The Liver is the largest gland in the body and is an important metabolic organ of many functions.
It has also an exocrine function, the secretion of bile, which is conveyed to the intestine by a system of ducts.
The organ is invested with a connective tissue sheath called the Glisson’s capsule most of which is covered by peritoneal serous membrane.
From the connective tissue sheath fibrous bands or septa arises and enter into the substance of the gland and divide the gland into numerous hexagonal or polygonal units, the hepatic lobules. These connective tissue septa from the capsule are collectively termed as the Glisson’s capsule and the interlobular columns of connective tissue convey blood vessels and ducts and are also referred to as the portal canals.
The interlobular connective tissue in all domestic animals and men, except in the pig and camel. In the liver of pig and camel, the lobular is very well seen, as each hexagonal or polygonal lobule is distinctly circumscribed by connective tissue.
At the hilus of the liver, the Glisson’s capsule forms a sheath for the hepatic artery and portal vein which enter the liver and the hepatic duct which comes out of the liver.
The branches of these structures are conveyed by the interlobular connective tissue septa or portal canal and these three (i.e. interlobular branches of hepatic artery, portal vein and hepatic duct) are fanned at the periphery of the lobules together, forming the portal triad.
In sections they appear as cords of cells radiating away from the central vein. The cells are arranged in regular laminae or plates extending radially form the central vein to the periphery. The plates are one cell thick and curved. They anastomose by interlaminar bridges and between them are broad, irregular spaces-lacunae, containing sinusoids.
Liver cells (hepatocytes) are polyhedral in shape and have a large round nucleus, which is vesicular and show a few chromatin and one or more prominent nucleoli, cytoplasm presents a variable appearance, depending upon the functional status. Glycogen and fat are both dissolved in the usual preparations. Some cells have two nuclei.
The plates of liver cells are separated form one another by the sinusoids. These are irregular tortuous blood spaces, which pursue a radial course in the lobule and connect the ends of the interlobular portal veins with the intralobular central veins. They also receive blood form the branches of hepatic artery. They fill all the interlaminar space and anastomose irregularly.
The sinusoids are lined by irregularly alternating cells of two different kinds. One of these has small dark nucleus. Its cytoplasm extends as a thin film along the sinusoid. The other lining cells are larger and have processes, which appear to extend from one wall to other of the sinusoid or extend into the sinusoid. These have large oval vesicular nuclei and prominent nucleolus. These are referred to as stellate cells of Kupffer. They are phagocytic cells and form a part of the reticulo endothelial system.
There is potential space between the Kupffer cells lining the sinusoid and hepatic cells. It is called as space of Disse, which is traversed by a reticular fibre network. It is considered by some workers that lymph might pass through these spaces to the lymphatic vessels in the portal canal, as within the lobule no lymphatic capillaries are present.
The hepatic artery entering at the hilus divides into branches to supply the capsule of Liver. Some enter the portal canals and by divisions interlobular arteries which supply capillaries to interlobular connective tissue and then become continuous with the intralobular capillaries of portal vein.
Some of these may open directly into the sinusoids. Portal vein like the hepatic artery enters the hilus and divides into number of branches, which pass the portal canals as the interlobular veins. Branches of interlobular veins enter the periphery of the hepatic lobules and break into a brush of capillaries. These intralobular capillaries anastomose to form a sinusoidal network.
The hepatic sinusoids between the laminae of hepatic cells. These sinusoids converge towards the center of the lobule where they unite to form the central vein. The central vein is the efferent vessels from the lobule. Several central vein join to form a sublobular vein.
Sublobular veins are found in the interlobular connective tissue between the opposed faces of lobules. Several sublobular veins join to form a collecting vein, which in turn join to form hepatic veins which pursue independent of the portal venous system and open in the posterior vena cava.
Between the opposing faces of hepatic cells are minute channels, the bile canaliculi. In sections they appear as fine round openings between the two opposed hepatic cells. They are like the intercellular secretory capillaries of other glands; and fine tubules. They are formed by indentations or grooves on the opposing faces of two or three adjacent cells.
The bile capillaries form continuous networks within the liver plates and interlaminar bridges. Because they are always enclosed between adjacent cells, they are as far removed as possible from sinusoids. The hepatic cells therefore have two different physiological surfaces through which they discharge their products.
On the surface of the lobules the bile capillaries pass by the canal of herring into the lobular bile duct. The canals of Herring are lined by cuboidal epithelium. The interlobular ducts are lined by columnar epithelium. The interlobular bile ducts unite to form the hepatic duct which leaves the liver at the hilus. The large ducts are lined by high columnar epithelium and have a connective tissue coat.
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