Flashcards in L25. Histology of the Liver and Pancreas Deck (34):
Describe the liver
A large gland (solid organ) that is located in the right upper quadrant under the diaphragm.
What does the liver consist of?
Hepatocytes make up the large proportion of the liver (75% of the weight)
What are the functions of the liver?
1. Storage of glycogen
2. Gluconeogenesis: break down of lactic acid, amino acid and glycerol into glucose
3. Synthesis of cholesterol
4. Lipoprotein production
5. Production of ketone (for energy)
6. Plasma protein production: colloid osmotic pressure of blood
7. Production of non-essential amino acids
8. Detoxification of metabolic wast (ammonia cycle)
9. Storage of iron for Hb production
10. Destruction of hormones
11. Destruction of toxins: alcohol and drugs
12. Makes bile
Describe the hepatocytes
Polyhedral (6 surfaces) cells that are about 25um across.
They have 1-2 nuclei
Are tetraploid or polyploid (have multiple copies of DNA)
HIGHLY SYNTHETIC CELLS:
Prominent rough ER (Golgi) - protein synthesis
Prominent smooth ER - fat metabolism
Many mitochondria -energy consuming
What is the turnover of hepatocytes?
Hepatocytes are of epithelial origin
They have a life span of 150 days
How to hepatocytes regenerate?
By cell division (not by stem cells)
MATURE hepatocytes divide in a tightly regulated manner
This means that liver has the capability to regenerate tissue (important in the transplant setting)
- limited regeneration as can't repair after severe hepatocyte damage (Hep B or C)
Describe the blood supply to the liver
TWO blood supplies
Systemic blood supply through the hepatic arteries
Venous blood supply from the portal vein system (from the gut)
Describe the oxygenation of the liver in terms of the blood supply
The liver receives a mix of oxygenated and deoxygenated blood (about 20% of oxygenated and 80% deoxygenated)
What is the significance of having a portal blood supply to liver?
The portal system arises from blood draining from the gut and into the liver. This is important as the liver acts as a screening process for what the blood has received from the gut. This includes all metabolites (digestive products), nutrients and toxins.
Describe the connective tissue of the liver
The liver has a very thin capsule called the GLISSON'S CAPSULE that surrounds the blood vessels and the ducts as well.
This is a relatively little amount of protective connective tissue
How are the hepatocytes supported by CT?
Made up of reticular fibres (collagen type III)
They act only as shape forming support - a scaffold rather than protective
What are the liver lobules? Describe their shape
Lobules are the building blocks of the lobes. They are polygonal in shape from 0.7-2mm.
They are made up of STACKS of hepatocytes in plates which are then stacked on top of each other to form the lobule.
Each plate has thousands off hepatocytes within it.
How are the lobules arranged within a lobe?
They are arranged edge to edge with one another with a very narrow space between them. There are open spaces surrounding some of the corner edges of each lobule and this is the site for the portal triads.
Describe the blood vessels associated with a single liver lobule.
The CENTRAL VEIN is located in the centre of the lobule running the length of it.
At some of the corners there is a portal triad (roughly 3-4 triads per lobule). Within this triad is the portal vein and the branches of the hepatic arteries and a bile duct.
Describe the blood flow through the lobules
Blood is always flowing from the outside in: from the portal vein and the hepatic arteries in the triads leading INTO the central vein which anastomose together eventually forming the hepatic veins which drain into the IVC
What are the liver sinusoids?
Both the veins and the arteries in the portal triads give rise to a network of sinusoids
They are branching vessels of the veins and arteries to provide efficient supply of the hepatocytes (each hepatocytes is in direct contact with at least 2 sinusoids).
These sinusoids are the first site of the mixing of the deoxygenated blood from the portal system and the oxygenated blood from the systemic system.
What is the space of Disse?
The sinuoids and the hepatocytes have a special relationship that enables a very efficient exchange and sensing of the hepatocytes to the contents of the blood.
