Download The_Ruminant_Gastrointestinal_Tract_3

The Ruminant Gastrointestinal Tract 3: Liver, pancreas, lesser omentum.
Development, Topography, and Function.
General. The digestive system develops in the embryo as a simple tube extending
the length of the body. Its epithelial lining is formed by the endoderm, its coats of
muscle, its blood and lymphatic vessels and the connective tissues supporting them
arise from the mesoderm. The nerves which innervate these tissues have their origin
in the neural ectoderm. Liver and pancreas develop as branching, tubular extensions
from the duodenum. The form of the digestive tube at an early embryonic age looks
like this:
Drawn by David
Stewart Geary
Liver. The liver of the newborn calf develops initially within the transverse septum of
the embryo. and, as it enlarges, pulls away from the septum as a separate organ. It
1
remains caudal to the diaphragm, with a narrow linear strip of its diaphragmatic
surface remaining adherent to the diaphragm. Initially, the liver’s long axis is in a
transverse plane. In this position, it most resembles the human liver. The left lobe is
left of the quadrate lobe; the right lobe is to the right. The quadrate lobe is essentially
central in position, between the umbilical fissure and the fossa for the gall bladder.
The caudate lobe is caudal to the right lobe. With growth of the rumen and reticulum,
the liver is pushed to the right and rotated so that its left lobe is ventral and its right
lobe is dorsal. The liver is displaced almost entirely to the right of the midline. On its
caudal, visceral, face, the caudate lobe is disposed in an inverted “L” shape. Its
caudate process forms the short arm of the inverted L, and the free end of the
process is directed right. The main part of the caudate lobe is cranioventral and is
medial to the porta of the liver; its craniomost extent reaches to the esophageal notch.
The papillary process is small. Note: The division of the liver into lobes has comparative
significance but is useful chiefly in permitting a more precise description of liver anatomy and the
location of disease processes.
Fig. A. Liver developing
within septum transversum
and separating from it. Diagr.
Lehrbuch der
Enwicklungsgeschichte der
Haustiere, Zietzschmann and
Krolling,1955: Verlag Paul
Parey, Berlin and Hamburg
Fig. B. Bovine embryo, 34
days, 12 mm length. Body wall
cut away in part. Fig. 1.
Lehrbuch der
Enwicklungsgeschichte der
Haustiere, Zietzschmann and
Krolling,1955: Verlag Paul
Parey, Berlin and Hamburg
diaphragm
pleural
cavity
stomach
mesonephros
heart
s
lung
septum
transversum
liver
dev’ng liver
peritoneal
Fig. 23. Bovine liver, mature, parietal (diaphragmatic) face and
cavity
visceral face. From Sisson, modified. Numbering of this figure and those
following continues from Ruminant Gastrointestinal Tract 2.
2
Parietal Face
Visceral Face
esophageal
impression
portal vein,
hepatic art.,
common bile duct
umbilical
fissure
b
diaphragm
esophagus
lesser omentum,
line of hepatic
reflection
Biliary duct system. In cattle, right and left hepatic ducts receive the smaller bile ducts
from the right and left halves of the liver, respectively. They join, forming the common
hepatic duct which joins the cystic duct that leads from the gall bladder. The common
bile duct (ductus choledochus) that is formed by their union then passes to the
duodenum just beyond the sigmoid loop (Fig. 24).
3
Fig. 24. The biliary duct
system in cattle.
common bile duct
right hepatic duct
left hepatic duct
common hepatic duct
cystic duct
gall bladder
Pancreas. The right lobe of the pancreas lies in the mesoduodenum dorsal to the
descending duodenum and the left lobe lies within the deep wall of the greater
omenum, where the omentum is applied to and fused with the descending colon (see
Fig. 14, 360o rotation, and Fig. 20a to see the close relationship of the deep wall of the
greater omentum with the transverse colon; in ruminants and the horse, the deep wall
(with the left lobe of the pancreas) is regularly fused with the cranial aspect of the
transverse colon). Right and left lobes of the pancreas meet at the body of the
pancreas, which is where the greater omentum is continuous with the
4
mesoduodenum of the descending colon. The body of the pancreas is perforated by
the portal vein and forms the caudal margin of the omental foramen. The portal vein,
with the hepatic artery, which passes ventral to the vein, forms always the ventral
margin of the omental foramen in the domestic animals (Fig. 25).
Fig. 25. Relations at
the omental foramen of
the horse (shown here)
are very similar to the
relations in ruminants.
Fig. 26. Goat.
Pancreas, portal vein,
hepatic artery, common
bile duct. Dorsal view.
pancreas, right lobe
pancreas, left lobe
panc.
duct
5
hepatic artery
duodenum,
sigmoid loop
spleen
rumen
common bile duct
The pancreas arises in two places in the embryo: 1) as a ventral pancreatic bud from
a common origin with the hepatic bud (the hepatopancreatic bud of the dudodenum)
and 2) as a dorsal pancreatic bud farther distally, directly from the duodenum. It is the
branching of these two buds that gives rise to the duct system and the exocrine
pancreatic acini. The endocrine islet cells separate from the proliferating cells and
come to lie between the acini. The origin of the ventral pancreatic bud becomes the
pancreatic duct; the origin of the dorsal pancreatic bud becomes the accessory
pancreatic duct. Within the pancreas the duct systems usually communicate and in
cattle, and in sheep and goats, ordinarily only one duct persists.
