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Intrinsic nerve fibers in the liver parenchyma.

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Intrinsic Nerve Fibers in the Liver Parenchyma
YOUSSEF MIKHAIL AND ABDEL LATIF SALEH
Faculty of Medicine, E i n Shams University, Cairo, Egypt, U.A.R.
The hepatic plexus which forms the
largest offset from the coeliac plexus receives filaments from both vagi and right
phrenic nerve. The nerves which enter the
liver at the porta hepatis are reported to
accompany mainly the blood vessels and
bile ducts.
Alexander ('40) could trace no nerve
fibers into the parenchyma of the liver.
Mitchell ( ' 5 3 ) claimed that the liver is
supplied with terminal ganglia in or near
the viscus. He also mentioned that intrinsic fibers in the liver are scanty and mainly
confined to the vicinity of blood vessels,
with few occasional groups found among
the liver cells. It is surprising that in spite
of the rich nature of the hepatic plexus
the liver parenchyma is scarcely supplied
by nerve fibers. Therefore this investigation was done to study the intrinsic nerve
fibers of the liver and their mode of termination. The presence of terminal autonomic ganglia in the liver parenchyma
was also verified.
Holmes method. In the last series of animals, the portions of liver removed were
fixed and stained by the Bodian protargol
method. Several brands of protargol were
used from different firms as DembachRoussel, Merck and Bayer. The best results were achieved by the Merck and
B ayer preparations.
OBSERVATIONS
Rat
The structure of the liver as revealed in
the first series of experiments was identical
with the normal histology of the liver
which has been described in the literature
before repeatedly and adequately. However few important points were noticed
which deserved recording. The presence
of spherical groups of cells at variable
intervals in the liver parenchyma was
peculiar. These collections of cells varied
in their size and number of their constituent cells. They included clusters of small
rounded cells with few triangular forms or
even horse-shoe-shaped cells. Superficially
MATERIAL AND METHODS
these collections resembled terminal paraThe animals used in this investigation sympathetic ganglia. Only the cell bodies
comprised normal adult male albino rats, were evident while their processes did not
mice, guinea pigs, rabbits and dogs. The appear (fig. 1 ) .
animals were divided into 4 series; each
In sections stained with thionine similar
series included at least two animals from collection of cells were stained with thioevery species investigated. All the animals nine appearing dark blue while the adjawere killed by gas and portions from the cent liver cells were stainless. However,
different lobes as well as from the area the stain faded slowly and the appearance
of the porta hepatis were removed. In the of the cells was not convincing.
first series the portions of livers taken were
In the third series of experiments the
fixed in Bouin. After paraffin embedding, Holmes method helped to demonstrate the
sections 15 p to 20 p in thickness were cut outline of nerve cells and nerve fibers in
and stained with hematoxylin and eosin. the liver parenchyma. The nerve fibers
In the second series the material was fixed were seen surrounding the branches of
in 96% alcohol and after paraffin em- the portal vein and bile duct and even
bedding sections were stained with 1% penetrating the liver parenchyma running
thionine. In the third series the areas re- along the sinusoids of the liver (fig. 2).
In the last series of experiments the outmoved were fixed in 5% formol saline,
embedded in paraffin and stained by the line of nerve fibers and nerve cells was
317
318
YOUSSEF MIKHAIL AND ABDEL LATIF SALEH
_I
Fig. 1
- Ganglion
Liver of rat stained by H & E. ( X 650) showing a large spherical ganglion.
iwrve noer
Fig. 2 Liver of rat stained by Holmes method ( x 320) showing nerve fibers extending
along the sinusoids of the liver.
very clear. Nerve fibers of varying thickness surrounded the branc:hes of the portal vein, hepatic artery and bile ducts. The
nerves formed a meshwork of fibers which
surrounded the vessels on all sides. Nerve
fibers radiated from the big vessels to the
various lobules of the liver. They extended
between the cords of the liver cells for
variable intervals being rather tortuous
and confined mainly to the vicinity of the
liver sinusoids. Terminal ganglia were
found in the area of the porta hepatis and
in the various lobes of the liver. They were
found near to the portal canals and even
within the liver lobules. Occasionally the
ganglia formed large spherical groups
PERIPHERAL NERVES O F LIVER
Nerve cell
3119
Nerve cell
Fig. 3 Liver of rat stained by Bodian method ( X 950) showing a cluster of neurons and
nerve fibers among the liver cells.
