Congenital Megacolon (Hirschsprung Disease)
Congenital megacolon (Hirschsprung disease) results from a failure of parasympathetic ganglia to form in the wall of part or all of the colon and rectum because the neural crest cells fail to migrate. Most familial cases of Hirschsprung disease are due to mutations in the RET gene, which codes for a cell membrane tyrosine kinase receptor. This gene on chromosome 10q11 is essential for crest cell migration. The ligand for the receptor is glial cell-derived neurotrophic growth factor (GDNF) secreted by mesenchyme cells through which crest cells migrate. Receptor ligand interactions then regulate crest cell migration. Consequently, if there are abnormalities in the receptor, migration is inhibited and no parasympathetic ganglia form in affected areas. The rectum is involved in nearly all cases, and the rectum and sigmoid are involved in 80% of affected infants. The transverse and ascending portions of the colon are involved in only 10 to 20%. The colon is dilated above the affected region, which has a small diameter because of tonic contraction of noninnervated musculature.
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