the liver

بسم الله الرحمن الرحيم

Liver
The liver is the second largest organ in the bod, (the largest is the skin), weighing about 1-1.5 kg. It is situated in the abdominal cavity beneath the diaphragm. Most of it's blood supply comes from the portal vein (70%) and a small percentage come from the hepatic artery. All the absorbed material come from the intestine through the portal vein, except the lipid which pass through the lymphatic vessels. liver is an exocrine gland which secrete bile through the bile duct and also an endocrine gland with it's secretion pass directly to the blood stream .


The liver is covered by a thin connective tissue capsule Glison's capsule that becomes thicker at the hilum (porta hepatis), where the portal vein and the hepatic artery enter the organ and where the right and left hepatic ducts and lymphatics exit.

Histological structure : the main structural unit of the liver is the "Hepatocyte" , these cells are grouped in plates in such away to appear in light microscope as polygonal mass called ( liver lobule ) which have portal spaces at the periphery and in the center is vein (central or centrolobular vein),in certain animals as in pigs these lobules are separated from each other by a layer of C.T., this does not occur in humans where it is difficult to determine the exact limit between the lobules .
Liver lobule
Portal area


in some regions however the corners of the lobules are separated by C.T. containing vessels . these regions are called " portal spaces or portal areas or portal canals " about 3-6 in each lobule, each containing a venule which is a branch of the portal vein , arteriole which is a branch of the hepatic artery , a duct ( part of the bile duct system ) & lymphatic vessels.
hepatic artery
Portal vein
Bile duct



The hepatocytes are radially arranged as plates like the bricks of a wall in the liver lobules , these plates are directed from the periphery to the centre of the lobule , between the plates lies sinusoidal capillaries.
sinusoid
sinusoid
Hepatic plates
Kupffer cell

The sinusoidal capillaries are : irregularly dilated vessels composed of a discontious layer of fenestrate endothelial cells,The spaces between the endothelial cells together with the cellular fenestrate and a discontinous basal lamina, give these vessels great permeability.
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The sinusoids arise at the periphery where the hepatic artery and portal vein are emptying and these sinusoids run to end in to the central vein of the lobule

sinusoid is surrounded and supported by a delicate sheath of reticular fibers.

In addition to the endothelial cells, the sinusoids contain macrophages known as Kupffer cells. These cells are found in the luminal surface of the endothelial cells, within the sinusoids. Kupffer cells account for 15% of the liver cell population

The main functions of kupffer cells are: 1- To metabolize aged erythrocytes . 2 - Digest hemoglobin 3- Secrete proteins related to immunological processes . 4- And destroy bacteria that eventually enter the portal blood through the large intestine .



The hepatocytes are separated from the sinusoids by a space ( space of Disse ). which consist of extracellular fluid with reticular fibers and the microvilli of the hepatocyte float in it . The hepatocytes are polyhedral with six or more surfaces, each containing one or two nuclei, in sections stained with H&E the cytoplasm appear eosinophilic due to the presence of mitochondria, smooth and rough endoplasmic reticulum, Golgi, lysosomes & free ribosomes.


The Functional units of the liver are: 1- Classical lobule: is discussed before. 2- Portal lobule: which at it's center is the portal canal, it is triangular in shape, surrounding the portal canal is parts of three adjoining hepatic lobules, so all of which drain bile into one portal canal. The central veins lies at the tip of the angles of the triangle.

3- Hepatic Acinus: it is the region which irrigated (supllied) by the terminal branch of the interlobular veins. it is diamond in shape, in addition to the vein there is the terminal branch of the hepatic artery & the ductule, they lie between 2 classical liver lobules.

The hepatocyte has three important surfaces , the hepatocyte surfaces are important because they have a role in the transference of substances between the hepatocytes, blood vessel and bile canaliculi : Sinusoidal surface: separated from a sinusoids by a space of Disse this account for 70% of a total hepatocyte surface , the surface is covered by short microvilli and it is a site where materials transferred between the sinusoids and hepatocytes.


2. Canalicular surface: account for 15% of a hepatocyte surface across which the bile drains from the hepatocytes into the canaliculi which are 0.5 – 2.5 micrometer in diameter and lined by irregular microvilli , the cytoplasm of hepatocytes close to the canaliculi is rich in " actin filaments " which influence the diameter of the canaliculi and a rate of flow of bile.

