The Digestive SystemLecture 11

Histology of the Liver▪ The liver is completely surrounded by a

fibrous (Glisson's) capsule, containing

collage and elastic fibers.

▪ Septa pass inward from the fibrous capsule,

which divide the liver tissue into lobules.

▪ In the human liver, the fibrous septa are

indistinct, and the lobules are not clearly

demarcated from one another.

▪ Lobules are polygonal prisms which, on

section, show five, six or more sides.

1

Figure 1. Transverse section through the liver

stained with Mallory-azan, showing the liver

lobules. Low magnification. 2

▪ A central vein runs through the center of

each lobule.

▪ The central veins drain away from the

lobule (drain into sublobular and hepatic

veins).

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Figure 2. Schematic

drawing shows the

structure of the

liver. The liver

lobule in the center

is surrounded by the

portal space;

arteries, veins and

bile ducts occupy

the portal space. In

the lobule, note the

radial disposition of

the cords (plate

formed by the

hepatocytes. The

sublobular

(intercalated) veins

drain blood from

the lobules.4

▪ Between the periphery of the lobule and

the central vein, the parenchyma consists

of rows or cords of cells that run from the

periphery of the lobule to the central vein

and anastomose freely.

▪ The cells that make up these cords are

hepatocytes, the chief functioning cells of

the liver.

▪ Hepatocytes are cells that secrete bile,

while at the same time they also perform

many endocrine and metabolic functions.

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▪ Cords are separated by fine vascular

sinusoids, the liver sinusoids, through

which blood flows.

▪ These sinusoids are dilated vessels

composed of a discontinuous layer of

fenestrated endothelial cells.

▪ The fenestrae have no diaphragm.

▪ There are also spaces between endothelial

cells, which together with the cellular

fenestra and a discontinuous basal lamina,

gives these vessels great permeability.

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Endothelial cells of

sinusoid

Figure 3. Three

dimensional

aspect of the

normal liver. In the

upper center is

the central vein; in

the lower center is

the portal vein.

Note the bile

canaliculus, liver

cords (plates),

Hering’s canal,

Kupffer cells,

sinusoid, fat

storing cells, and

sinusoid

endothelial cells.

▪ A subendothelial space known as

perisinusoidal space (space of Disse)

separates the endothelial cells from the

hepatic cords.

▪ The fenestrae and discontinuity of

endothelium allow the free flow of plasma but

not cellular elements into the space of Disse,

permitting an easy exchange of molecules

(including macromolecules) from the

sinusoidal lumen to the hepatocytes and vice

versa.

▪ Blood flow into liver sinusoids comes from

terminal branches of both hepatic artery and

portal vein. 8

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▪ The liver, therefore, unusual in having both

arterial and venous blood supplies, as well

as separate venous drainage.

▪ The hepatic artery brings blood that is rich

in oxygen to support liver metabolism.

▪ The portal vein brings materials absorbed

from intestine.

▪ In turn blood from liver drains via hepatic

veins into inferior vena cava.

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▪ In addition to the endothelial cells, the liver

sinusoids contain macrophages known as

Kupffer cell, which are found on the luminal

surface of endothelial cells, within the

sinusoids.

▪ Kupffer cells account for 15% of liver cells

population, their main functions:

a. Metabolize aged erythrocytes.

b. Digest hemoglobin.

c. Secrete protein related to immunological

process and destroy bacteria that

eventually enter the portal blood through

the intestine. 11

Figure 3. Three

dimensional

aspect of the

normal liver. In the

upper center is

the central vein; in

the lower center is

the portal vein.

Note the bile

canaliculus, liver

cords (plates),

Hering’s canal,

Kupffer cells,

sinusoid, fat

storing cells, and

sinusoid

endothelial cells.

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▪ In the space of Disse, fat-storing (stellate)

cells, contain vitamin A-rich lipid inclusions

(fig. 3); their function:

a. uptake, storage, and release of retinoids.

b. synthesis and secretion of extracellular

matrix proteins and proteoglycans.

c. secretion of growth factors, and cytokines.

d. regulation of sinusoidal lumen diameter.

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Figure 3. Three

dimensional

aspect of the

normal liver. In

the upper center

is the central

vein; in the lower

center is the

portal vein. Note

the bile

canaliculus, liver

cords (plates),

Hering’s canal,

Kupffer cells,

sinusoid, fat

storing cells, and

sinusoid

endothelial cells.14

▪ The main blood vessels and ducts run

through the liver within a branched

collagenous framework termed the portal

tracts.

▪ These tracts also contain the bile ducts

that transport bile away from the liver to be

secreted into the small intestine.

▪ The portal spaces located in the corners of

the lobules, contain connective tissue, bile

ducts, lymphatics, nerves, and blood

vessels.

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Figure 5. Liver, transverse section stained with

hematoxyline and eosin, showing liver lobules. High

magnification.

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▪ In human each lobule is associated with 3

to 6 portal spaces.

▪ Each lobule has a venule (branch of portal

vein), an arteriole (branch of hepatic artery),

a duct (part of the bile duct system), and

lymphatic vessels.

▪ The duct, which is lined by cuboidal

epithelium, carries bile synthesized by the

hepatocytes and eventually empties into

the hepatic duct.

