Histology Flashcards
What are the 5 functions of the GIT
Digestion Absorption Secretion Movement Protection
Name the 4 layers of the stereotypical GIT walls
Mucosa Submucosa Muscularis externa Serosa * Plus glands in the wall and external glands
What 3 sub layers does the the mucosa consist of?
- Epithelium
- Lamina propria
- Muscularis mucosa
What are the functions of each of the sub layers of the mucosa?
1. Epithelium • Simple columnar or stratified squamous • Can include endocrine cells, • Renewed every 5-6 days 2. Lamina propria • Loose CT containing: -> nerves, small blood vessels and immune cells. 3. Muscularis mucosa • Thin layer of SM • Forming boundary of mucosa + • Facilitates mixing
What are the two types of epithelium found in the GIT?
Where are each of them found?
Simple columnar and stratified squamous
- Stratified squamous is found in the oesophagus and the rectum the rest is simple columnar
Where are blood vessels found in the mucosa?
The lamina propria - they are small (mainly capillaries and lymphatics)
Where is the submucosa located and what is it made up of?
Adjacent to mucosa
It consists of dense, irregular connective tissue
Blood vessels and nerves also present
sometimes contains immune cells/glands (oesophagus + rectum - the Brunner’s gland of the duodenum is submucosal)
What is the role of the submucosa?
strength/elasticity
What are the two layers of the muscularis externa?
- inner circular layer → squeeze gut
2. Outer longitudinal layer → shorten & lengthen gut
What is the primary function of the muscularis externa?
Responsible for gut movement: mixing, propulsion, evacuation (through peristalsis)
Esp important in the stomach stomach where it mixes the chyme together with the acids and enzymes
What is the role of the serosa?
it provides a non stick surface of gut and allows organ movement
What is the serosa composed of?
fluid-secreting simple squamous epithelium (mesothelium)
What is a potential complication arising from surgery that results from the serosa?
Formation of adhesions restricting movement (tissue must be kept moist in surgery to prevent organs sticking!)
Where is serosa found?
Surrounding intraperitoneal organs
What is the enteric nervous system?
It is the 3rd component of the ANS (along with the SNS and the PNS) which supplies the gut
Can the gut function without central nervous system input?
ENS has autonomic motor neurons and autonomic sensory neurons in gut wall
It Doesn’t need CNS to function
How do the number of neurons in the gut compare to those in the spinal cord?
As many in the gut wall as the spinal cord
What are the roles of the ENS?
it is responsible for mixing, peristalsis and secretion
Where are the ENS nerve plexus and what inputs do they provide?
Myenteric ganglia/plexus
Located between the inner circular and outer longitudinal muscles
It provides motor innervation to both layers of the muscular layer, having both parasympathetic and sympathetic input,
Submucosal ganglia/plexus
Located in the submucosa and only parasympathetic fibers and provides secretomotor innervation to the mucosa nearest the lumen of the gut.
What is the epithelium of the oesophagus made of and why?
Non-keratinized squamous epithelium
It is tough, replaceable, resistant to mechanical abrasions by food
Why could the oesophagus not absorb nutrients even if it wanted to?
It has an extremely thick mucosa which provides protection
What is the breakdown of the muscles of the oesophagus?
Upper 1/3 skeletal (voluntary)
Middle 1/3 mixed
Lower 2/3 smooth muscle (involuntary)
What can happen to the epithelium of the oesophagus if exposed to prolonged GERD?
Squamous epithelium converts to columnar mucous form -> Barrett’s oesophagus (metaplasia.) -> cancerous
What is the primary role of submucosal glands in the oesophagus?
Aid in lubrication
What happens to the mucosa as it transitions from oesophagus to the stomach?
Goes from squamous epithelium → highly glandular mucosa (in stomach)
What controls the release of material (acidic chyme) into the intestine?
Pyloric sphincter -
What are the 4 regions of the stomach?
Cardiac region, fundus, body and antrum
What are the functions of the stomach?
Initiatedigestion (mainly of proteins)
Produce chyme (a thick liquid) by mechanical and chemical breakdown
Regulate release of chyme into the intestine
Only minimal absorption
What is the structure of the stomach?
- Simple columnar secretory epithelium
- Prominent muscularis mucosae
- Thick submucosa
- Thick muscularis externa (third, oblique layer in parts)
What is the role of the pyloric sphincter?
important to control as it allows only small amounts of acidic chyme into the small intestine
What are the folds of the stomach called?
rugae
What is found within the gastric pits?
Gastric glands
How many glands per gastric pit?
3-5
What is the arrangement of the glands?
