RESUMEN
The myenteric plexus of the colon was studied ultrastructurally in a colony of an Ls Ls strain of mice manifesting a piebald coat color mutation associated with a high incidence of genetically determined aganglionic megacolon. Ultrastructural studies were histochemically supplemented by the Maillet technic and stains for acetylcholinesterase and catecholamines. The development of megacolon did not appear to require total aganglionosis, since ostensibly aganglionic areas contained rare ganglion cells. In the distal narrowed segment, both cholinergic and adrenergic fibers in the muscularis, submucosa and mucosa were somewhat reduced. In the mouse, the dilated portion showed an abrupt increase in adrenergic fibers. These findings are related to the pathophysiology of the disorder. The increasing degenerative changes seen in myenteric plexus structures from the fetus to adult suggest that aganglionic megacolon may be an abiotrophy, wherein the congenitally deficient myenteric plexus may be unusually predisposed to postnatal injury and degeneration.