LIVER
The liver is the largest gland in the body, weighing about 1-1.5 kg.
It is situated in the abdominal cavity beneath the diaphragm.
The liver is the organ in which nutrients absorbed in the digestive
tract are processed and stored for use by other parts of the body. It
is thus an interface between the digestive system and the blood.
Have double blood supply: Most of its blood (70-80%) comes
from the portal vein, arising from the stomach, intestines, and
spleen; the smaller percentage (20-30%) is supplied by the hepatic
artery, so it have double blood supply.
Bile production and secretion from the liver is passed into the
intestinal tract for digestion of lipids.
Detoxification (removal of toxic substances), filtration of the
blood, removing bacteria and other foreign particles that have
gained entrance to the blood from the lumen of the intestine.
The liver also has the very important function of producing plasma
proteins, such as albumin, other carrier proteins, coagulation
factors.
Stroma
The liver is covered by a thin connective tissue capsule (Glisson's
capsule) that becomes thicker at the hilum, where the portal vein and
the hepatic artery enter the organ and where the right and left hepatic
ducts and lymphatics exit. These vessels and ducts are surrounded by
connective tissue all the way to their termination (or origin) in the portal
spaces between the liver lobules. At this point, a delicate reticular fiber
network that supports the hepatocytes and sinusoidal endothelial cells of
the liver lobules is formed. The structural units of the liver are called
liver lobule.
Liver Lobules
:
1. Classic Liver Lobule: (this is based on the direction of the blood
flow)
It is a hexagonal mass of tissue primarily composed of plates of
hepatocytes which radiate from the region of the central vein (present in
the center) toward the periphery. The plates are separated by hepatic
sinusoids. At the corners of the lobule, there is the portal canals [portal
areas, portal tracts, portal spaces or portal triad].Each portal canal
contains: a portal venule (a branch of portal vein), a hepatic arteriole (a
branch of hepatic artery), bile duct and lymphatic vessels. These
structures are surrounded by loose connective tissue.
The arterial and venous blood flows centripetally in each lobule (from
the portal areas to drain in the central vein), whereas the bile flows
centrifugally towards the portal areas.
In the lobule, various zones of hepatocytes can be identified. These are
the centrolobular, periportal, and mid zones.
1. The centrolobular zone: is composed of hepatocytes surrounding
the central vein, and is the zone most distant from the oxygenated
arterial blood supply.
2. The periportal (perilobular zone): at the periphery of the lobule, is
closely related to the portal tracts. The outermost layer of
periportal hepatocytes immediately adjacent to the portal tract is
called the limiting plate; it is the first group of hepatocytes to be
damaged in liver disorders that primarily involve the portal tracts.
3. The mid zone: is the zone of hepatocytes between the centrolobular
and periportal zones.
2. Portal Lobule: (this is based mainly on the direction of bile flow)
It is a triangular region with a portal triad at its center and a central vein
at each of its three corners. It contains portions of three adjacent classic
liver lobules all of which drain bile into one portal canal.
3. Hepatic Acinus (of Rappaport): (it is based on changes in oxygen,
nutrient, and toxin content as blood flowing through the sinusoids is
acted on by hepatocytes)
It is that region which is supplied by a terminal branch of the portal vein
and hepatic artery and drained by a terminal branch of the bile duct. It is
a diamond-shaped region, contains portions (triangular sections) of two
adjacent classic liver lobules (whose apices are the central veins).
Hepatocytes:
They are the main cells of the lobules. They are epithelial cells (derived
from embryonic endoderm).
They are arranged in plates that anastomose freely and separated from
each other by spaces filled by hepatic sinusoids and are disposed radially
around the central vein. Hepatocytes are large polyherdral cells (20-30
µm in diameter). Hepatocytes have three important surfaces:
1- Sinusoidal surfaces: are separated from the sinusoidal vessel by
the space of Disse. They account for approximately 70% of the
total hepatocyte surface. They are covered by short microvilli,
which protrude into the space of Disse. The sinusoidal surface is
the site where material is transferred between the sinusoids and the
hepatocyte.
2- Canalicular surfaces: are the surfaces across which bile drains
from the hepatocytes into the canaliculi. They account for
approximately 15% of the hepatocyte surface.