Simple tubular glands - penetrate into the lamina propria
What are the different cells of the gastric glands?
Surface cells Neck cells Regenerative stem cells Parietal cells Chief cells Enteroendocrine cells
What do surface and neck cells do?
They secrete mucous which provides lubrication and protection (not goblet cells)
What do parietal cells do?
They produce HCl which activates stomach enzymes and breaks down food
What do chief cells do?
release pepsinogen at the base of the gland, this is then converted to pepsin (pepsinogen is a zygomen of pepsin/pro-enzyme)
What do enteroendocrine cells do?
They produce hormones (gastrin) which regulates acid secretion
How is pepsin activated?
The acid envrionement in the stomach activates some pepsinogen to pepsin which then cleaves activates more pepsinogen
Pepsin is only active at low pH (pH2)
How is the stomach protected from its contents?
- Mucosa is protected from pepsin and acid by a layer of mucus
- Mucus is constantly being released
- Breakdown of the mucus layer leads to gastric ulcers, as the mucosa is digested Stomach protection
Which section of the gut is responsible for most of the absorption of the products of digestion?
Small intestine (85%)
What cells make up the epithelium of the small intestine?
Simple columnarepithelium (with microvilli)
What 3 structures increase the surface area of the SI?
i. ↑ SA 1) plicae circularis = folds 2) villi = extension of the mucosa 3) microvilli = on simple columnar ep.
1. Microvilli = MOST important in increasing the SA (20x) to absorb; ALSO have brush border enzymes
What are found at the base of the villi?
Tubular glands (crypts of leiberkuhn) secrete fluid and mucous
What is the lamina propria?
CT of the lamina propria extends into the core of each villus
- Contains smooth muscle (muscularis mucosae) blood vessels, lymphatics and immune cells
- Muscularis mucosae can move the villus to prevent any unstirred layers (aids absorption)
What are the epithelial cells of the SI and what do they do?
- Enterocytes = fluid transport, adsorption
- Goblet cells = mucous secretion
- Enteroendocrine cells = hormones
- Paneth cells = secrete anti-microbial peptides (deep in glands)
- Stem cells = to renew epithelium
What is the role of the doedenum?
Neutralize the acidic chyme and active pepsin which it receives from the stomach (?)
Also most enzymes which are used for digestion are added to the duodenum as is bile
What is a Brunner’s gland?
These are glands found in the submucosa which release alkaline mucous, the pH is raised to around 7.3 which inactivates pepsin
(if the Brunner’s glands are not working a duodenal ulcer may result)
How much fluid is absorbed in the small intestine?
8.5L per day
What features does the ileum have?
Short villi
Lots of goblet cells
Peyer’s patches
What separates the colon from the rest of the body?
Longitudinal muscle (muscularis externa) is in three distinct strips (taenia coli)
What is seen in the mucosa of the large intestine?
They have no villi, straight tubular glands and columnar epithelium
What are the differences in epithelia of the SI and LI
- Paneth cells rare
- Higher proportion of goblet cells
- Differences in enteroendocrine cell types
What type of epithelium is seen in the anus?
stratified squamous
What happens at the recto-anal junction?
Abrupt transition to become stratified squamous epithelium of the anal canal
What is in the feces?
water, roughage, bacteria, fat, inorganic stuff, bile pigment
What neutralizes the bacterial acid?
HCO3
How much water is lost per day in the feces?
100ml
What are the primary functions of the liver?
Metabolism Absorption of fats Protein synthesis Storage Detox
What are the tissue types in the liver?
Hepatocytes (75% of total weight)
Connective tissue (the liver has relatively little CT)
Vessels
What is the structure of a hepatocyte?
6 sided
Can have 1 - many nuclei (needed because of high transcription rate)
Prominent rough ER
Prominant smooth ER (fat metabolism)
Many mitochondria
Has microvilli on apical and basolateral surface
Is the liver a regenerative organ?
Yes
Hepatocyte lifespan is about 150 days (there are not hepatic stem cells, rather they are replaced by cell division)
Capacity to regenerate is very good (can even remove a lobe)
Extensive damage caused by Hep B/C and alcohol abuse may cause permanent damge (cirrhosis and atrophy)
Describe the blood supply of the liver:
Hepatic and Systemic
This allows the liver to process the products of digestion as well as destroy toxins before they are able to enter into the circulation
What is the capsule that invests the liver?
Glissons capsule which surrounds and invests liver, it also surrounds blood vessels and ducts
What type of collagen in seen in hepatocytes, what function does it serve?
