Histology Lecture 4 -- Liver and Pancreas Flashcards
Anatomic location of the liver
Under the diaphragm in the upper right quadrant of the abdomen
What is the largset gland in the body?
Liver
What is the liver covered by?
A dense connective tissue capsule (Glisson’s capsule) lined by peritoneum
Lobes of the liver
- Large right lobe
- Smaller left lobe
- 2 smaller lobes in between
- Anterior = quadrate lobe
- Posterior = caudate lobe
What lies between the quadrate and caudate lobes of the liver?
At the center of the liver = hilum-like indentation –> portal fissure / porta hepatis
3 structures of the porta hepatis / portal fissure
- Hepatic artery
- Portal vein
- Bile duct
Define portal spaces
Spaces corresponding to the sites where branches of the hepatic artery, (hepatic) portal vein and bile duct are found, surrounded by connective tissue
How does the liver differ from other glands?
Connective tissue elements are sparse; most of the liver is made up of rather uniform parenchymal cells (hepatocytes)
What kind of gland is the liver?
Exocrine and endocrine
What functional cell of the liver performs its glandular functions?
Hepatocyte
4 exocrine secretion products of the liver
- Bile
- Bilirubin
- (Also, phospholipids and cholesterol)
Contents of bile
Bile salts and conjugated bilirubin
Where are the endocrine products of the liver released into?
Blood
5 endocrine products of the liver
- Albumin
- Prothrombin
- Fibrinogen
- Lipoproteins
- Transferrin
7 functions of the liver
- Endocrine secretion
- Exocrine secretion
- Storage of carbohydrates as glycogen and triglycerides
- Metabolic functions
- Detoxification and conversion/ inactivation (i.e. of drugs)
- Blood reservoir
- Fetus = hemopoietic organ
2 metabolic functions of the liver
- Gluconeogenesis
- Transformation of ammonia into urea
3 structure forming the prominent features of liver histology
- Portal spaces
- Cords of liver cells and sinusoids
- Central veins
Location of the portal spaces
At some corners of the hepatic lobule
How can hepatic (liver) lobules be distinguished?
By joining the portal spaces together
NOTE: In human liver, spaces are not actually joined together, but in pig liver, connective tissue of portal spaces delineate the outer perimater of the anatomical liver lobules (arrows)
5 contents of portal spaces
- Lymphatic vessels
- Nerves
- Branches of the hepatic artery (usually seen as a small muscular artery/arteriole)
- Branch of the portal vein (usually a small vein/venule)
- Branch of the bile duct (interlobular bile duct)
Appearance of interlobular bile ducts
Regular ducts lined by a columnar or cuboidal epithelium
Describe the positions of cords (plates) of liver cells and sinusoids
From the perphery of the lobule, they radiate toward the middle of the lobule, where they converge on the central vein. Cords alternate with sinusoids.
Describe the blood supply of sinusoids
- Branches of the hepatic artery deliver arterial blood into the sinusoid
- Branches of the hepatic portal vein send venous blood rich in nutrients from the intestine into the same sinusoids
- Both types of blood mix in the sinusoid
Describe the blood flow from the sinusoids
- As blood flows towards the central vein, it gives up oxygen and nutrients to the liver cells and picks up metabolized and synthesized products made by liver cells (endocrine)
- Blood empties into the central vein of the lobule
What forms hepatic veins and where do they go?
- Central veins converge –> hepatic veins
- Do not run in the portal spaces
- Join to leave the posterior aspect of the liver and enter the IVC
Describe the general morphology of liver cells
- Polyhedral shape
- 2 distinct surfaces
- Those facing other liver cells to make up the cord (or wall of cells)
- Those facing toward the sinusoids (usually the cord is 1 cell thick, so there are 2 sinusoid-facing sides)
Describe the histology of the sinusoid-facing sides of liver cells
- Microvilli
- Face an extracellular space (perisinusoidal space; space of Disse)
Describe the histology of the hepatocyte-facing side of liver cells
- Hemispherical depressions on surface –> Match similar depressions on the adjacent liver cell
- Two halves form a “bile canaliculus” between liver cells
- Golgi region faces this –> bile secretion into small intercellular duct
Describe the direction of bile flow
Toward the portal space (opposite to the direction that blood flows in the sinusoids)
Where do the bile canaliculi drain into?
Form a chicken wire network –> drain into cholangioles (canals of Hering)
Describe the histology of cholangioles
- At first lined in part by hepatocytes and by low cuboidal cells
- Then become entirely lined by low cuboidal cells
Where do the cholangioles empty into?
Interlobular ducts in the portal space
What is the portal lobule centered on?
Portal space
What is the anatomical liver lobule centered on?
Central vein
Define the hepatic acinus
- Tissue supplied by a terminal branch of the portal vein and hepatic artery and drained by a terminal branch of the bile duct
- No anatomical landmark limit
What does each acinus comprise?