scattered at wide intervals in the liver
parenchyma. These ganglia were formed
from large clusters of cells which were
surrounded by numerous nerve fibers from
all sides. The processes related to the
cells in the middle of the ganglia were not
evident while those related to the peripheral cells were apparent. Even post ganglionic fibers were often seen radiating
from the cells and extending between the
cords of liver cells.
Frequently the terminal ganglia were
formed from small clusters of cells which
were found near the walls of the central
veins of the liver lobules (fig. 3). The nerve
fibers and the processes connected to these
neurons extended for variable intervals
between the cords of liver cells and along
the sinusoids of the liver.
Often isolated nerve cells one or more
were seen in between the liver cells. They
varied in their shape but usually they appeared either rounded, triangular or pyriform in shape (fig. 4). They were usually
multipolar and their dendrites varied
greatly in length. Cells which possessed
long and short dendrites were frequently
encountered while cells with pure short
dendrites were scarcely seen. The dendrites extended in between the liver cells
often interlacing with the dendrites of a
neighbouring nerve cell forming a peripheral plexus. The short dendrites often
ended in knob like swellings which were
very close to the hepatic cells. In the terminal plexus it was often difficult to differentiate between long dendrites and post
ganglionic fibers. Similarly the terminal
nerve fibers in the plexus could not be
identified as afferent and efferent by any
of the techniques employed. Occasionally
it was possible to differentiate the post
ganglionic fibers from other processes as
they sprouted from the nerve cells and
extended for long distances among the
liver cells. Since only fibers of vagus origin
effected a synaptic connection with terminal ganglia, the numerous ganglia encountered in the liver of the rat were regarded
as totally of parasympathetic nature.
YOUSSEF MIKHAIL AND ABDEL LATIF SALEH
320
Multipolar nerve cell
Fig.4
Liver of rat stainetd by Bodian method ( X 1200) showing a multipolar nerve cell.
Mouse
The parenchyma of the liver of the
mouse was permeated by nerve fibers
which extended along the portal canals.
The terminal ganglia in the liver were
however small, and scattered at wide intervals. Small collections of nerve cells
were found near the walls of the central
veins. Isolated nerve cells were often seen
among the liver cells which appeared
either triangular or oval in shape (fig. 5).
Their processes often joined together forming a fine delicate plexus in the liver parenchyma.
Guinea pig
The distribution of nerve fibers and cells
in the liver of the guinea pig was similar
to the rat. The ganglia were however
numerous, and scattered irregularly in
the liver parenchyma. The ganglia were
densely populated with neurons while the
adjacent liver cells appeared far less
crowded (fig. 6 ) .
Rabbit
The innervation of the liver of the rabbit showed the same variations recorded
in the rat. The ganglia were however irregular in shape and distribution. Amidst
the liver cells multipolar and occasionaIly
bipolar forms of nerve cells were seen.
The dendrites of the nerve cells ran tortuously in between the liver cells (fig. 7).
Dog
The parenchyma of the liver of the dog
was richly supplied by nerve fibers. The
terminal ganglia were large, spindle shaped
321
PERIPHERAL NERVES O F LIVER
though detached clusters of neurons were
seen (fig. 8).
DISCUSSION
Nerve
cell
Nerve
cell
Fig. 5
Liver
of
mouse
(Bodian
method)
( x 780) showing two multipolar nerve cells.
The ultimate ramification of autonomic
nerve fibers in glandular structures is one
of the problems in neurology. The terminal intraglandular nerve fibers are notorious by their ability to evade all the
methods of staining. In the present investigation the Bodian protargol method
has given encouraging results specially
after using the protargol of Merck and
Bayer. The parenchyma of the liver contrary to the previous observations was
richly supplied with nerve fibers which
radiated along the vessels, and bile ducts
and even extended along the sinusoids in
the liver lobules. Terminal ganglia were
seen in the livers of all animals investigated. Their cells were multipolar and
they possessed long and short dendrites.