Hepatocytes are supported by reticular fibres (collagen type III)
These fibres are thin and act mainly as a scaffold
What is a liver lobule?
It is a functional unit of the liver
It has a polygon shape and is 2mm tall x 0.7mm acrross
What are the portal triads?
Three components
Branch of the hepatic portal vein (with deox blood from the gut)
Branch of the hepatic artery (oxygenated blood)
Bile ducts
The portal triad supplies multiple lobules - it is located at the edge of the lobule
What is the central vein?
The sinusoids converge on the central vein which carries blood to the hepatic vein and then to the system venous system.
What are the liver sinusoids?
The venous and arterial networks give rise to sinusoids, they are conduits that allow blood flow into the middle of the lobule and the central vein.
They carry a mixture of oxygenated and deoxygenated blood (20:80)
What is the direction of blood flow in a lobule?
Outside In
What is the Space of Disse?
Each hepatocyte has sinusoids above and below, fenestrations in the sinusoid wall allow plasma to flow out into the extracellular space. Thus the hepatocytes are directly bathed in by plasma.
This communication is enhanced by the microvilli (increased surface area) on the surface of the hepatocytes
What is the classic lobule model?
centre of lobule is the central vein with triads in the periphery i. Focus on direction of blood flow = outside is where the blood is coming from
What is the portal lobule model?
centre of lobule is the portal triad, central vein is the periphery i. Focus on direction of bile flow
What is the acinar lobule model?
i. Physiological context: different zones have different oxygenation and metabolic function ii. Zone 1 close to portal triad = high in O2, high in toxins, high in nutrients iii. Zone 3 is close to central vein = low in O2, low in toxins, low in metabolites iv. Important if things go wrong – do not get reflected in typical hexagonal lobule; often follow patterns related to oxygenation and metabolic exposure RATHER than geometry
What is bile?
Greenish salty fluid
Bile salts act as surfactants to break down fat (emulsify) in the gut
How is bile made?
What happens to it?
Synthesised from cholesterol
It is resorbed in the small intestine and recycled
What gives bile its colour?
Bile pigments = dark green colour due to bilirubin (breakdown of Hb) 1. Damaged and old red blood cells are destroyed in the spleen and liver
Where is bile collected?
Each hepatocyte makes bile -> travels along channels (canaliculi) formed between hepatocytes and flows OUTWARDS (opposite to blood flow) to be collected into bile ducts in the portal triads b. Not visible in section normally
How is bile transported?
. The biliary transport system = bile is stored in gall bladder if Sphincter of Oddi closed
What happens to bile in gall bladder?
The gall bladder concentrates the bile, removing water and salts (≈90% resorbed)
What cells are on the epithelium of the gall bladder and why?
Gall bladder epithelium = simple columnar epithelium as it simply absorbs
What controls the release of bile from the gall bladder?
a. Fat in duodenum stimulates specialized cells of duodenum mucosa to release hormone (CCK) into blood b. CCK causes gall bladder to contract and sphincter of Oddi to relax – bile released into duodenum
What is the pancreas and where is it?
solid organ with exocrine and endocrine glandular components a. The pancreas is a large gland nestled in the loops of the intestine with ducts joining common bile duct at the Sphincter of Oddi [also important in controlling pancreatic secretions]
What is the function of the exocrine pancreas?
Function of the exocrine pancreas = fluid contains proteases, lipases, amylases, nucleases and bicarbonate ions a. Bicarbonate ions neutralize acid from the stomach whilst enzymes participate in digestion b. Exocrine pancreas = basal purple area (rough ER!!!) and luminal pink area (GRANULES of zymogens) i. Pancreatic glands formed from acini (balls of cells with a central duct) -> ducts joint to form large duct
What is a pancreatic acinus?
Football shaped mass of secretory cells = acinus
Cells point inwards and empty into a duct
Each of the ducts of the acinus convalesce to form the pancreatic duct
What is the function of endocrine pancreas?
Function of the endocrine pancreas = 1-2% total pancreas volume comprising ≈1 million clusters of hormone-secreting cells (islets of Langerhans) which have RICH vascular supply (secretes directly into blood stream inuslin + glucagon)
What is a gall stone?
precipitation of cholesterol and/or bile salts
What is a zymogen?
A package of enzyme secreted from the pancreas in an innactive form
What activates the zygomen?
Other enzymes and bile salts?
What happens if there is a blockage at sphincter of oddi?
The blockage will prevent the movement of bile, given that the common bile duct and pancreatic duct coalesce before entering into the duodenum it can cause bile to enter into the pancreas and activate the zygomen. This will in turn cause auto-digestion of the pancreas.
