Associated Organs
Introduction
In
Chapter 33 we looked at the development of the gastrointestinal tract as a tube
and mentioned a number of buds that sprout from the tube and its associated mesenchyme.
These develop into a number of organs (Figure 34.1).
Lung bud
As
the oesophagus develops and elongates during week 4 the respiratory
diverticulum buds off from its ventral wall (Figure 34.1). To create two
separate tubes a septum forms between the respiratory bud and the oesophagus
called the tracheoesophageal septum (see Figure 32.1). This creates the
oesophagus dorsally and the respiratory primordium ventrally (see Chapter 32).
In
the fifth week the spleen starts to develop from a condensation of mesenchymal
cells between the folds of the dorsal mesogastrium (Figure 34.2). With the
rotation of the stomach and duodenum the spleen is moved to the left side of
the abdomen, explaining the adult location of the splenic artery, a branch of the
coeliac trunk. The gastrosplenic ligament between the stomach and spleen
is an adult remnant of the dorsal mesogastrium, as is the splenorenal
ligament between the spleen and left kidney (Figure 34.3).
The
spleen begins to create red and white blood cells in the second trimester and
is an important site of haematopoesis during the foetal period. After birth it
stops producing red blood cells and concentrates on its adult functions of the
lymphatic and im tems, and of removing old red blood cells from circulation.
Beginning
as an epithelial outgr entral wall of the distal end of the foregut the liver
bud, or hepatic diverticulum (Figure 34.1), appears at the end of
week 3. Growing rapidly during week 4 the liver bud grows into the septum
transversum, a sheet of mesodermal cells located between the pericardial
cavity and the yolk sac stalk. The septum transversum will contribute to the
diaphragm (see Chapter 19) and the ventral mesentery here. Both the liver bud
and septum transversum integrate to form parts of the liver. The liver bud
grows within the ventral mesentery, and retains a connection with the foregut
that will become the bile duct. A cranial part of the liver bud will
form the liver, and a caudal bud will form the gallbladder (Figure
34.4).
The
liver is formed from cells of different sources. The liver bud from the foregut
will form hepatocytes and the epithelial lining of the bile duct. The vitelline
and umbilical veins will form hepatic sinusoids. Cells of the septum transversum
will form the stroma and capsule (connective tissues) of the liver and also
haematopoietic cells, Kupffer cells, smooth muscle and connective tissue of the
biliary tract. The lesser omentum between the stomach and the liver, and the
falciform ligament between the liver and the anterior abdominal wall are the
adult structures of the ventral mesentery.
By
week 10 of development the liver accounts for around 10% of the embryonic
weight. At birth this reduces to 5% of total body weight. A main embryological function
of the liver is haemat- opoiesis, with the liver producing red and white blood
cells.
With
the rotation of the stomach and duodenum the route of the common bile duct to
the duodenum is altered from anterior to the foregut to a posterior course (Figure
34.5), and is joined by the pancreatic duct at the ampulla of Vater.
Eventually the bile duct passes behind the duodenum and bile is formed by the
liver in week 12.
Two
pancreatic buds develop from the foregut (duodenum) giving dorsal and ventral
buds (in the fourth and fifth week, respectively) within the mesentery. The
dorsal bud is larger, and the ventral bud is a bud from the hepatic
diverticulum (Figure 34.4).
With
the rotation of the duodenum to the right the ventral bud moves dorsally (much
like the movement of the bile duct entrance to the duodenum) to rest below and
behind the dorsal bud (Figure 34.5). In week 7 the duct systems of the buds
fuse and the adult main pancreatic duct forms from the main duct of the ventral
bud and the distal part from the dorsal bud. Occasionally, the proximal part of
the duct of the dorsal bud persists as an accessory duct that opens into the
duodenum a little proximal to the main duct.
The
uncinate process and most of the head of the pancreas forms from the ventral
bud, and the rest forms from the dorsal bud. Exocrine and endocrine cells are
all derived from endoderm, taking separate differentiation pathways. The islets
of Langerhans (endocrine cells) form in the th d insulin is secreted from
the fourth to fifth month.
