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Case 8: Histology Part 2 - Coggle Diagram
Case 8: Histology Part 2
Small IntestineOutline the features of the Small Intestine
- Small intestine is the site of most digestion
- Small intestine has cells called Enterocytes which produce digestive enzymes, hormones and mucous and absorb nutrients
- Nutrient and Water absorption takes place in the small intestine
- Small intestine is adapted to provide a large surface area for absorption
- Small Intestine is 4-7 meters long and is divided into three sequential segments:
- Duodenum
- Jejunum
- Ileum
- Wall of the Small Intestine consists of four layers:
- Mucosa
- Submucosa
- Muscularis
- Serosa or Peritoneum
Outline the functions of the Small Intestine
- Secretion
- Digestion (Mechanical and Chemical)
- Small Intestine is the site of digestion
- Absorption
- Small intestine is the site of absorption of nutrients
- Propulsion
- Motility mixes the chyme with digestive enzymes and pancreatic secretions and exposes nutrients to the intestinal mucosa for absorption
- Small intestine propels unabsorbed chyme into the Large Intestine
Structure and Function of the Small IntestineExplain the relationship between the structure and function of the Small Intestine
- The internal surface of the Small Intestine has four degrees of folding in order to amplify the absorptive surface area of the mucosae
- The following four features contribute to the increased surface area
- Small intestine has permanent longitudinal folds that are plicae circulares (or circular folds)
- Plicae circulares slow down the transit of chyme through the small intestine, which gives the nutrients more time to be digested
- And for the cells of the small intestine called Enterocytes more time to absorb nutrients
- Intestinal Villi which are finger-like projections of the Mucosa, covering the small intestine
- Each villus consists of a layer of Enterocytes surrounding a central core of blood capillaries and a lymphatic vessel called a Lacteal
- Shape and the length of the Intestinal Villi differs amongst the regions of the small intestine
- Microvilli are evaginations on the apical domain of the enterocyte
- Crypts of Lieberkühn are formed by the invagination of the mucosa between the adjacent villi
- Crypts of Lieberkühn increase the intestinal surface area
DuodenumDescribe the features of the Duodenum
- Duodenum is the initial and smallest segment of the small intestine
- It acts as a mixing bowl because it receives chyme from the Stomach and digestive enzymes from the Liver, Gallbladder and Pancreas.
- Duodenum is arched into a "C" shape as it curves around the Pancreas where only the Proximal Portion of the Duodenum is Intraperitoneal
- Remainder of the Duodenum sits posteriorly to the peritoneal cavity and is Retroperitoneal
- Duodenum has a Major Duodenal Papilla, where secretions from the Gallbladder and the Pancreas enter into the small intestine
- Duodenal Submucosa contains specialized glands called Brunner's (Duodenal) Glands
- Brunner's Glands secrete alkaline mucous to protect duodenum from acidic chyme
- Duodenum has a few Plicae Circulares (Circular Folds) and small villi
JejunumDescribe the features of the Jejunum
- Jejunum is located between the Duodenum and the Ileum
- Jejunum makes a sharp bend as its beginning
- Jejunum is in the Peritoneal Cavity which means that it is Intraperitoneal
- It has numerous Plicae Circulares (Circular folds) and Long villi
- Majority of chemical digestion and nutrient absorption takes place in the Jejunum
IleumDescribe the features of the Ileum
- Ileum is the final and longest segment of the small intestine
- Ileum ends at the Ileocecal Valve
- Ileocecal Valve is the sphincter controlling flow from the Ileum into the Cecum of Large Intestine
- Ileocecal valve prevents the material in large intestine from flowing backward into the Ileum, critical for preventing access to bacteria from large intestine
- Ileum has a few Plicae Circulares (Circular folds) and none distal portion
- Villi are relatively stumpy