3. Intercellular surface: is the surface between the adjacent hepatocyte, account for 15% of the hepatocyte surface , they are specialized for cell to cell communication by communicating junctions.


Biliary tract : Bile produced by the liver cells flows through bile canaliculi which lies between the liver cells , the bile canaliculi are the first portion of the intrahepatic bile duct system , The cell membranes near these canaliculi are firmly joined by tight junctions . The bile canaliculi form a complex anastomosing network progressing along the plates of the liver lobule and terminating in the region of the portal spaces .
Bile canaliculi
Bile canaliculi
Central vein

The bile flow therefore progresses in direction opposite to that of the blood (from the center of the lobule to the periphery). The bile canaliculi anastomose with each other to form the bile ductules.



The ductules are lined by cuboidal cells with clear cytoplasm & few organells , bile ductules end in to the bile ducts ( interlobular duct ) which lie in the portal area & lined with columnar or cuboidal epith. , they gradually enlarge and fused to form the R & L hepatic ducts which leave the liver through it's hilum (porta hepatis) . the R & L hepatic ducts join together forming common hepatic duct which meet the cystic duct of gall bladder to form the common bile duct. Bile canaliculi bile ductules interlobular ducts R&L hepatic duct Common hepatic duct which join the cystic duct


Blood supply of the liver :A ­– portal venous system: 1. Portal vein divide in to branches called interlobular venules runs between the hepatic lobule 2. The interlobular venules penetrate the lobules to empty in the Sinusoidal capillaries. 3. The sinusoid run radially and meet with each other to form the Central vein. 4. The central veins of the lobules join together to form two large veins R & L hepatic veins which opens in to the inferior vena cava.

B – Arterial system: The hepatic artery branch and give interlobular arterioles that lies in between the liver lobule and open in the sinusoids, so sinusoids contain a mixture of portal venous and arterial blood

Function of the liver:1- Bile synthesis & secretion – bile is an alkaline secretion containing Water, ions & bile pigments " Exocrine function". 2-Protein synthesis like albumin & blood clotting factors "Endocrine function". 3-Detoxification of toxic substances like alcohol & drugs.4- Storage of materials like glycogen & vitamin.5- Change lipid and amino acid into glycogen.

Gall bladderThe gall bladder is a hallow , pear –shaped organ attached to the lower surface of the liver ,the parts of gall bladder are fundus,body, neck , & cystic duct ,the cystic duct communicates with the common hepatic duct to form common bile duct . fibromuscular coat
mucosa
epithelium
lamina propria
serosa

Histological structure: The wall of the gall bladder consist of: 1- Mucosa which composed of simple columnar epith. & lamina properia. The epith. cells are tall columnar cells with basal nuclei, their cytoplasm rich in mitochondria, microvillus is present at the apical surface of the cells, these cells are specialized for absorption of water from the bile. The epith. of the gall bladder is thrown into folds or Plicae which are evident when the gall bladder is empty & they are flattened when the gall bladder is distended.

fibromuscular coat

mucosa
epithelium
lamina propria
perimuscular layer
serosa
Near the cystic duct the epith . invaginate into the lamina properia forming tubulo – alveolar glands with awide lumen, their cells secrete mucous which is responsible for the mucous present in the bile. The lamina properia contains loose C.T. & some diffuse lymphatic tissue. There is no muscularis mucosa in the gall bladder.2- The muscular coat consist of inner fibromuscular layer which contain Smooth muscle fibers arranged irregularly in many directions, many elastic fibers are present between the smooth muscle bundles of this coat. The outer perimuscular or subserosal layer consist of areolar tissue & it may contain groups of fat cells. it conveys arteries, Veins, lymphatics & nerves to the organ.3- C.T. serosa which cover the muscular coat.


The main function of gall bladder is to store the bile produced by the liver. it also serves to concentrate the bile through reabsorption of large quantities of water & mineral salts through it's mucosal layer & emptying the concentrated bile in to the common bile duct.