Sinusoids have a FENESTRATED wall, the gaps large enough for plasma to diffuse out and bathe the hepatocytes directly (not large enough for RBCs) hence the extracellular space of the hepatocytes is made up of plasma from the sinusoids.
Describe the surface of the hepatocytes that is related to the sinusoidal surfaces.
The hepatocytes have microvilli to increase the surface area for exchange.
What is the difference between a classic lobule and a hepatic lobule?
A classic lobule has a central vein at the centre and it supplies the lobule
The hepatic lobule (hepatic acinus) is a METABOLIC UNIT that puts the portal triad at the centre to emphasise that one portal triad supplies MULTIPLE LOBULES a metabolic organisation
(Different ways of looking at it)
Describe the concept of zonation in the lobules from the portal triads to the central vein. Relate this to the pathological problems that can happen in lobules
As the blood from the portal triads run from the outside in there is a zone formation of different contents of the blood as it passes through.
Zone 1 is closest to the portal triad and hence receives the most oxygenated blood (protected from infarction) but also receives the most toxins. Thus pathology involving toxins to the liver affect this zone.
Zone 2 is in the middle
Zone 3 is the closest to the central vein and hence receives the least toxins (protected from toxic damage) but also the least oxygenation and pathology involving ischaemia and infarction occurs here first.
Describe bile production and the function of bile
Bile is synthesised by hepatocytes from cholesterol and bile salts act as surfactants to emulsify the fat in the gut to allow for lipid digestion. Bile is reabsorbed in the ileum and recycled.
Describe bile pigments
Bile is a dark green colour due to the bile pigments. These pigments are a breakdown product of haemoglobin called BILIRUBIN.
Old RBCs are destroyed in the spleen and liver
How is bile transported from the hepatocytes?
Bile is produced in the liver and drain into very small spaces in between the hepatocytes. These small spaces form a reticular system of drainage that form CANNALICULI channels carrying blood from the INSIDE TO THE OUTSIDE forming the bile duct in the portal triad.
The bile ducts join to make the common bile duct
What is the gall bladder?
A muscular sac that is attached to the liver and the duodenum.
Bile from the common bile duct heads to the SPHINCTER OF ODDI at the major duodenal papilla but as the sphincter is normally closed, bile backs up into the cystic duct and into the gall bladder.
Describe the epithelium of the gall bladder and the function of it
It is an active organ made up of simple columnar eptihelium.
It has an absorptive function to pull water out: concentrates the bile
it also has smooth muscle strands in the walls and so acts as a contractile organ to squeeze bile out on demand
What are gall stones?
Spontaneous precipitates of cholesterol and bile salts that can lodge in the gall bladder or in ducts
Describe the hormonal signalling in the gall bladder
Fat in the duodenum stimulates specialised cells to release CCK into the blood. It circulates and acts in the gall bladder to contract and the spincter of oddi to relax so bile squirts into the duodenum.
Describe the pancreas
A large gland (epithelial origin) nestled in the loops of the small intestine
Describe the ducts of the pancreas and where they lead
Ducts drain the pancreas called the pancreatic duct along the body of the pancreas and it eventually joins the common bile duct draining to the spinchter of oddi.
Another secondary pancreatic duct from the uncinate ligament drains into the minor duodenal papilla
What does the pancreas release?
Pancreatic juice containing proteases, lipases, amylases, nucleases and bicarbonate ions
Describe the pancreatic acini
Pancreatic glands are formed from acini: football shaped organisation of cells with a duct in the middle of them.
The cells are granular in the centre (lighter staining with zygmogens) and darker staining on the outisde (nuclei and other machinery)
What is meant by the dual function of the pancreas
Endocrine: insulin production and secretion in Islets of Langerhans
Exocrine: production of digestive enzymes
Describe Zymogen activation and need for them
The pancreatic enzymes are released in active form to prevent any autodigestion of the pancreatic tissue. These inactive forms are called Zymogens.
They are activated by other enzymes present only in the duodenum and also activated by bile.