Published on 22/03/2015 by admin
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Chapter 320 Normal Development, Structure, and Function
Chris A. Liacouras
The primitive gut is recognizable by the 4th wk of gestation and is composed of the foregut, midgut, and hindgut. The foregut gives rise to the upper gastrointestinal (GI) tract including the esophagus, stomach, and duodenum to the level of the insertion of the common bile duct. The midgut gives rise to the rest of the small bowel and the large bowel to the level of the mid-transverse colon. The hindgut forms the remainder of the colon and upper anal canal. The rapid growth of the midgut causes it to protrude out of the abdominal cavity through the umbilical ring during fetal development. The midgut subsequently returns to the peritoneal cavity and rotates counterclockwise until the cecum lies in the right lower quadrant. The process is normally complete by the 8th wk of gestation.
The liver derives from the hepatic diverticulum that evolves into parenchymal cells, bile ducts, vascular structures, and hematopoietic and Kupffer cells. The extrahepatic bile ducts and gallbladder develop 1st as solid cords that canalize by the 3rd mo of gestation. The dorsal and ventral pancreatic buds grow from the foregut by the 4th wk of gestation. The two buds fuse by the 6th wk. Exocrine secretory capacity is present by the 5th mo.
Cis-regulatory genomic sequences govern gene expression during development. Modules of cis sequences are linked and allow a cascade of gene regulation that controls functional development. Extrinsic factors have the capacity to influence gene expression. In the gut, several growth factors, including growth factor-β, insulin-like growth factor, and growth factors found in human colostrum (human growth factor and epidermal growth factor), influence gene expression.
Propulsion of food down the GI tract relies on the coordinated action of muscles in the bowel wall. The contractions are regulated by the enteric nervous system under the influence of a variety of peptides and hormones. The enteric nervous system is derived from neural crest cells that migrate in a cranial to caudal fashion. Migration of the neural crest tissue is complete by the 24th wk of gestation. Interruption of the migration results in Hirschsprung disease. Newborn bowel motor patterns are different from adults. Normal fasting upper GI motility is characterized by a triphasic pattern known as the migrating motor complex. Migrating motor complexes occur less often in neonates and they have more nonmigrating phasic activity. This leads to ineffective propulsion, particularly in premature infants. Motility in the fed state consists of a series of ring contractions that spread caudad over variable distances.
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