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 and embryos homozygous for the lethal spotted (ls) allele that are born with aganglionosis coli and serve as a model for the human birth defect, Hirschsprung disease. Neuroblasts, derived from the vagal neural crest, colonize the gut in a cranial-to-caudal manner. However, migration of neuroblasts in ls/ls gut is impaired when neuroblasts reach the ileocecal junction and attempt to colonize the large intestine. To learn 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 embryos (E10.5-E13.5) were treated with a histochemical substrate for the transgene product and examined by light and electron microscopy.
Results: Five stages of ileocecal development were defined based on distinctive gross, light, and electron microscopic features. At each stage, neuroblasts had different ultrastructural features than other mesenchymal cells. Initial colonization of the colon was different from other parts of the gut, in that a string of "pioneer" neuroblasts populated the mesenteric border of the proximal colon before circumferential invasion. 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 the 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: This study supports the hypothesis that impaired migration of neuroblasts in ls/ls embryos is not limited to the presumptive aganglionic segment, but begins at the ileocecal junction. Migration of neuroblasts from the ileum into the proximal colon follows a different pattern than movement of neuroblasts through the small intestine. The biological bases for these differences may account for the defects observed in ls/ls mice and/or may affect the pathogenesis of human Hirschsprung disease.