Parenchyme from two adjacent lobules
Describe the 3 regions of the acinus of the liver
3 poorly defined concentric regions of hepatic tissue:
- Zone 1 = richest oxygen supply
- Zone 3 = poorest
Describe the histology of sinusoids
- Lined by fenestrated endothelial cells
- Large macrophages (Kupffer cells)
- Some reticular fibers under the endothelium
- Fat-storing Ito cell associated to sinusoid, but located in the space of Disse (which communicates directly to the pores of the sinusoidal endothelium)
Describe the histology of the central vein
- No more than endothelium
- A little connective tissue lining the ends of the liver cords as they come towards the lobule centre
- Vein receives numeroud openings of the sinusoids
Describe the histology of the wall of the gall bladde
- Mucosa = simple columnar epithelium with a few microvilli and a lamina propria
- Tunica muscularis of non-oriented smooth muscle
- Thick connective tissue coat covered by a serosa
- NOTE: No muscularis mucosa at the borderline of the mucosa and submucosa
Describe the anatomic location of the pancreas
- Large encapsulated gland located retroperitoneally at the level of L1 - L3
- Enclosed on the right by the duodenal loop
- Head against the 2nd part of the duodenum
- Body and tail extending to the spleen on the left side
What kind of gland is the pancreas?
Mixed gland:
- Large compound acinar exocrine portion
- Small spheres of endocrine portion scattered among the exocrine portions appearing as circular islands = Islets of Langerhans
- Exclusively serous
Describe the histology of serous pancreatic acini
- Pyramidal cells with round basal nuclei
- Basophilic cytoplasm at the base
- Eosinophilic zymogen granules
- Small lumen
- Duct cells enter the luminal space and line part of the lumen with thin flattened cells (centroacinar cells)
Describe the nuclei of the centroacinar cells
Prominent and appear in the center of the acinus of the pancreas
How must one identify the pancreas to differentiate it histologically from the parotid gland?
Confirming the nuclei of centroacinar cells lining the lumen since their nuclei are characteristic of the pancreas
Type of duct that arises from the centroacinar cells of the pancreas
Intercalated ducts
Describe the histology of the duct system of the pancreas
- Beginning = centroacinar cells
- Intercalated ducts lined by cells similar to centroacinar cells –> join with intralobular collecting ducts, but are not striated
- Rest of duct system is as describe the salivary glands (see that lecture)
What stimulate the hormones produce by enteroendocrine cells in the duodenal mucosa?
- Food in the gastric antrum
- Passage of the acidic stomach chyme into the duodenum
2 hormones produced by enteroendocrine cells in the duodenal mucosa in response to a meal
- Secretin
- Cholecystokinin
Define secretin and its role in the pancreas
- 27 amino acid polypeptide
- Stimulates the centroacinar duct cells to secrete a bicarbonate-rich fluid that forms part of the exocrine secretion and neutralizes the chyme
Define cholecystokinin and its role in the pancreas/duodenum
- 33 amino acid polypeptide
- Cause secretion of the enzymes by the acinar cells
- Stimulates contraction of the smooth muscle in the gall bladder
Contents of pancreatic juice
- Bicarbonate fluid of the ducts
- Many enzymes that were present in the inactive zymogen form in the zymogen granules
- Pancreatic lipase
- Pancreatic amylase
- Trypsin
- Chymotrypsin
- Carboxypeptidase
- Ribonuclease
- Deoxyribonuclease
- Trypsin inhibitor
Purpose of pancreatic lipase
Hydrolysis of glycerol and fatty acids
Purpose of pancreatic amylase
For starch and glycogen breakdown
Purpose of trypsin inhibitor
Prevent premature activation of trypsin (otherwise it would digest the digestive system)
During stimulation of acinar secretion in the pancreas, what happens to the granules?
Granules serially fuse and are exocytosed together
Describe the histology of the islets of Langerhans
- Scattered between the acini of the exocrine pancreas
- Paler-stained circular bodies of various sizes
- Not dileneated by noticeable amounts of connective tissue
- Reticular fibers are present
- Made of pale stained cords of cells that contain spherical nucleu
- Network of fenestrated capillaries between the cords
Purpose of fenestrated capillaries between the cords in the islets of langerhans
Receive the endocrine secretions of the cord cells
Describe the 5 cell types present in the cords of the endocrine pancreas
- Cannot be distinguished without immunocytochemical staining
- A (alpha) cells = 20% of islet, located at periphery
- B (beta) = 70% of islet, central
- D (delta) = 5% of islet, variable position
- G cells = 1%
- PP (or F) cells = 1%
What do A (alpha) cells secrete in the pancreas?
Glucagon
What do B (beta) cels secrete in the pancreas?
Insulin
What do D (delta) cells secrete in the pancreas?
Somatostatin
What do G cells produce?
Gastrin
What do PP (or F) cells produce?
Pancreatic polypeptide (unknown function)
Describe the nuclei of the flattened epithelial cells lining the branches of the small intralobular duct leading to the pancreatic acini
Fusiform
Describe the histologic appearance of pancreatic B cells
- Abundant
- Pale, vaguely granulated cytoplasm
Describe the histologic apperance of pancreatic A cells
- Less abundant than B cells
- Cytoplasm contains fine acidophilic granules
Describe the inactivation/elimination functions of the liver
- Glucuronyl transferase conjugates glucuronic acid to bilirubin AND other drugs
- Endocytosis of hormones and cholesterol (LDL particles)
4 kinds of drugs that glucuronyl transferase can conjugate glucuronic acid into
- Barbiturates
- Antihistaminics
- Anticonvulsants
- Insecticides
Give an example of substance conversion in the liver
Tetra-iodothyronine (T4; thyroxine) –> Tri-iodothyronine (T3)
The central vein of the liver lobule drains into what?
Sub-lobular vein
The sub-lobular vein drains into what?
Hepatic vein