In cattle, the pancreatic duct usually undergoes involution and only the accessory
pancreatic duct is present. Occasionally a small pancreatic duct is also present, which
probably is due to a failure of the two systems to unite completely. The accessory
pancreatic duct passes from the caudal end of the right lobe of the pancreas and joins
the descending duodenum distal to the common bile duct. In the small ruminants, it is
the accessory pancreatic duct that undergoes involution. The pancreatic duct of the
sheep and goat is present, well developed, and joins the common bile duct 2 – 3 cm
proximal to that duct’s union with the duodenum just caudal to the sigmoid loop. The
sigmoid loop itself is brought about by the greater growth of the cranial part of the
duodenum relative to the limited growth of the common bile duct. Thus, to find the
common bile duct in ruminants, one first finds the caudal end of the sigmoid loop.
Fig. 27. 8mm sheep embryo. Diagrammatic. From Lehrbuch der Entwicklungsgeschichte
der Haustiere, O. Zietzschmann, O. Krölling, 1955; Verlag Paul Parey.
dorsal pancreas
ventral pancreas
liver
gut
heart
6
accessory pancreatic duct from
caudal end of right lobe of pancr.
caudate process
of caudate lobe
caudal vena
cava
Fig. 28. Visceral
face of bovine liver.
The probe is in the
omental foramen.
From Sisson.
portal vein
common bile duc
at end of sigmoid
umbilical vein
remnant
Lesser omentum.
7
Fig. 29. The mammalian embryo at an early stage of development. Dorsally, a
dorsal mesogastrium, which becomes the greater omentum (green), passes to
the greater curvature of the stomach; it is continuous caudally with the
mesentery (orange. Ventrally, the ventral mesogastrium (yellow) passes from
the lesser curvature of the stomach and the first part of the duodenum to the
abdominal face of the diaphragm and the ventral body wall up to the umbilical
stalk. It ends caudally in a free border. The heavy dashed line indicates the
position of the septum transversum, which forms the ventral part of the
diaphragm and is a cranial attachment of the ventral mesogastrium.
8
septum
transversum
caudal free
border
Fig. 30. From the hepatopancreatic bud, the hepatic (liver) and ventral
pancreatic buds grow into the part of the ventral mesogastrium that extends
to the first part of the duodenum. Actually, different from the diagram above,
most of the liver growth takes place within the septum transversum and then
gradually separates from the septum. The growth of the liver separates the
ventral mesogastrium into the ventral falciform ligament, which conveys the
umbilical vein from the umbilicus to the liver, and the more dorsal lesser
omentum, which extends from the liver to the lesser curvature of the stomach
and the first part of the duodenum. Both the falciform ligament and the lesser
omentum end caudally in a free border. After birth, the umbilical vein
becomes non-functional in most animals and in mature ruminants and the
dog and cat usually is lost entirely. In 80% of horses, it remains patent, small
and draining veins of the ventral body wall. In ruminants, the falciform
ligament itself is also usually lost except for a narrow line on the
of the liver
leadsthe
from
the omentum
umbilical fissure.
On diaphragmatic
the completion surface
of development,
wethat
expect
lesser
to extend from the
liver to the stomach and first part of the duodenum and to end in a free caudal border.
Fig. 31. Omentums and
omental foramen. In
the lower right figure,
the double arrow is in
the omental foramen.
lesser
omentum
line of atttachment
to the stomach
atttachment
to liver
free caudal border (at
omental foramen)
9
lesser omentum
greater omentum
Fig. 32. Relations at the
omental foramen of the
horse (shown here) with
the liver in place are
similar to the relations in
ruminants.
10
The falciform ligament containing the remnant of the umbilical vein is usually entirely
absent but for a thin line on the parietal face of the liver. If intact, we would expect to
see the ligament extending from the liver’s umbilical fissure (see Fig. 23) and parietal
face to the body wall from the esophagus to the umbilicus. Like this:
Fig. 33. The position of an
intact falciform ligament.
The writer has seen a completely intact falciform ligament in the sheep and only once.
Fig. 28, from Sisson, shows a (rare) remnant of the umbilical vein in the ox.
Displaced abomasum and abomasal torsion. The abomasum joins the cranial part
of the duodenum at the pylorus. Together, these two parts of the gastrointestinal tract
form a continuous tube, held on the one end by the omaso-abomasal junction and, on
the other end, by the common bile duct. The duct is strong and does not stretch. The
result is that with displacement of the abomasum, these two parts are relatively fixed
in position but the tube of smooth muscle of abomasum and cranial part of the
duodenum is more mobile. The same is true in abomasal torsion. Left displacement of
the abomasum and torsion of the abomasum are shown in Figs. 33a – c.
Fig. 33a. Ruminant
stomach, left view.
omasum
abomasum
11
Fig. 33b. Left abomasal
displacement.
Fig. 33c. As 33b, left
abomasal displacement,
in color.
Fig. 33d. One form
of abomasal torsion.
12
Addendum: A common procedure for surgical correction of displaced abomasum is to
put the animal in dorsal recumbency. This frees the displaced abomasum, allowing it
to “float” ventralward toward the surgical incision, which is a linear ventral abdominal
one. Freed of the weight of the rumen, gas may be removed and the abomasum
begins to contract, reducing its size. The incision is closed with the surgeon grasping
the abomasal wall with the suture, which fixes the abomasum in a fairly normal,
ventral, position as the abdomen is closed. This usually effectively avoids future
displacement. Understanding the anatomy and physiology of the ruminant
gastrointestinal tract is useful, necessary, in clinical diagnosis and in a number of
clinical procedures, including surgical correction of abomasal displacement and
torsion, traumatic gastritis, relief of bloat, and grain overload. The developmental
aspects of ruminant anatomy are presented chiefly to give an understanding of how
things “got this way.” This may also help to explain those occasional circumstances in
which development has been incomplete or altered.
13
14