Histology - Liver and Pancreas Flashcards
Liver is covered by…
a capsule of connective tissue
What is the porta hepatis?
The hilum of the liver (entrance of portal veins, hepatic arteries, hepatic duct and common bile duct)
Blood from the portal veins and hepatic arteries is collected by..
the hepatic vein, which drains into the IVC
Functions of the liver
- Endocrine secretion i.e. directly to blood (albumin, fibrinogen, prothrombin, transferrin, VLDL/LDL)
- Exocrine secretion i.e. to gallbladder/intestine (bile acids, bilirubin, phospholipids, cholesterol)
- Inactivation/elimination of substances
- Conversion of substances (T4 to T3)
- Hemopoiesis (erythrocyte production during fetal development)
- Storage of glycogen and lipids
The liver inactivates substances via..
glucoronyl transferase, an enzyme that conjugates glucoronic acid to bilirubin (and eliminates drugs)
Liver eliminates hormones and cholesterol by…
endocytosis (LDL particles)
Hepatic lobules are separated by connective tissue areas called…
portal spaces
What is found in the portal spaces of each lobules?
hepatic arterioles, portal venules and bile ducts
What is at the centre of each hepatic lobule
central vein
Across the lobule, there are small vessels connecting the central vein to the portal spaces, called..
sinusoids
Blood from central veins drains into … which drain into …
sub-lobular veins, which drain into the hepatic vein
Main cell of the liver, that make up the liver lobules
Hepatocytes
Small spaces between hepatocytes
Bile canaliculi (collect and transport bile produced by the hepatocytes towards small bile ducts)
Small bile duct branches that collect bile from canaliculi and drain into bile ducts
cholangioles
In what direction does blood drain in the lobules?
In what direction does bile drain the lobules?
Blood from portal venules and hepatic arterioles drains from the periphery to the central vein.
Bile from the hepatocytes drains from the centre to the periphery (opposite direction).
In the lobules, the “plates” formed by hepatocytes are covered by…
endothelial cells (i.e. lining the sinusoids) and Kupffer cells (fixed macrophages)
Function of Kupffer cells
Remove old erythrocytes from the blood (transfer them to hepatocytes, where they get converted to bilirubin)
Type of epithelium that forms the bile ducts
Columnar or cuboidal simple epithelium
Portal venules and hepatic arterioles (portal space) open directly into…
the sinusoids (blood from veins and arteries combines)
Sinusoids are lined by…
loose endothelial cells (no tight junctions)
What is the name of the space between the endothelial lining of sinusoids and the hepatocytes?
Space of Disse
Type of junction between hepatocytes
Tight junctions (seal bile canaliculi)
Sinusoids do not have a basement membrane. Instead, they are surrounded by…
reticular fibres
What is characteristic about the hepatocyte borders that face the space of Disse?
Microvilli (brush border)
Purpose of endocytic activity of hepatocytes
Inactivation of substances (endocytosis of hormones, LDL, HDL, chylomicrons).
The endocytosed particles are taken up into endocytic vesicles, which fuse with endosomes in the cell and sent the contents to lysosomes, which destroy the contents. Ligand receptors are recycled.
What is LDL
low density lipoprotein (bad cholesterol)
The rough endoplasmic reticulum (RER) of hepatocytes synthesizes…
Apolipoproteins
The smooth endoplasmic reticulum (SER) of hepatocytes stores..
fatty acids, triacylglycerols, phospholipids and cholesterol
What is the role of the Golgi in hepatocytes?
Apolipoproteins and fatty acids/triacylglycerols/phospholipids/cholesterol are sent to the Golgi.
The Golgi combines them to form apolipoprotein complexes (VLDL).
Purpose of exocytic activity of hepatocytes
They release apolipoprotein complexes (VLDL) via exocytosis into the space of Dessi.
What are VLDL
Very low density lipoproteins .
How is VLDL transformed after being released into the circulation by the liver? Name the enzymes!
VLDL is transformed to IDL (intermediate density lipoprotein) by lipoprotein lipase.
IDL is then transformed into LDL (low density lipoprotein) by hepatic lipase.
LDL is taken to extra-hepatic cells, that require cholesterol for growth.
This process prevents free cholesterol from circulating in the blood and causing atherosclerosis
Unique organelle in liver and kidney cells
Peroxisome
What is the function of the peroxisome?
It contains the enzyme catalase, which converts hydrogen peroxide (H2O2) to water and oxygen.
Oxidation of long-chain fatty acids also takes place in the peroxisome.
Also catabolizes alcohol.
Also participates in cholesterol synthesis.
This H2O2 comes from the oxidation of long-chain fatty acids, but is toxic to cells and hence must be transformed
Which organelle synthesizes bile in the hepatocytes?
Smooth endoplasmic reticulum (SER)
How does hepatitis (viral infection) affect the liver?
It loosens the tight junctions of the bile canaliculi, causing jaundice (because bile enters the circulation via sinusoids).
How long does it take for hepatocytes to renew (turnover)
150 days (very low mitosis rate, unlike the stomach and intestines)
If a piece of the liver is removed, can it grow back?
Yes: Cells will divide actively until the organ reaches its normal size
What happens o the liver when it undergoes repeated damage?
Repeated damage to the liver results in the proliferation of connective tissue, i.e. cirrhosis (iireversible)
The pancreas is a mixed..
exocrine and endocrine gland
What is the exocrine part of the pancreas composed of? What is the endocrine pancreas composed of?