PancreasThe pancreas is a soft, elongated organ behind the stomach, it consist of head, body & tail. the head lie in the duodenal loop and the tail extend to the spleen, it's surface appears lobulated ,the pancreas is a mixed exocrine – endocrine gland,

It is covered with loose C.T. from which septa pass into the gland subdividing the pancreas into a number of lobules which are incomplete so the demarcations between the lobules are not clear, each lobule is composed of spherical clusters (acini) of secretory exocrine cells,between these acini, there are the endocrine portion(pancreatic islets).

Pancreatic acini (serous acini): The pancreatic acini are all of serous type, each acinous consist of pyramidal shaped protein-secreting cells (zymogenic cells) surrounding a small lumen, the cells have broad base & narrow apical surface coverd by short microvilli, the basal part of the cells are deeply basophilic stained due to the presence of rough endoplasmic reticulum, while the apical part containing acidophilic granules called zymogen granules. .
centroacinar cell
interlobular duct
interlobular C. T. septa
pancreatic islet

The nuclei of the acinous cells lie in the basal part, they are spherical in shape &each contains nucleoli. Near the lumen of the acinous there is special cell called centroacinar cell, it appear as small rounded epith. cell, they differ from the acinar cells in having a cytoplasm devoid of secreting granules or basophilic substances. this cell is considered as the beginning of the excretory duct system


The excretory duct system begin from small cells inside the acini called centroacinar cells, these cells lies in the beginning of the intercalated duct which is the first part of the pancreatic ductal system, the intercalated ducts are lined by simple cuboidal epith. , they fuse to form larger interlobular duct lined with columnar epith.which lie in the septa between the lobules, the interlobular ducts united to form the main pancreatic duct lined by tall ciliated columnar to sratified epith. which open into the duodenum at the "ampulla of vator"


This exocrine portion of pancreas is similar to the parotid ,it is pure serous gland but the differences between them as follows : 1- the pancreas is covered by loose c.t. while in parotid is covered by thick c.t. capsule . 2- absence of striated duct in pancreas 3- presence of islets of Langerhans in pancreas. 4- presence of centroacinar cells in pancreas. 5- the alveolus of the parotid is smaller than in pancreas.. 6- the cells of interlobular duct in pancreas is columnar epith. While in parotid the cells are pseudosratified or stratified columnar. 7- no myoepithelial cells surrounding the pancreatic acini.

Function of pancreatic exocrine part: The secretion of the pancreatic juice consists of three main enzymes: 1. Trypsin: it brakes down protein to amino acid. 2. Amylase: it converts starch to maltose. 3. Lipase: it convert the fat to fatty acid & glycerol. The pancreatic enzyme is controlled by a hormons secretin & cholecystokinin which are produced by the enteroendocrine cells of the intestinal mucosa (duodenum , jejunum).


The endocrine portion of the pancreas (islets of langerhans): are multihormonal endocrine microorgans; they appear as rounded clusters of cells embedded within the exocrine pancreatic tissue. They are highly vascularized aggregations of secretory cells paller than the acini, thin C. T. capsule separate the endocrine part from the dark staining exocrine part, the endocrine cells are arranged in cords & clumps, between the cords are fine C. T. with capillaries. The size of the islets varies from one area to the other, the number of islets also varies in different parts of the pancreas, it is more in the tail region than other part.

The cells in the islet are 4 types:1- A or Alpha cells: (glucagon – secreting cells) these are large cells, generally these cells are situated more peripherally in the islet & their cytoplasm contain acidophilic granules, they secret glucagon which increase the level of sugar in the blood. C. T. capsule
Alpha cells

2- B or Beta cells: (insulin secreting cells) they more in number than alpha cells, they contain basophilic granules, these cells are located deeper or more in the center of the islets & are the common cell type of pancreatic islets & constitute approximately 70 % 0f their mass. The cell secretes insulin which decreases the level of sugar in the blood,.

Beta cells

centroacinar cells


3- D or Delta secrete the somatostatin hormone which decreases and inhibit secretory activities of both Alpha and Beta cells through local action within the pancreatic Islet. 4- F-cells: Its secretion is unknowen.

Blood Supply of islet: The islets contain a complex of network of capillaries which arise from small arterioles outside the islets, they are fenestrated capillaries & they merge with the capillaries supplying the exocrine component of pancreas.