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Figure 3. Three

dimensional

aspect of the

normal liver. In

the upper center

is the central

vein; in the lower

center is the

portal vein. Note

the bile

canaliculus, liver

cords (plates),

Hering’s canal,

Kupffer cells,

sinusoid, fat

storing cells, and

sinusoid

endothelial cells.18

Hepatocyes▪ Are polyhedral has six or more surfaces (fig.

8).

▪ Wherever two hepatocytes abut, they delimit a

tubular space between them called the bile

canaliculus, which are limited only by the

plasma membranes of the two hepatocytes

and have few microvilli in their interiors.

▪ At the periphery, bile canaliculi enter bile

ductules (hering's canal), which in turn end in

the bile ducts.

▪ Hering's canals are lined by cuboidal

epithelium. 19

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Bile canaliculus

Hering’s canal

Bile duct

▪ Bile ducts are lined by cuboidal or

columnar epithelium and have a distinct

connective tissue sheath.

▪ They gradually enlarge and fuse, forming

right and left hepatic ducts, which

subsequently leave liver.

▪ Hepatocytes contain one or two round

prominent nuclei with one or two nucleoli,

and peripherally dispersed chromatin.

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Figure 8. Glycogen in liver cells. Stain: periodic

acid-schiff with blue counterstain for nuclei. Oil

immerstion. 23

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25

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▪ Nuclei vary greatly in size and more than half

the hepatocyte contains twice the normal

(diploid) complement of chromosomes.

▪ The extensive cytoplasim is strongly

eosinophilic due to numerous mitochondria.

▪ Has extensive free ribosomes and RER.

▪ The RER forms aggregates dispersed in the

cytoplasm; these are often called basophilic

bodies.

▪ SER is distributed diffusely throughout the

cytoplasm. 28

▪ Several proteins are synthesized on

polyribosomes in the basophilic bodies.

▪ The SER is responsible for the process of

oxidation, methylation, and conjugation

required for inactivation or detoxification of

various substances before their excretion

from the body.

▪ Numerous Golgi complexes are also present,

which are involved in the formation of

lysosomes and secretion of plasma proteins,

glycoproteins, and lipoproteins.

▪ The cytoplasm contains also number of lipid

droplet. 29

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▪ Lysosomes and peroxisomes are also

cytoplasmic components.

Peroxisomes are involve in oxidation of

excess fatty acids, breakdown of hydrogen

peroxide, breakdown of excess purines to uric

acid , and participation in synthesis of

cholesterol, bile acids, and some lipids used

to make myelin.

▪ Bile secretion is an exocrine function.

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▪ Between 70% and 90% of bilirubin is derived

from degeneration of hemoglobin of

circulating red blood cells, which occurs

mainly in the spleen but also throughout the

rest of the peripheral mononuclear phagocyte

system including Kupffer cells within the liver.

▪ After transportation to the hepatocytes,

bilirubin is conjugated in the SER to

glucuronic acid, forming water soluble

bilirubin glucuronide.

▪ Bilirubin glucuronide is secreted into the bile

canaliculi. 33

Vessels and Nerves of the Liver▪ The liver receives arterial blood from the

hepatic artery and venous blood from the

portal vein.

▪ Both divide into right and left branches before

entering the porta hepatis.

▪ Within the liver further branches form the

segmental vessels.

▪ The Hepatic artery provides 20% of afferent

supply.

▪ Segmental artery provides interlobular artery.

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▪ Some of these arteries irrigate the portal

spaces, others divides and give rise to

arterioles, which enter the sinusoids at their

periphery or nearer the central vein.

▪ They provide an adequate amount of oxygen

to hepatocytes.

▪ The portal vein provides 80% of the afferent

blood supply.

▪ Segmental veins branch repeatedly and send

small portal venules to the portal spaces.

▪ These venules branch into the distributing

veins that run around the periphery of lobule.

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▪ From the distributing veins, small inlet

venules empty into sinusoids.

▪ The sinusoids run radially, converging in the

center of the lobule to form the central vein.

▪ As the central vein runs along the lobule, it

receives more and more sinusoid and

gradually increases in diameter.

▪ At its end, it leaves the lobule at it base by

merging with the larger sublobular vein.

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▪ The sublobular veins gradually converge and

fuse, forming the two or more large hepatic

veins that empty into the inferior vena cava.

▪ Lymph vessels from the liver enter several

lymph nodes in the portahepatis.

▪ Efferent vessels from these nodes pass to the

coeliac nodes.

▪ A few vessels pass from the bare area of the

liver through the diaphragm to the posterior

mediastinal lymph nodes.

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Hepatic artery Segmental arteries

Interlobular arteries Some irrigate

Portal space.

Others divide and

give rise to arterioles which enter sinusoids either

near periphery or near central vein; providing O2

to the hepatocytes.

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Portal vein Segmental arteries - Divide

Small portal venules (irrigate Portal

spaces) – Divide Distributing veins (round

periphery of lobules Small inlet venules

(empty into sinusoids) Central veins

Sublobular veins (converge and fuse) 2 to 3

Hepatic veins Inferior vena cava.

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