Lec :1&2 Histology Dr.Rana Mustafa
Liver is the second largest organ (next to the skin)& it is the heaviest gland in the body, its w.t. 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 (70-80%) and the remaining small percentage (20%-30%) come from the hepatic artery.
All the absorbed material comes from the intestine through the portal vein pass to liver, except the lipid which passes through the lymphatic vessels.
Liver is an exocrine gland which secrete bile through the bile duct and also endocrine gland which it's secretion pass directly to the blood stream.
Histological structure :
The liver is enclosed by a connective tissue capsule (Glisson's capsule) and visceral peritoneum, except for small area where there is no peritoneum callec bare area.
At the porta hepatis, the C.T. capsule become thickened at the hilum & sends a branching network of septa into the substance of the liver to provide its main support; so, vessels, nerves, and bile ducts follow these septa to the interior of the liver.
Each lobe in the liver is separated into many tiny lobules, which form the functional unit of the liver liver lobule or hepatic lobule.
The classical hepatic lobule:
Is polygonal mass as seen under light microscope which consist of many hepatic cells are radially disposed & are arranged like the bricks of a wall, these celullar plates are directed from the periphery of the lobule to its center (toward the central vein) that located at the center.
These hepatic plates are separated from each other by a vascular channels called hepatic sinusoids.
In certain animals as pigs , the hepatic lobule are separated by C.T. which limitting the boundaries of each liver lobule, such demarcation is not present in human making its difficult to estabish the exact limits between different lobules, except in certain area at the periphery of each lobule, there is region called (portal spaces or portal area or hepatic canal) .
Portal area located at the corners of the lobules, each of which consisting of C.T. containing a venule (which is branch of portal vein), arteriole (which is branch of hepatic artery), duct (which is branch of bile duct) & lymphatic vessels, there is (3 to 6) portal areas per each lobule & these contents of portal area named as hepatic triad, some times there is more than one section of these structure & some times absence of one of them.
Is the main structural unit of the liver, these arranged in plates which directed from the periphery to the center of the lobule & anastomose freely forming sponglike structure
Such hepatocytes are polyhedral in shape with 6 or more surfaces.
In histological sections stained with hematoxylin &eosin
(H&E) , the hepatocyte is esinophilic mainly because of large no. of mitochondria & smooth & rough endoplasmic reticulum,it has one or two nuclei with one or two nucleoli, in addition to Golgi complex, lysosomes & free ribosomes.
The hepatocyte has(3) important surfaces,the hepatocyte surfaces are important because they have a role in a transfer 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.
Canalicular surface: account for 15% of a hepatocyte surface across which the bile drains from the hepatocytes into the canaliculi which 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.
Intercellular surface:is the surface between the adjacent hepatocytes , account for 15% of the hepatocyte surface , they are connected by communicating junctions.
Whenever two hepatocytes are separated, they delimit a tubular space between them known as the bile canaliculus,they limited only by the plasma membrane of two hepatocytes &have a small microvilli in their interiors.
So, bile canaliculi form a complex anastomosing network progressing along the plates of liver lobule & terminating in the region of the portal space.
The bile flow therefore progresses in direction opposite to that of the blood ( from the center of the lobule to the periphery).
At the periphery, bile enters the bile ductule or Hering's canals which after a short distance cross the limitting hepatocytes of the lobule & end into the bile duct in the portal area, such duct lined by cuboidal or low columnar epithelium & have a distinct C.T. sheath .
Then the bile ducts gradually enlarge& forming the RT< hepatic ducts which subsequently leave the liver through its hilum then unite together forming common hepatic duct that join the cystic duct to form the common bile duct which open into the ampulla of vator in the 2nd part of duodenum together with main pancreatic duct.
Biliary tract arranged as follows: Bile canaliculi→bile ductule→interlobular bile duct →RT< hepatic ducts→common hepatic duct→join cystic duct→common bile duct .
are irregularly dilated vessels composed of a discontinous layer of fenestrated endothelial cells, have no diaphragm, with spaces between the endothelial cells which together with a discontinous basal lamina, give these vessels great permeability.
Asubendothelial space known as space of Disse separates the endothelial cells from the hepatocytes, such space contains extracellular fluid supporting by reticular fibers containing floating microvilli of hepatocytes.
The fenestrae &discontinuity of endothelium allow easy exchange of molecules from the sinusoidal lumen to the hepatocytes &vice versa.
The space of Disse (perisinusoidal space) contains fat storing cells called Ito's cells or stellate cells which contain vitamin A rich lipid inclusions or vesicles whichactivated in liver cirrhosis.