- Submucosa contains aggregated Lymphoid Nodules
Small Intestine HistologyDescribe the histology of the small intestine
- The Histology of the small intestine:
- Plicae circulares
- Villi
- Lymphatic Vessels
- Mucosa
- Mucosa is lined by a Simple Columnar Epithelium with Villi and microvilli
- Submucosa
- Muscularis externa
- Serosa
Describe the histology of the Villus
- Villus has the following structures:
- Simple Columnar epithelium containing Enterocytes
- Goblet cells
- Crypts of Lieberkühn
Small Intestine: Intestinal Blood, Lymphatic and Nerve SupplyOutline the Intestinal Blood, Lymphatic and Nerve Supply of the small intestine
- Rich Blood supply, Lymphatic Supply and Nerve Supply is derived from the Submucosal plexus
- Central Lymphatic Vessel called a Lacteal is present in the Lamina Propria of the Villus
- Lacteal absorbs dietary fats int he small intestine
- A Capillary Villus Plexus supplies the intestinal villi and the upper portion of the Crypts of Lieberkühn
- Pericryptal Capillary Plexus supplies the lower portion of the Crypts of Lieberkühn
- Myenteric Plexus and Submucosal Plexus form the Intrinsic Enteric Nervous System
- Intestinal motility such as Mixing of chyme and Movement of chyme is controlled by the Intrinsic Autonomic Nervous System
- Intrinsic Autonomic Nervous System is made up of the Submucosal Plexus of Meissner, and Myenteric Plexus of Auerbach
- Contract of the Muscularis externa is responsible for the mixing the intestinal contents and for peristaltic movements
Villi and Crypts of LieberkühnDescribe the Villi and Crypts of Lieberkühn
- Villi are the finger-like projections of the Mucosa, covering the entire internal surface of the small intestine
- Crypts of Lieberkühn (Intestinal Crypts) are the invaginations of the mucosa formed between adjacent villi
- Intestinal Mucosa and the Crypts of Lieberkühn are lined by the Simple Columnar Epithelium
- Simple Columnar Epithelium contains five major cell types:
- Enterocytes or Absorptive cells
- Goblet cells
- Paneth Cells
- Intestinal Stem cells
- Enteroendocrine cells
Describe the function of each of the five major cell types in the Simple Columnar Epithelium
- Cells in the Simple Columnar Epithelium provide intestinal protection
- Enterocytes absorb nutrients
- Goblet cells produce a viscous gel-like substance that lubricated and protects the small intestine
- Paneth cells produce anti-microbial proteins such as lysosomes and defensins that permeability bacteria and Tumour Necrosis Factor when there is tissue injury or infection
- Intestinal Stem cells replace superficial cells that have been damaged or destroyed
- Enteroendocrine cells monitor the contents of the gastric lumen and secrete hormone such as Secretin, CCK in response to their sensing receptors
MicrovilliOutline the structure and features of the Microvilli
- Microvilli are the evaginations of the Villi, and are densely packed on the Apical Surface of the Enterocytes
- This results in the formation of the Brush Border
- Structure of the microvilli has ACTIN cytoskeleton
- Brush Border Enzymes called Proteases are as follows:
- Integral membrane proteins
- Sucrase, Maltase, Lactase, Enterokinase
List the Functions of the Microvilli
- Chemical Digestion with the use of Proteases
- Absorption, microvilli increase the intestinal surface area for amplified absorption
- Lipid Processing
- Transport amino acids, Monosaccharides and Fatty Acids
Peyer's Patches in the Ileum
- Peyer's Patches are part of the GALT.
- Follicle associated epithelium is formed by Enterocytes and M cells
- Dome contains B Lymphocytes, Membrane Attack Complex, Plasma Cells
- Germinal Centre contains Plasma Cells which produce IgA and IgG
- IgA is secreted via transcytosis through enterocytes to the gut lumen
- Entry is via the post capillary high endothelial venule
- Cells and Antibodies leave via the Lymphatic Vessels
Intestinal Glands-Brunner's Glands pr Submucosal Glands
- Brunner's glands are located in the Duodenum of the small intestine
- They produce large quantities of mucous when chyme arrives from the stomach.
