Liver Histology and Physiology Flashcards
How many lobes is the liver divided into?
- 4 different lobes
* Not necessarily functional, but anatomic
What important biochemical processes is the liver involved with?
• First to receive blood from digestive systems, so it gets nutrients AND toxins first
• Synthesizes most of the blood proteins, glycoproteins and lipoproteins
• Stores glucose from the gut temporarily in the form of glycogen
• Metabolizes many lipid-soluble molecules and toxins
• Involved in urea formation
○ All these things need intimate connection/association with the blood, so the liver is organized accordingly
What is the name of the connective tissue capsule surrounding the liver?
• Glisson’s capsule
What is the Liver lobule?
- The smallest individual functional unit, or the dead end of the hepatic portal vein and hepatic artery
- Either classical lobule, portal lobule or acinar lobule
What is the classic liver lobule?
• Hexagonal-shaped structure of anastomosing plates of hepatocytes arranged radially around a central vein
• At the six vertices of the lobule run the interlobular vessels carrying incoming blood from both the hepatic portal vein and the hepatic artery
○ Blood is not yet mixed in these vessels
• Also at each vertex runs a bile duct and a lymphatic space called the space of Mall
Does the bile secretion pathway involve the blood supply initially?
- No, bile secretion is separate from the blood supply
- Network of tiny bile canaliculi run within the plates of hepatocytes, sealed from exposure to the blood by zonulae occludens between hepatocytes
- The network of canaliculi is peripherally connected to the bile ducts running along each vertex of the lobule
Describe how blood ends up washing both surfaces of the plates of hepatocytes
- Think of each hepatocyte having both ends touching plasma
- Interlobular vessels and the bile duct form the portal triad which is surrounded by loose connective tissue stroma (these are at the vertices of the hexagonal structure)
- Branches from interlobular vessels called the distributing or perilobular branches extend peripherally around the edges of the lobules, distributing blood into the sinusoids between the plates of hepatocytes
- Blood percolates through the narrow sinusoids between the plates en route to the central vein
- It flows over both surfaces of the plates which are usually one hepatocyte cell thick, hence two sides of most hepatocytes are exposed to plasma
The portal lobule differs from the classic lobule how?
- Classic lobule is a structure organized around the central vein
- Portal lobule is a triangular shape between three central veins (veins being the vertices of the triangle)
- This area is where a group of bile canaliculi feed
- “bile secretory” functional unit
How is the acinar lobule different than the other two lobules?
• Classic and portal lobules
• Another structural definition based on function
• Short axis lies between two portal triads
○ Think of it as a horizontal football with the laces touching the bottom vertex of a portal triad
○ The points of the football are between two different central veins
• Funcitonal in terms of blood delivery
• Distributing branches of the interlobular vessels run along the edges of a classic lobule but run along the short axis of the acinar lobule
• Relevant because exposure to toxins leads to degeneration nearest the distributing vessels
○ However, Lack of oxygenation or nutrients affects hepatocytes nearest the central vein
What are zones 1,2 and 3 used for?
- These are areas that have different histological evidence of different pathology
- For instance, glycogen tends to accumulate more eficiently in hepatocytes of the central zone of the acinar lobule which is nearest blood supply
What do hepatocytes do?
• Each hepatocyte is capable of performing all funcitons, there is no division of labor between cells
• Metabolically very versatile for nutrient uptake and detoxification purposes and bile formation
• Take up glucose and store as glycogen
• Break down glycogen to release glucose during fasting state
• Produce major blood proteins:
○ Albumin, clotting factors like fibrinogen and prothrombin
• Produce lipoproteins - mainly VLDL
• Conjugate and detox lipid-soluble toxins and bilirubin from spleen
• Excess cholesterol put into bile
Characterize the endothelial cells in the sinusoids
• Fenestrated. They must be to allow all proteins and lipoproteins and plasma components to push through without losing the RBC themselves
• All this plasma stuff goes into the space between endothelial cells and the hepatocyte - the perisinusoidal space
○ Aka - space of disse
• Certain silver stains will highlight this space by staining the reticular fibers within it
What are Kupffer cells?
- They are between the hepatocytes and sinusoids, acting as a monocyte-lineage-derived filter
- Especially for bacteria and carbon compound blobs which are visible microscopically
- Play major role in defense and particulate removal from the blood
What is bile?
- Composed primarily of bile salts which are typically cholate derivatives
- Smaller amounts of cholesterol, phospholipids, bilirubin glucouronides and proteins are also present
- Also there are electrolytes
- Polymeric IgA is a major protein in bile that is transcytosed from hepatocytes
- Interestingly, vesicular fusion for all components EXCEPT proteins is thought to be the mechanism for bile stuff getting out into the canaliculi
How does the gallbladder concentrate the bile?
- Tight junctions and lots of active salt transport from apical to basolateral side
- Water will follow osmotically