There was no chance for confusing these
terminal processes with connective tissue
fibers as they were always traced to the
cells from which they originated. AS the
Ganglion
Ganglion
Fig. 6
Liver of guinea pig (Bodian method) ( X 450) showing two spherical ganglia.
322
YOUSSEF MIKHAIL AND ABDEL L A T I F S A L E H
Nerve cell
__
Liver of rabbit stained by Bodian method ( x 950) showing isolated nerve cells.
-I
Fig. 7
~~
Clusters of nerve cells
Fig. 8
~
Nerve fibers
Liver of dog (Bodian method) ( x 550) showing detached clusters of neurons.
323
PERIPHERAL NERVES OF LIVER
nerve cells were numerous and irregularly
scattered in the liver parenchyma it was
therefore not surprising to find that the
processes connected to these cells were
plentiful. They often joined together forming an intricate plexus in the liver parenchyma. These ganglion cells formed
synapses with preganglionic fibers of the
vagus nerve only. Therefore the terminal
plexus in the liver contained preganglionic
parasympathetic fibers, the intrinsic ganglion cells and their processes as well as
the post ganglionic sympathetic fibers.
The terminal plexus in the liver parenchyma as shown in this investigation is
homologous with the Auerbach and Meissner plexuses in the alimentary canal and
with the cardiac and pulmonary plexuses.
The presence of terminal ganglia and
the abundance of nerve fibers in the liver
parenchyma may have a regulating effect
on the hepatic function. The sympathetic
fibers supplying the intra hepatic vessels
has been shown to have a constricting effect upon them. Carnot et al. ('30) reported
on the effects of vagus stimulation on intra
hepatic circulation. Wakim ('42) found
that parasympathetic stimulation did not
elicit any appreciable effect on the flow of
blood through the liver.
Concerning the nervous regulation of
biliary secretion, Hillyard ('30) failed to
observe any change in the output of bile
after carbohydrate, fat and protein meals
in normal and completely denervated
livers of dogs. Tanturi and Ivy ( ' 3 8 )
noticed a direct effect of vagus stimulation on the secretion of bile in the dog and
the monkey but not in the cat or the rabbit.
The latter investigators found that even
after 5 days from sectioning the vagus
nerve in the neck, stimulation of the peripheral end of the nerve had an excita-
tory secretory effect. Stimulation of the
central end of the divided nerve, while the
other vagus was intact resulted in increase
of biliary secretion. The various results
showed the direct and indirect effects of
vagus stimulation on bile production. The
close association between the hepatic cell
and the terminal nerve fibers and cells as
shown in this investigation may support
the possibility of a nervous regulation of
the functional activity of the liver.
SUMMARY
The parenchyma of the liver of the rat,
mouse, guinea pig, rabbit and dog was
found to be richly supplied with nerve fibers. Terminal parasympathetic ganglia
were found and they varied greatly in their
size, shape and number of constituent
cells. The neurons were either rounded or
triangular in shape and were usually multipolar. Their dendrites and post ganglionic
fibers interlaced together forming a terminal plexus in the liver parenchyma.
LITERATURE CITED
Alexander, W. F. 1940 The innervation of the
biliary system. J. Comp. Neur., 72: 357-370.
Carnot, P., R. Gayet and F. Merklen 1930 Exploration graphique des modification de la
pression veineuse porte likes a des excitations
vaso constrictives. C. R. SOC.Biol., 104: 12601266.
Hillyard, L. V. 1931 The effect of denervation
of the liver on secretion of bile. Amer. J.
Physiol., 98: 612-614.
Mitchell, G . A. 1953 Anatomy of the Autonomic nervous system. E. & s. Livingstone Ltd.
Tanturi, C. A. and A. C. Ivy 1938 A study of
the effects of vascular changes in the liver and
the excitation of its nerve supply on the formation of bile. Amer. J. Physiol., 121: 61-74.
Wakim, K. G. 1942 Effect of stimulation of
autonomic nerves on intra heDatic circulation
of blood i n intact animals. Proc. SOC. Exp.
Biol., 49: 307-311.
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