Large Intestine (Large Bowel)Outline the Large Intestine
- Large intestine runs along border of the abdominal cavity, surrounding the small intestine and other abdominal organs like a frame
- Large intestine has a larger diameter than the small intestine
- Large intestine receives material from the small intestine that was not digested or absorbed, and it serves as a passage way for faeces or faecal matter to exit the body
Outline the function of the Large Intestine
- Active in absorbing Water and Electrolytes, it also absorbs Vitamin B and K
- And it is critical in maintaining Fluid, Electrolyte and Acid-Base Homeostasis
- Plays a role in the Secretion of mucous, Propulsion and Defecation
- Houses numerous bacteria that perform important functions such as synthesising Vitamins
- Does not produce Enzymes
Histology of the Large IntestineDescribe the Gross Anatomy of the Large Intestine
- Large Intestine has the following gross anatomical structures:
- Vermiform appendix
- Caecum
- Ileocecal valve
- Ascending Colon
- Hepatic Flexure
- Splenic Flexure
- Taenia coli
- Haustrum
- Transverse Colon
- Descending Colon
- Epiploic appendages
- Sigmoid colon
- Rectum
Histology of the Large IntestineDescribe the Tissue Layers of the Large Intestine
- Large intestine is made up of the following layers:
- Mucosa
- Mucosa is lined by a Simple Columnar Epithelium with apical Microvilli
- Mucosa has Crypts of Lieberkühn or Tubular Glands that are orientated perpendicular to the long axis of the Colon with a large percentage of Goblet Cells
- Tubular glands are deeper than the glands of the small intestine
- Submucosa
- Submucosa contains Blood Vessels and large Lymphoid Nodules
- Muscularis externa
- Serosa
Describe the Histological Features of the Large Intestine
- Large intestine has the following histological features:
- Mucosa of the Large intestine lacks Villi and its cells lack Microvilli
- These structural adaptations indicate that nutrient absorption is not the primary function of the large intestine
- Mucosa is rich with Goblet Cells that secrete protective and lubricating mucous
- Muscularis Externa of the Large intestine is made up of two smooth muscle layers: Inner Circular Layer and an Outer Longitudinal Layer
- Muscularis Externa of the Large intestine is unique in that its Longitudinal layer is not continuous throughout most of its length
- Longitudinal Layer of smooth muscle is gathered into 3 bands or ribbons of muscle called Taeniae Coli
- The constant tension of the 3 bands of muscle bunches the Colon into pockets called Haustrum
- Serosa or Visceral Peritoneum of the Large intestine contains fat-filled pouches known as Epiploic Appendages
Regulation of the GITOutline the Regulation of the GIT
- There are three types of regulation that take place in the GIT:
- Neural Regulation
- Extrinsic central nervous system
- Intrinsic enteric nervous system
- Humoral
- GIT Hormones
- Paracrine mediators
- Autonomous smooth muscle activity
Nerve Supply (Enteric Innervation
- Alimentary canal is innervated by the Autonomic Nervous System
- Autonomic nervous system consists of an Extrinsic Components (Parasympathetic and Sympathetic Innervation) and Intrinsic (Enteric) Component
- Enteric innervation begins in the Oesophagus and extends down to the anus.
- Enteric Innervation has 2 distinct neuronal circuits:
- Submucosal plexus of Meissner
- Submucosal plexus of Meissner monitors the contents of the gastric lumen
- And based on those sensing responses, it receives stimulation that can affect GI secretions and blood flow
- Myenteric plexus of Auerbach
- Myenteric plexus of Auerbach is located between the circular and longitudinal layers of the Muscularis externa
- Stimulation of the Myenteric plexus of Auerbach affects the contraction of the muscle throughout the GIT
Intrinsic Nervous System of the GIT
- Parasympathetic Nervous System increases the motility of the GIT and the secretions via the Vagus Nerve or Pelvic Nerve
- Sympathetic Nervous System tends to slow the GIT motility and secretion via sympathetic ganglia
- Ganglia in the Myenteric and Submucosal plexus receives input from the Sympathetic Nervous System, Parasympathetic Nervous System Mechanoreceptors and Chemoreceptors
Interstitial Cells of Cajal (ICC) are the pacemaker cells of the GIT
- Myenteric plexus-ICCs and Intramuscular-ICCs are electrically coupled to smooth muscles via the gap junctions
- Spontaneous activation of the pacemaker current
- Results in the electronic conduction of slow waves via gap junctions
- Depolarization and Activation of L-type Ca+ Channels in Smooth Muscle cells
Humoral Regulation of the GIT
- GIT regulatory substances are classified as Hormones, Paracrine and Neurocrine
- Hormones are secreted by an endocrine cell of the GIT into the blood stream to a target cell
- Cholecystokinin, Gastrin, and Secretin
- Paracrine are secreted by an endocrine cells of the GIT and act on cells that are near to the secretory cell
- They travel via diffusion
- Neurocrine are secreted by a neuron of the GIT via an action potential
Functions
- Secretion of enzymes of digestion
- Secretion of fluid and electrolytes
- Contraction or relaxation of smooth muscle wall and sphincters
