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chapter 15 summary Junqueira's Basic Histology
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Alimentary System The primordial gut forms from the dorsal part of the umbilical vesicle, which is incorporated into embryo.
The endoderm of the primordial gut gives rise to the epithelial lining of the alimentary tract, except for the
cranial and caudal parts, which are derived from ectoderm of the stomodeum and cloacal membrane,
respectively.
The muscular and connective tissue components of the alimentary tract are derived from splanchnic
mesenchyme surrounding the primordial gut.
The foregut gives rise to the pharynx, lower respiratory system, esophagus, stomach, proximal part of the
duodenum, liver, pancreas, and biliary apparatus.
Because the trachea and esophagus have a common origin from the foregut, incomplete partitioning by the
tracheoesophageal septum results in stenoses or atresias, with or without fistulas between them.
The hepatic diverticulum, the primordium of the liver gallbladder, and biliary duct system, is an outgrowth of the
endodermal epithelial lining of the foregut.
Epithelial liver cords develop from the hepatic diverticulum and grow into the septum transversum. Between the
layers of the ventral mesentery, derived from the septum transversum, primordial cells differentiate into hepatic
tissues and linings of the ducts of the biliary system.
Congenital duodenal atresia results from failure of the vacuolization and recanalization process to occur after
the normal solid developmental stage of the duodenum.
Usually the epithelial cells degenerate and the lumen of the duodenum is restored. Obstruction of the
duodenum can also be caused by an annular pancreas or pyloric stenosis.
The pancreas develops from pancreatic buds that form from the endodermal lining of foregut. When the
duodenum rotates to right, the ventral pancreatic bud moves dorsally & fuses with the dorsal pancreatic bud.
The ventral pancreatic bud forms most of the head of the pancreas, including the uncinate process.
The dorsal pancreatic bud forms the remainder of the pancreas. In some fetuses, the duct systems of the two
buds fail to fuse, and an accessory pancreatic duct forms.
The midgut gives rise to the duodenum (the part distal to the entrance of the bile duct), jejunum, ileum, cecum,
appendix, ascending colon, and right one half to two thirds of the transverse colon.
The midgut forms a U-shaped umbilical loop of intestine that herniates into the umbilical cord during the sixth
week because there is no room for it in the abdomen. While in the umbilical cord, the midgut loop rotates
counterclockwise 90 degrees.
During the 10th week, the intestine returns to the abdomen, rotating a further 180 degrees.
Omphaloceles, malrotations, and abnormal fixation of the gut result from failure of return or abnormal rotation
of the intestine.
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