Exocrine: Compound acinar gland
Endocrine: Islets of Langerhans
The exocrine pancreas produces a…
slightly alkaline fluid and pro-enzymes (zymogens) that will eventually digest food in the intestine
Why is the fluid produced by the exocrine pancreas slightly alkaline?
To neutralize the very acidic chyme coming from the stomach to the duodenum.
Both the pancreas and the salivary glands derive from…
evaginations of the endodermal lining of the embryonic intestine
The bile duct and main pancreatic duct converge and open into…
the hepatopancreatic ampulla of the duodenum
The main pancreatic duct and its branches is part of…
a) the endocrine pancreas
b) the exocrine pancreas
b) the exocrine pancreas
Describe the branches of the main pancreatic duct
Main pancreatic duct -> interlobar ducts -> intralobular ducts -> intercalated ducts -> acini
The pancreas is covered by..
a capsule of connective tissue
The exocrine pancreas consists of….
serous acini
What is the shape of the cells of the serous acini in the pancreas? Describe their appearance
Pyramidal cells with round nuclei at the base.
Filled with little red dots (zymogen granules).
The intercalated ducts (intralobular) connect to the serous acini. The cells of the ducts that “penetrate: the acini and can be seen at their centre are called…
centro-acinar cells
What penetrates the islets of Langerhans (endocrine pancreas)?
Fenestrated capillaries
Islets of Langerhans are surrounded by…
a thin layer of fibrocytes (CT) and reticular fibres
What cell type makes up the tiny intercalated ducts?
Squamous cells
What type of cells make up the small intralobular ducts?
Cuboidal cells
Name 2 hormones that regulate the exocrine pancreas, and how
Secretin: stimulates the secretion of H2O and bicarbonate
Cholecystokinin (CCK): promotes exocytosis?
What cells secrete CCK?
Enteroendocrine cells of the duodenum
What enzyme converts trypsinogen to its active form? What is it converted to?
Trypsinogen is converted to trypsin by enterokinase
Name the 4 pro-enzymes contained in zymogenic granules of the serous acini
Trypsinogen
Pro-carboxypeptidase
Pro-elastase
Pro-phospholipase A
What is the role of trypsin?
It converts all the other pro-enzymes to their active forms
Name the active forms of the pancreatic enzymes
Carboxypeptidase
Elastase
Phospholipase A
The pancreas also secretes active enzymes (not just pro-enzymes). Name them (4)
Lipase
Amylase
RNAse
DNAse
Sometimes the pancreas has to secrete a trypsin inhibitor to avoid excessive enzyme activation. What cells secrete this inhibitor?
Centro-acinar cells
Early activation of pro-enzymes in the pancreas can cause…
pancreatitis
2 types of cells that make up the Islets of Langerhans. Are they acidophilic (eosinophilic) or basophilic?
B (beta) cells (basophilic - pale)
A (alpha) cells (acidophilic - dark)
There are a lot more B cells than alpha cells!
What hormone is produced by the large A (alpha) cells of the islets of Langerhans?
Glucagon
What is the function of glucagon (hormone produced by A cells) (2)
Glycogen breakdown
Gluconeogenesis
It plays a crucial role in maintaining normal concentrations of glucose in blood and has the opposite effect of insulin
Compare the actions of insulin vs glucagon
Insulin: Promotes glucose uptake by cells, decreases blood glucose concentration
Glucagon: Promotes glycogen breakdown into glucose, increases blood glucose concentration
When is glucagon secretion stimulated? why?
When blood glucose levels begin to fall below the normal range, in order to breakdown glycogen stored in the liver (or to activate hepatic gluconeogenesis) because the brain has an absolute dependence on glucose as fuel.
What hormone is produced by the B (beta) cells of the islets of Langerhans?
Insulin
What is the function of insulin (produced by B cells of the liver)? (2)
Uptake of glucose by cells
Lower glucemia (blood sugar)
How do the alpha and beta cells release their respective hormones into the bloodstream?
They form vesicles containing insulin or glucagon and release the content by exocytosis. The hormones then enter the capillaries via fenestrations in the endothelium.
There are also some D (delta) cells and PP cells in the islets of Langerhans. What hormones do they produce?
D (delta) cells: Somatostatin
PP cells: pancreatic polypeptide
What is the role of somatostatin (D cells of pancreas)? (2)
Inhibits the release of glucagon and insulin
Inhibits HCl secretion by parietal cells
What is the role of pancreatic polypeptide (PP cells of the pancreas)?
Stimulates activity of the chief cells in the stomach.
Chronic ingestion of alcohol can lead to acute pancreatitis. Why?
Chronic alcohol ingestion can obstruct the pancreatic duct system. This blockage increases pressure in the duct system and can cause premature activation of digestive enzymes like trypsin inside the pancreas instead of the intestine. Once activated, these enzymes start digesting pancreatic tissue, leading to inflammation, swelling, and acute pancreatitis.
What type of diabetes develops before the age of 15?
Type I DM, i.e. insulin-dependent diabetes mellitus
Note: More recent data suggests this can also affect humans.
What causes Type I diabetes mellitus (insulin-dependent diabetes mellitus)?
Autoimmune destruction of B-cells of the islets of Langerhans in the pancreas.
Another name for type I diabetes mellitus.
Insulin-dependent diabetes mellitus
