AGGREGATION CHIMERAS DEMONSTRATE THAT THE PRIMARY DEFECT RESPONSIBLE FOR AGANGLIONIC MEGACOLON IN LETHAL-SPOTTED MICE IS NOT NEUROBLAST AUTONOMOUS

The lethal spotted (ls) mouse has been used as a model for the human d isorder Hirschsprung's disease, because as in the latter condition, Is /ls homozygotes are born without ganglion cells in their terminal colo ns and, without surgical intervention, die early as a consequence of i ntestinal obstruction. Previous studies have led to the conclusion tha t hereditary aganglionosis in ls/ls mice occurs because neural crest-d erived enteric neuroblasts fail to colonize the distal large intestine during embryogenesis, perhaps due to a primary defect in non-neurobla stic mesenchyme rather than migrating neuroblasts themselves. In this investigation, the latter issue was addressed directly, in vivo, by co mparing the distributions of ls/ls and wild-type neurons in aggregatio n chimeras. Expression of a transgene, DbetaH-nlacZ, in enteric neuron s derived from the vagal neural crest, was used as a marker for ls/ls enteric neurons in chimeric mice. In these animals, when greater than 20% of the cells were wild-type, the ls/ls phenotype was rescued; such mice were neither spotted nor aganglionic. In addition, these 'rescue d' mice had mixtures of ls/ls and wild-type neurons throughout their g astrointestinal systems including distal rectum. In contrast, mice wit h smaller relative numbers of wild-type cells exhibited the classic ls /ls phenotype. The aganglionic terminal bowel of the latter mice conta ined neither ls/ls nor wild-type neurons. These results confirm that t he primary defect in ls/ls embryos is not autonomous to enteric neurob lasts, but instead exists in the non-neuroblastic mesenchyme of the la rge intestine.