The sinusoidal capillaries are lined in addition to the endothelial cells by macrophages known as Kuffer cells which acount 15% of liver cell population, lie on the luminal surface of sinusoidal capillaries & most of them located at the peripheral region of the lobule where they are very active in phagocytosis, their functions are:
metabolize the aged RBC.
secrete proteins related to immunological processes.
destroy bacteria that eventually enter theportal blood through the large intestine.
Blood supply of the liver :
A – portal venous system:
1. Portal vein divide in to branches called interlobular venules run between the hepatic lobules ( Liver lobules ).
2. The interlobular venules penetrate the lobules to empty in the
3. The sinusoid run radially and meet with each other to form the
4. The central veins of the lobules join together to form two large veinsRT & LT 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 the sinusoids contain a mixture of portal venous blood 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.
The 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 .
The wall of the gall bladder consist of:
1- Mucosa which composed of simple columnar epith. & lamina propria.
The epith. cells are tall columnar cells with basal nuclei, their
cytoplasm rich in mitochondria, microvilli are 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.
Near the cystic duct the epith. invaginate into the lamina propria
forming tubulo – alveolar glands which 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- Muscular coat consist of:
inner fibromuscular layer which contain smooth muscle fibers arranged irregulary in many directions,with many elastic fibers are present between the smooth muscle bundles of this coat.
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.
Main functions of gall bladder is to:
1- store the bile produced by the liver.
2- concentrate the bile through reabsorbtion of large quantities of water & mineral salts through it's mucosal layer
3-and emptying the concentrated bile in to the common bile duct.
The pancreas is a soft, elongated organ behind the stomach, it consist of head, neck, body & tail. the head lie in the duodenal loop and the tail extend to the spleen,
The pancreas is a mixed exocrine – endocrine gland, it's surface appears lobulated & 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 portions between these acini, there are the endocrine portion (pancreatic islets).
The pancreas has rich capillary network, essentialfor the secretory process.
Pancreatic acini (serous acini) exocrine portion:
The pancreatic acini are all of serous type, each acinus consist of pyramidal shaped cells (zymogenic cells) surrounding a small lumen, the cells have broad base & narrow apical surface covered by short microvilli, the apical part containing acidophilic granules called zymogen granules,the nuclei of the acinus cells lie in the basal part, they are spherical in shape &each contains nucleoli.
Another characteristic feature of exocrine portion of pancreas is that , the initial portion of intercalated duct are penetrate deep into the center of acini forming special cell called centroacinar cell, it appear as small flattened epith.in which their nuclei surrounded by pale cytoplasm.
The excretory duct system of exocrine portion: begin from small cells inside the acini called centroaciner 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 which lie in the septa between the lobules, the interlobular ducts united to form the main pancreatic duct lined by tall ciliated columnar epith. which open in to the duodenum at the "ampulla of vator".
This exocrine portion of pancreas is similar to the parotid which is pure serous gland but the differences between them as follows :
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 that in pancreas.
6- The nucleus in the cells of the parotid centrally located while in the
pancreas the nucleus lie near the base of the cell.
7- The cells of interlobular duct in pancreas is columnar epith. While
in parotid the cells are pseudosratified or stratified columnar.
8- Presence of adipose tissue in parotid gland.
Function of pancreatic (exocrine part):
The exocrine portion secrete about 1500-3000ml of alkaline fluid per day containing water, ions &several enzymes which include:
1. Trypsin: it breakes 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 two hormons secretin & cholecystokinin which are produced by the endocrine cells of digestive mucosa in response to the entry of gastric acid into the duodenum (acidic chyme) so, these hormons neutralize the acidic PH to allow the pancreatic enzymes to act at their optimal neutral PH.
The endocrine portion of the pancreas (islets of langerhans):
They are highly vascularized aggregations of secretory cells paller than the acini, a 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:
A cells or alpha cells: (glucagons – secreting cells) these are large
cells, generally these cells are situated more peripherally in the islet & their cytoplasm contain acidophilic granules, constitute approximately 20% of the islets mass, they secret glucagon which tend to increase the level of blood sugar.
B cells or Beta cells: (insulin - secreting cells) they are 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 % of their mass . these cell secretes insulin which decreases the level of blood sugar.
D cells or gamma cells (somatostatin secreting cells): are very
few in human less than 5% , variable in position, secrete somatostatin hormon which inhibits the release of the other islet cells hormons.
F cells: its rare, variable in position , secrete pancreatic polypeptide
which control of gastric secretion.
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.