MIGRATION OF GANGLION-CELL PRECURSORS IN THE ILEOCECA OF NORMAL AND LETHAL SPOTTED EMBRYOS, A MURINE MODEL FOR HIRSCHSPRUNG DISEASE

BACKGROUND: Dopamine-beta-hydroxylase-nlacZ transgenic mice are useful for studies of enteric neurodevelopment. Expression of the transgene provides a histochemical marker for neuroblasts in wild-type embryos a nd embryos homozygous for the lethal spotted (Is) allele that are born with aganglionosis coli and serve as a model for the human birth defe ct, Hirschsprung disease. Neuroblasts, derived from the vagal neural c rest, colonize the gut in a cranial-to-caudal manner. However, migrati on of neuroblasts in ls/ls gut is impaired when neuroblasts reach the ileocecal junction and attempt to colonize the large intestine. To lea rn more about neuroblast migration through this specific region of the intestinal tract, a detailed light and electron microscopic study of neuroblast colonization of the developing ileoceca from wild-type, ls/ +, and ls/ls embryos was conducted. EXPERIMENTAL DESIGN: The ileoceca from wild-type, ls/+, and ls/ls, dopamine-beta-hydroxylase-nlacZ embry os (E10.5-E13.5) were treated with a histochemical substrate for the t ransgene product and examined by light and electron microscopy. RESULT S: Five stages of ileocecal development were defined based on distinct ive gross, light, and electron microscopic features. At each stage, ne uroblasts had different ultrastructural features than other mesenchyma l cells. Initial colonization of the colon was different from other pa rts of the gut, in that a string of ''pioneer'' neuroblasts populated the mesenteric border of the proximal colon before circumferential inv asion. Subsequently, neuroblasts were arranged in intersecting linear groups of contiguous cells that radiated around the cecum and proximal colon. In ls/ls embryos, transition from neuroblast extension along t he mesenteric border to cecal invasion was delayed profoundly. However , the ultrastructural features of neuroblasts and adjacent mesenchyme were indistinguishable in ls/ls and wild-type embryos. CONCLUSIONS: Th is study supports the hypothesis that impaired migration of neuroblast s in ls/ls embryos is not limited to the presumptive aganglionic segme nt, but begins at the ileocecal junction. Migration of neuroblasts fro m the ileum into the proximal colon follows a different pattern than m ovement of neuroblasts through the small intestine. The biological bas es for these differences may account for the defects observed in ls/ls mice and/or may affect the pathogenesis of human Hirschsprung disease .