Mechanism of Protection in the GITList and outline the Mechanisms of protection in the GIT
- Mucous Blankets
- Antimicrobial secretions
- Immune Defense Mechanisms
- Peristalsis
- Normal Flora
- Mucous Blanket prevents abrasion and autodigestion int he stomach
- Antimicrobial Secretions
- Lysozymes which lyse bacteria are produced by Serous cells in the Salivary glands and Paneth cells
- Defensins which lyse bacteria
- HCl acid which destroys disease causing organisms
- Immune Defense Mechanisms
- Payer's patches in Ileum elicit an immune response to foreign antigens
- Lymphoid follicles in Duodenum, Jejunum and Large intestine elicit an immune response to foreign antigens
- IgA secreted by plasma cells in the Intestinal villi
- Tumour necrosis Factor-alpha which is proinflammatory and recruits inflammatory cells
- Peristalsis
- Motilin produced by endocrine cells in the upper small intestine stimulates gastrointestinal motility
- Enteric system and Muscularis externa perform peristalsis in the small intestine and prevent colonisation of bacteria
- Normal flora in the large intestine prevent overgrowth of pathogenic bacteria
Digestive GlandsOutline the function of Digestive Glands
- There are three main digestive glands:
- Salivary Glands (Parotid, Submandibular and Sublingual Glands)
- Exocrine Pancreas
- Salivary Glands
- Function of the digestive glands is to secrete products that lubricate, protect, digest and absorb
Salivary GlandsOutline the types of Salivary Glands
- There are three types of Salivary Glands:
- Parotid gland
- Parotid gland is a serous producing gland with Serous Acinus
- Serous Acinar Cell has Zymogen granules in the apical region
- Sublingual Gland
- Sublingual gland is a mucous producing gland with Serous-secreting cells
- Mucous acinar cell has mucous product in the apical regions
- And an irregularly shaped basally located nucleus
- Submandibular Gland
- Submandibular glands are mixed glands with Serous Acinus and Serous-secreting cells
- Serous demilunes are the serous cells at the distal end of mucous tubuloalveolar secretory unit of certain salivary glands
- Myoepithelial cells help by contracting in order to release secretory product into ductal system
Function of Salivary GlandsList the functions of the Salivary Glands
- Lubrication
- Salivary glands secrete mucous and water saliva
- Protection
- Salivary Glands secrete antibacterial proteins such as Lysozyme, Lactoferrin and IgA
- Chemical Digestion
- Amylase and Lingual Lipase enzymes to digest Carbohydrates and Lipids
Explain the mechanisms of how saliva is produced
- Parasympathetic nerve stimulates the secretion of NaCl through active transport, H2O through passive transport into lumen
- Sympathetic nerve stimulates the secretion of protein rich saliva into lumen
- This is done by the Acinar cells
- Mucous cells in the glands secrete Mucin and Serous cells will secrete enzymes into the lumen
- As a result, the Saliva in the Intercalated duct is Isotonic
- In the striated duct the Saliva becomes Hypotonic as the NaCl is actively transported into the capillary
- Plasma cells are pump IgA into the saliva
PancreasOutline the Pancreas
- Pancreas has an Exocrine and an Endocrine component
- Endocrine component is made up of Islets pf Langerhans that are responsible for the regulation of Glucose metabolism
- Exocrine component occupies most of the gland
- Exocrine Components has the following structures:
- Acinar cells have strong basophilic cytoplasm rich in rough endoplasmic reticulum
- The apical side of the cells is filled with Zymogen Granules that contain a variety of digestive
- Pale staining Centroacinar cells are located on the terminal portion of the ducts within the acinus
- Centroacinar and Ductal cells secrete aqueous HCO3- which neutralises H+ delivered to the duodenum
- There are NO myoepithelial cell sin pancreas
Pancreatic DuctsDescribe the types of Pancreatic Ducts
- There are two types of Pancreatic Ducts:
- Intralobular Ducts
- Intralobular ducts are formed by a cuboidal epithelium within the lobules
- They receive secretions from the intercalated ducts
- Intercalated Ducts
- Intercalated ducts have are made up of flattened cuboidal epithelium that extends up into the lumen of the acinus to form what are called Centroacinar cells
- They receive secretions from the Acini
Regulation of Pancreatic SecretionOutline the regulation of Pancreatic Secretions
- Pancreatic secretions are regulated by the following:
- Cholecystokinin
- Secretin
- Vagal Stimulation
- Cholecystokinin is released in response to the presence of proteins and fats in the intestinal lumen
- Cholecystokinin acts on Acinar cells which then secrete Digestive enzymes:
- Pancreatic amylase, lipase
- Nuclease
- Proteases
- CCK is the most important stimulant for enzymatic secretion
- Secretin is secreted in response to the presence of H+ in the lumen of the intestine
- Secretin stimulates Ductal cells to secretion an aqueous secretion of Na+, HCO3- and H20
- The HCO3- rich pancreatic juice neutralises acidic chyme
- Secretin is a major stimulant of the aqueous HCO3- rich secretion
- Vagal Stimulation (Parasympathetic) results int he secretion of Acetylcholine which stimulates the release of enzymatic secretions