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Histology of the Liver

The liver is the largest internal organ of the human body, weighing approximately 1.5 kg. Embryologically it develops from the foregut and it spans the upper right and part of left abdominal quadrants. Anatomically the liver consists of four lobes: two larger ones (right and left) and two smaller ones (quadrate and caudate). The liver consists of the following major histological components: Parenchyma, which is represented by hepatocytes Stroma, which is a continuation of the surrounding capsule of Glisson. It consists of connective tissue and contains the vessels. The capsule is also covered by a layer of mesothelium, arising from the peritoneum covering the liver. The connective tissue of the stroma is type III collagen (reticulin), which forms a meshwork that provides integrity for the hepatocytes and sinusoids. Sinusoids, which are capillaries travelling between hepatocytes Spaces of Disse (perisinusoidal spaces), which are located between the hepatocytes and the sinusoids. Structure In histological terms, the liver consists of a large number of microscopic functional units that work in unison to ensure the overall, proper activity of the entire organ. There are three possible ways of describing one such unit, as given below: Hepatic (classic) lobule Portal lobule Liver acinus Hepatic (classic) lobule The classic lobule is the traditional description and the one that you have most likely heard of the most. It consists of hexagonal plates of hepatocytes stacked on top of each other. Within each plate, the hepatocytes radiate outwards from a central vein. As they extend towards the periphery, the hepatocytes are arranged into strips, similar to the spokes of a cartwheel. Hepatic sinusoids travel between the strips of hepatocytes, draining into the central vein. One portal canal is located at each corner of the hexagonal classic lobule, making a total of six for each lobule. These portal canals are composed of the portal triads, which are surrounded by loose stromal connective tissue. A periportal space (space of Mall), where lymph is produced, is sandwiched between the connective tissue of the portal canals and the hepatocytes. While connective tissue is present around the portal canals, the interlobular quantity is very small in humans. This can make routine histological visualizations of the classic lobule difficult. Portal lobule While the classic lobule view focuses on the blood supply and hepatic mass arrangement, the portal lobule view underlines the exocrine function of the liver i.e. bile secretion. In this case, each functional unit is a triangle, having a central axis through a portal field and the imaginary vertices through the three different but closest portal canals surrounding it. The area covered by the triangle represents the hepatic regions that secrete bile into the same bile duct. Liver acinus The focus of this description is the perfusion, metabolism and pathology of hepatocytes, providing a more accurate description of the physiology of the liver. A liver acinus functional unit is in the shape of an oval. The short axis is represented by a shared border between two adjacent lobules together with the portal canals. The long axis is an imaginary line between two adjacent central veins. Hepatocytes These large and polyhedral (six surfaces) cells make up 80% of the total cells of the liver. They can contain between two and four nuclei, which are large and spherical, occupying the centre of the cells. Each nucleus has at least two nucleoli. The typical lifespan of a hepatocyte is five months. The adjacent hepatocytes leave a very small space between them known as bile canaliculi which are almost 1.0-2.0 μm in diameter. The cell membranes near these canaliculi are joined by tight junctions. The cytoplasm is acidophilic in routine H&E staining, dotted with basophilic regions represented by rough endoplasmic reticulum (rER) and ribosomes. In addition, hepatocytes contain the following organelles: Smooth endoplasmic reticulum (sER), which is essential in toxin degradation and conjugation, as well as cholesterol synthesis. Mitochondria (up to 1000/cell) Golgi network, which is composed of approximately 50 small Golgi units. They contain granules with very low density lipoprotein and bile precursors. Peroxisomes, which contain oxidases and catalases. These enzymes are responsible for detoxification reactions taking place in the liver, for example, that of alcohol. Glycogen deposits, which are lost in during H&E preparations, leaving irregular stained areas. Lipid droplets Lysosomes, which are responsible for iron storage under the form of ferritin. Perisinusoidal space (space of Disse) This space is situated between the layers of hepatocytes and the sinusoidal endothelial cells. The hepatocytes extend villi into the perisinusoidal space, increasing the extent and rate of material exchange, together with the microvilli.