
- Anion Exchanger exchanges Bicarbonate ions (HCO3-) for Chloride ions (Cl-)
- Chloride ions (Cl-) are then transported out of the parietal cells into the lumen of the stomach via the Cl- Channel on the apical membrane of the cell
- Outflow of Bicarbonate ions (HCO3-) results in an elevation of the blood pH. This is known as the Alkaline tide
- This helps to maintain the intracellular pH in parietal cells
- Hydrogen ions (H+) that were formed when the Carbonic acid (H2CO3) spontaneously dissociated in the parietal cells are transported into the lumen of the stomach via the proton pump (H+/K+ ATPase)
- Proton pump uses energy from ATP hydrolysis to exchange H+ with Potassium ions (K+)
- Potassium ions are then transported into the parietal cells
- As a result, Hydrogen ions (H+) and Chloride ions (Cl-) are present in the stomach lumen
- And the opposite charges of the two ions, causes the two ions to be attracted to each other
- Resulting in the formation of Hydrochloric acid
HCl and Bicarbonate SecretionRole of HCl and Bicarbonate secretion in pH of Mucous Blanket
- HCl and Pepsin are secreted by Parietal cells and Chief cells respectively into the gastric pits
- This results in an aggressive component
- Carbonate ions produced by the Surface-Mucous cells combine with the Hydrogen ions (H+) from the Gastric juices
- This results in the formation of Carbonic Acid. in the mucous blanket.
- The Carbonic acid then spontaneously dissociates into Carbon Dioxide and Water (H20)
- Carbon Dioxide (CO2) causes an increase in the pH from 2 to 7 in the mucous blanket
- Therefore, the mucous blanket is a protective layer of the internal surface of the stomach against the acidic Gastric Juice
The Regulation of HCl SecretionDescribe the way in which of HCl secretion can be increased
- There are three ways in which HCl acid production can be increased:
- Acetylcholine (ACh)
- Acetylcholine is produced by an enteric neuron
- The ACh then binds to an Acetylcholine receptor on the surface of the parietal cell.
- This stimulates the HCl acid production by the parietal cell
- Gastrin
- Gastrin is produced by the G cell, an Enteroendocrine cell
- Gastrin binds to a Gastrin receptor on the surface of the parietal cell.
- This simulates the HCl acid production by the parietal cell
- Histamine
- Histamine is produced as follows: G cell produces hormone Gastrin into the blood.
- An increase in the Blood Gastrin levels stimulates the Enterochromaffin-like cell (ELC) to secrete Histamine
- Histamine binds to the Histamine receptor on the surface of the parietal cell
- This simulates the HCl acid production by the parietal cell
- Cimetidine is a Histamine receptor antagonist.
- Cimetidine is used to treat heartburn and peptic ulcers by blocking the secretion of HCl acid
The Regulation of HCl SecretionDescribe the way in which of HCl secretion can be decreased
- Somatostatin is a hormone which is secreted by the Delta or D cells
- Somatostatin can decrease the secretion of HCl acid from parietal cells in three ways:
- Somatostatin inhibits the G cells from secreting hormone Gastrin
- This inhibits the Gastrin from binding to the Gastrin receptor on the surface of the parietal cells
- As a result, this inhibits the secretion of HCl acid.
- If the Blood Gastrin levels increase, Somatostatin inhibits the Enterochromaffin cell from secreting Histamine
- This inhibits the Histamine from binding to the Histamine receptor on the surface of the parietal cell
- As a result, this inhibits the secretion of HCl acid.
- Somatostatin directly binds to the Parietal cells and inhibits the secretion of HCl acid.
Chef Cells of the StomachOutline the structure of a Chief Cells
- Chief cells have the following structure:
- Apical Microvilli
- Zymogen granules containing Pepsinogen
- Mitochondria
- Golgi Apparatus
- Rough Endoplasmic Reticulum in the basal region
- Nucleus
- Basal Lamina
Describe the function of Chief Cells
- Chief cells are cells located in the lower part of the Gastric Glands
- Chief cells are responsible for storing an inactive precursor pro-enzyme called Pepsinogen within the Zymogen Granules
- Chief cells are responsible for the secretion of Pepsinogen into the Gastric Gland lumen.
Describe the activation of Pepsinogen into Pepsin
- Chief cells secrete an inactive precursor pro-enzyme called Pepsinogen
- Pepsinogen is the precursor for Pepsin
- Parietal cells secrete Hydrochloric acid (HCl) which creates an acidic environment in the stomach
- Once the Pepsinogen encounters the acidic environment, HCl cleaves Pepsinogen into Pepsin
- Pepsin (Proteolytic Enzyme) begins the digestion of Proteins into Peptides
- pepsin also acts on the Pepsinogen forming Pepsin. This is a positive feedback loop.
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