The lobes of the liver are divided into roughly 100,000 liver lobules, which are the functional units of the liver. Lobules are separated by a sparse connective tissue stroma. A typical lobule is 6 sided (hexagonal). It is roughly 2mm long by 1mm in diameter, and is composed of:
- Central vein- This passes through the core of the lobule and is surrounded by radiating sheets of hepatocytes. The sinusoidal channels converge on this vein, draining into it.
- Hepatocytes – Form plates one or two cells thick which are irregular, branched and interconnected surrounding sinusoids, running from the periphery to the centre of the lobule. Hepatocytes secrete plasma proteins, clotting factors and other products into the blood via the sinusoids.
- Sinusoids - Lined by fenestrated endothelium that separates the hepatocytes from the bloodsteam. It does allow blood plasma into the space between the hepatocytes and endothelium. The sinusoids filter blood that has come from the intestines. This allows the liver to remove glucose, amino acids, iron, vitamins and other nutrients. Hepatic macrophages (Kupffer cells) are also present in the sinusoids, which remove bacteria and cell debris.
- Hepatic triads are areas present at each corner of the lobule. They contain two blood vessels and a bile ductule. The vein is a branch of the hepatic portal vein, and the artery is a branch of the proper hepatic artery. They both supply the blood to the sinusoids giving them a mix of nutrient rich, deoxygenated blood and freshly oxygenated blood from the celiac trunk. When the blood collects in the central vein it drains into the left and right hepatic veins. These leave the liver at the superior surface immediately draining into the inferior vena cava.
Figure: Liver Lobule
Image courtesy of http://en.wikipedia.org/wiki/Image:Hepatic_lobule.JPG. This image is in the public domain and thus free of any copyright restrictions.
Fixed phagocytes called Kupffer cells are also present in the lobules. They live within the sinusoids and throw processes across the vessel. They destroy pathogens and remove cell debris such as old red blood cells. They are also responsible for storing irons and other heavy metals absorbed from the digestive tract.
Hepatocytes are the main functional cell of the liver, forming 80% of the cell population. They are generally about 20 - 30 mkm in diameter. Many cells are binucleate, turning tertraploid with age. They are highly metabolically active so they have a packed cytoplasm, full of organelles, with large central nuclei and distinguished nucleoli. Organelles with important functions include:
- Many mitochondria (800-1000 per cell) which generate energy and use ammonium ions to form urea.
- Lysosomes act as storage sites for Iron ions.
- Smooth endoplasmic reticulum is where detoxification of drugs occurs
- Many Peroxisomes are present (200-300 per cell) these are involved in gluconeogenesis, metabolism of alcohol and lipids.
- Glycogen deposits are used for energy and carbohydrate metabolism.
- Lipid droplets are also present involved in protein and lipoprotein synthesis and secretion
Hepactocytes have 3 important borders
- Sinusoidal – the border where material is transferred between the hepatocyte and the sinusoids. It is separated from the sinusoidal vessel by the space of Disse.
- Canalicular – the border where bile drains from hepatocytes to canaliculi. Cananliculi’s are formed by a two shallow channels on exactly opposing cell walls forming a tube.
- Intercellular – the borders between adjacent hepatocytes.
Hepatocytes secrete the bile they make into narrow channels called the bile caniculi between the plates of hepatocytes. Bile flows through the canaculi and into the bile ductules present in the triads. These then merge to form the left and right hepatic ducts which converge forming the common hepatic duct. The common hepatic duct is joined by the cystic duct from the gallbladder, forming the bile duct.
Hepatocytes have different metabolic functions, depending on their closeness to the portal tracts. Hepatocytes close to the portal tracts receive highly oxygenated blood and contain enzymes involved in oxidative reactions. They make and store glycogen and synthesise proteins. Hepatocytes closest to the central veins are furthest from the oxygenated blood supply, so they do not carry out oxidative activities, instead being involved in detoxifying reactions. Hepatocytes inbetween have intermediate metabolic properties.