In: Biology
Write in detail on the histological the structure of liver and its tissue and cells and function?
Liver histology:-
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).
Histologically speaking, it has a complex microscopic structure, that can be viewed from several different angles. Physiologically speaking, the liver also performs many essential functions and it is your best friend when you are enjoying some beers with your friends. This article will examine every histological component of the liver, its macroscopic and microscopic vascular supply, and the biliary system.
Functions and physiology:-
The liver performs several important functions in the human body,
such as given below:
Plasma protein synthesis - albumins, lipoproteins,
glycoproteins, prothrombin, fibrinogen
Vitamin storage and modification - vitamins A, D, and K
Iron storage and metabolism - transferrin, haptoglobin, hemopexin,
ferritin
Drugs and toxins degradation
Bile production
Carbohydrate metabolism
Histological components:-
The liver consists of the following major histological
components:
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:
Classic lobule
Portal lobule
Liver acinus
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 canal 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.
The perisinusoidal space contains a specific type of cell called Ito, or hepatic stellate, cells. Their role is the storage of hepatic vitamin A inside lipid droplets, which is subsequently released as retinol. However, Ito cells are also responsible for hepatic fibrosis, since they are the ones secreting large amounts of collagen during liver injury.
Vasculature :-
Liver:-
The liver, as an organ, receives blood from two different sources.
The major one is via the hepatic portal vein (75%), which carries
venous blood from the intestines, pancreas and spleen. Despite the
lack of oxygen, this blood contains high amount of nutrients,
endocrine secretions, broken down erythrocytes, but also ingested
toxins. The second major source is via the hepatic artery (25%),
which brings oxygenated blood to the liver.
Together with the bile duct, the hepatic portal vein and hepatic artery form the portal triad. Those structures supply blood to the sinusoids and the hepatocytes, subsequently draining into the central vein followed by the sublobular veins. The second drainage pattern is via the hepatic veins, which end up in the inferior vena cava.
Parenchyma :-
Interlobular vessels connect the portal triads and sinusoids,
transporting the blood into the latter. The hepatic sinusoids have
a discontinuous epithelium due to the presence of fenestrae and
gaps between endothelial cells and constitute a low-resistance
vascular channel. A basement membrane is also absent. The sinusoids
appear as pale spaces between the hepatocytes in routine
hematoxylin and eosin (H&E) staining. The endothelial cells
display flat and condensed nuclei, with poorly stained
cytoplasm.
The name of the blood vessels travelling through the portal canals are called interlobular vessels. They send blood into the sinusoids, either directly if they are really small, or by branching into distributing vessels first, which in turn empty into the sinusoids via inlet vessels. From the sinusoids, the blood drains into the central vein, which occupies the central axis of the classic liver lobule. The endothelial cells forming the central veins are surrounded by a small quantity of connective tissue fibers.
As they travel through the parenchyma, the central veins become larger, subsequently emptying into the sublobular veins. The endothelial lining of the sublobular veins is surrounded by a high quantity of connective tissue fibers, consisting of a layer of both collagenous and elastic fibers. Several sublobular veins then converge into larger and valveless hepatic veins, which ultimately empty into the inferior vena cava.
The sinusoids also receive arterial blood from the hepatic arteries. The latter have a thick muscular wall and also supply the connective tissue and various structures in the portal canals.
In addition, sinusoids contain a specific cell type called Kupffer cell, containing ovoid nuclei. These monocyte derivatives of the mononuclear phagocytic system are part of the sinusoid lining from which they extend processes into the lumen. Therefore, Kupffer cells continuously sample the blood travelling through the sinusoids, phagocytosing antigens, microorganisms, and damaged red blood cells.
Biliary tree :-
This is a system consisting of channels that carries bile from the
source, this being the hepatocytes, all the way to the gallbladder
and intestines. The epithelial cells lining this system are called
cholangiocytes. They range in shape from cuboidal in small
ductules, to columnar in large ductules. Cholangiocytes have
several histological features, such as:
Few organelles
Presence of tight junctions between the cells
Microvilli on their apical domains
Primary cillium
The hepatocytes secrete bile into the bile canaliculi, which are
the smallest channels of the biliary tree. The secretion into these
canaliculi is an active process. They partially loop around the
hepatocytes and become the canals of Hering close to the portal
canal.
These canals are lined by both hepatocytes and cuboidal cholangiocytes. The flow of bile is opposite to that of the blood and the canals of Hering are capable of contracting and assisting this flow towards the portal canal. The canals of Hering also posses hepatic stem cells, which represent the source of hepatocytes and cholangiocytes.
The canals of Hering flow into intrahepatic bile ductules in the periportal space (of Mall), which are completely lined by cholangiocytes. In turn, the intrahepatic bile ductules flow into interlobular bile ducts, which are lined by cuboidal to columnar cholangiocytes and form part of the portal triad. As these ducts travel towards the porta hepatis, they increase in diameter and become invested inside dense connective tissue composed of elastic fibers. Finally, these interlobular ducts join to form the right and left hepatic ducts, which in turn merge to form the common hepatic duct. This duct is lined by columnar epithelial cells. The cystic duct branches from the common hepatic duct, carrying bile to and from the gallbladder.