3- The intercellular surfaces: are the surfaces between adjacent
hepatocytes that are not in contact with sinusoids or canaliculi.
They account for about 15% of the hepatocyte surface.
Hepatic sinusoids: are vessels that arise at the periphery of a lobule
and run between adjacent plates of hepatocytes. They receive blood from
the vessels in the portal areas and deliver it to the central vein. Sinusoids
are larger than capillaries, more irregular in shape and their lining cells
are directly related to the epithelial cells with no intervening c.t. The
lining of the sinusoids consists of a discontinuous layer of fenestrated
endothelium also contain Kupffer cells in their endothelial lining. They
lack basal lamina.
Kupffer cells: are phagocytic cells (fixed macrophages) derived from
blood monocytes and located in the lining of hepatic sinusoids. Their
main functions are to metabolize aged erythrocytes, digest hemoglobin,
secrete proteins related to immunologic processes, and destroy bacteria
that enter the portal blood through the large intestine.
Space of Disse: is the perisinusoidal space located between hepatocytes
and the endothelium of hepatic sinusoids. It contains:
microvilli of the hepatocytes
plasma, reticular fibers
fat-storing cells (Ito cells).
Blood plasma enters this space through openings between the
endothelial cells that are too small for blood cells to pass. Blood-borne
substances thus directly contact the microvilli of hepatocytes. These
cells absorb nutrients, oxygen, and toxins from, and release endocrine
secretions into, these spaces.
It functions in the exchange of material between the bloodstream and
hepatocytes, which do not directly contact the blood stream.
Fat-storing (Ito) cells: are stellate cells lie in the space of Disse and
have the ability to accumulate lipid droplets. They are the main source of
vitamin A storage in the body.
Gallbladder:
It is a hollow, pear-shaped organ attached to the lower surface of the
liver. It can store 30-50 ml of bile. The gallbladder absorbs water from
the bile and stores the bile in a concentrated form. The wall of the
gallbladder consists of the following layers:
1. Mucosa: is composed of a simple columnar epithelium and a
richly vascularized lamina propria. The mucous membrane is
thrown into folds that are particularly evident in the empty bladder.
There is no gland in the gallbladder except in the neck region.
2. Thin muscular layer: is composed of a thin layer of bundles of
smooth muscle fibers oriented in several directions . The muscle
contracts
and empties
the
gallbladder
in
response
to
cholecystokinin released by entero endocrine cells (I-cells) in the
intestinal mucosa when dietary fat enters the small intestine.
3. External adnentitia or Serosa: covers its free surface.
Pancreas:
The pancreas is a mixed exocrine-endocrine gland that produces
digestive enzymes and hormones. The enzymes are stored and released
by cells of the exocrine portion. The hormones are synthesized in
clusters of endocrine epithelial cells known as islets of Langerhans. The
pancreas is a retroperitoneal gland. It has a head, body, and tail. The
head is lodged in the concavity of the C-shaped duodenum and its
narrower boy and tail extend to the hilus of the spleen.
A thin capsule of c.t. covers the pancreas and sends septa into it,
separating the pancreatic lobules. These septa convey blood and
lymphatic vessels into and out of the parenchyma and house the
interlobular ducts.
The exocrine portion of the pancreas
is a compound acinar serous gland. The exocrine pancreatic acinus is
composed of several serous cells surrounding a lumen.
Islets of Langerhans (endocrine pancreas):
They appear as rounded clusters of cells embedded within exocrine
pancreatic tissue. Most islets are 100-200 µm in diameter and contain
several hundred cells, small islets also are found. There may be more
than one million islets in the human pancreas, with a slight tendency for
islets to be more abundant in the tail region.
In sections, each islet consists of lightly stained polygonal or
rounded cells, arranged in cords separated by a network of blood
capillaries. A fine capsule of reticular fibers surrounds each islet.
Using immunocytochemical methods four types of cells A, B, D,
and F have been located in the islets. The ultrastructure of these cells
resembles that of cells synthesizing polypeptides.
A or α-cells ≈ 20% produce the hormone glucagon whose effects are
opposite those of insulin.
B or β-cells are the most numerous ≈ 70%. Produce insulin and tend to
be concentrated in the center of the islet.
D or delta cells 5% produce somatostatin.
F or PP cells 1-2% secrete pancreatic polypeptide.