DIGESTIVE SYSTEM III
continued
5. The liver cells or hepatocytes are arranged in an interconnecting network of
plates that are one or two cells thick.
http://www.finchcms.edu/anatomy/histology/organology/digestive/images/ff828.jpg
a. The sinusoids lie between
these plates allowing the blood
to percolate between adjacent
plates.
b. As blood passes through the
sinusoids, nutrients, toxins,
other wastes are transferred to
the hepatocytes where they are
processed.
c. Eventually the blood that
passes through the lobule
tissues empties into the central
collecting vein and is returned
to the large hepatic veins that
exit the liver.
d. From there the blood will
empty into the inferior vena
cava and be returned to the
heart.
5. The fact that the blood enters the lobule from its periphery, passes
through the tissues and empties into a central vein means that the blood
composition and the composition of the hepatocytes will show a gradient.
a. There will be higher
concentrations of both
nutrients and systemic tissue
wastes in more peripheral
hepatocytes of the lobule
and a lower concentration in
more central hepatocytes.
b. Differences such as these
are responsible in part for
the fact that the physiology
of the peripheral hepatocytes
is different than that of the
central hepatocytes.
D. Liver cells - hepatocytes - functional cells of the liver
1. As seen with the light microscope.
a. Polyhedral, six or more
surfaces.
b. 20-30 mm in "diameter”
c. One or 2 nuclei.
d. Cytoplasm is eosinophilic
when stained with H&E. This
is due to large numbers of
mitochondria and some
smooth ER.
e. Surface of each cell is
adjacent to blood sinusoid
walls with a space between
the liver cell and the
endothelial cells - called
space of Disse.
http://www.finchcms.edu/anatomy/histology/organology/digestive/images/ff837.jpg
http://med-ed.med.virginia.edu/med-ed/histology/imagedisplay.cfm?file=Cell0341
S.C. Kempf 2/14/00
2. As see with the
electron
microscope ultrastructure
a. Central
nucleus
b. Numerous
mitochondria
c. Abundant
rough and
smooth ER
Figure in
web notes
S.C. Kempf 2/14/00
d. Perinuclear golgi
bodies, lipid droplets,
glycogen granules,
lysosomes,
peroxisomes
e. Sometimes
autophagosomes are
present
f. The plasmalemma of
hepatocytes forms
microvilli that extend
into the space of Disse.
g. Reticular supportive
fibers are present on
the hepatocyte side of
endothelial cells.
http://www.finchcms.edu/anatomy/histology/organology/digestive/images/ff837.jpg
http://www.lab.anhb.uwa.edu.au/mb140/
S.C. Kempf 2/14/00
h. Where adjacent
hepatocytes abutt, there
are small canals (bile
canaliculi) that are
formed from
invaginations of the
plasmalemma of adjacent
cells.
i. Microvilli extend into
these canals.
j. Bile acids are secreted
into the canaliculi.
k. synthesized blood
proteins are exocytosed
into the space of Disse
http://med-ed.med.virginia.edu/med-ed/histology/imagedisplay.cfm?file=Cell0345
http://med-ed.med.virginia.edu/med-ed/histology/imagedisplay.cfm?file=Cell0346
k. The canaliculi form an
anastomosing network that empties
into the bile ductules (Hering's
canals).
l. The bile ductules empty into the
the interlobular bile ducts of the
triads and eventually these connect
to the hepatic ducts that empty into
the common bile duct.
E. Another common cell type found associated with the hepatocytes and the
sinusoids are the Kupfer cells.
1. Specialized macrophages (phagocytes) that are derived from
monocytes.
2. Phagocytose worn out erythrocytes, bacteria and particulate matter.
http://med-ed.med.virginia.edu/med-ed/histology/imagedisplay.cfm?file=Cell0342
http://med-ed.med.virginia.edu/med-ed/histology/imagedisplay.cfm?file=Cell0343
F. The other common cell type in the liver is the endothelial cells that line the
sinusoids.
3. Functions of hepatocytes
a. Protein synthesis
* Albumin, prothrombin, fibrinogen which are components of blood
plasma
* Interesting fact is that these proteins do not accumulate in the
cytoplasm of hepatocytes in groups of vesicles. Rather, the vesicles
are exocytosed into the blood as synthesis occurs.
b. Bile acid synthesis
* Only 10% of bile acids released from the common bile duct at any
given time were actively synthesized and released into the bile
canaliculi.
* The other 90% are recycled from the intestine as components of
the blood. These are absorbed by hepatocytes and re-introduced
into the bile canaliculi.
c. Storage of metabolites.
* Lipid and carbohydrate in form of glycogen are stored in
hepatocytes.
* This is reserve energy for use between meals.
* Also vitamin storage.
d. Conversion of lipids and amino acids to glucose.
e. Amino acid deamination that results in production of glucose and lipid
(from non-nitrogenous parts) and urea (from the removed amine group).
f. Detoxification & inactivation - various drugs and toxic substances can
be inactivated by hepatocytes
E. Gall bladder
1. This is a storage organ for
bile secretions of the liver.
2. The gall bladder also
concentrates bile by
reabsorption of water.
3. Connects to common bile
duct via cystic duct.
4. The wall of the gall bladder shows
some similarity to the structure of
the digestive tract in general, but
lacks a submucosa.
a.
has mucosa composed of
columnar epithelium and
lamina propria.
b.
Muscularis externa
c. well developed serosa or
adventitia depending on
location
however,
d. no muscularis mucosae
e. lacks a submucosa
f. smooth muscle layer
(muscularis externa) - inner
layer is longitudinal, outer
layer circular
http://www.lab.anhb.uwa.edu.au/mb140/
5. Mucus secreting tubuloacinar glands can be
found near the connection of the gall bladder
with the cystic duct.
a. These are formed as epithelial invaginations
into the lamina propria, - sort of like
crypts of Lieberkuhn.
6. The smooth muscle (muscularis externa)
surrounding the mucosa contracts to expel
stored bile into the intestine via the common
bile duct.
a.
ampulla of Vater
This contraction is caused by the
hormone cholecystokinin that is produced
by enteroendocrine cells in the crypts of
the small intestine.
7. Gall bladder contents are expressed into a
dialated portion of the duodenum called the
ampulla of Vater.
8. Flow of bile out of the common bile duct is
controlled by the sphincter of Oddi, a ring of
smooth muscle that surrounds the common
bile duct where it connects to the ampulla of
Vater.