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).
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.