The dopamine β-hydroxylase promoter has been shown to direct expression of the reporter gene product, β-galactosidase, to enteric neurons and putative embryonic neuroblasts in transgenic mice (; ). In this paper, expression of the transgene, DβH-nlacZ, in the gastrointestinal tract is characterized in more detail in wild-type mice and mice which are also homozygous for the lethal spotted allele (ls). Expression of the transgene in wild-type embryos was first detected in scattered mesenchymal cells in the proximal foregut on embryonic day 9.5, and progressed distally until embryonic day 13.5 when the entire length of the gut was colonized by such cells. Several observations suggest that the mesenchymal cells which express the transgene (MCET) are, in fact, enteric neuroblasts, probably derived from the vagal neural crest. (1) The presence of MCET in progressively more caudal portions of the embryonic gut correlated with the neurogenic potential of isolated gastrointestinal segments grafted under the renal capsule. (2) Mitotic activity of MCET was demonstrated by incorporation of [³H]thymidine in utero. (3) The migratory behavior of MCET and/or their precursors was revealed in anastomotic subcapsular grafts of gut from transgenic and non-transgenic embryos; enteric ganglia of the latter were populated by MCET from the former. (4) Enteric expression of the transgene postnatally was restricted to intrinsic neurons that coexpressed other phenotypic markers of neuronal differentiation.

The pattern of transgene expression in ls/ls mice was identical to that seen in ls/+ and +/+ mice until embryonic day 12.5. At that stage, the cranial-to-caudal progression of gastrointestinal colonization by cells that expressed the transgene extended into the proximal colon of wild-type embryos. In dramatic contrast, caudal expression of the transgene in ls/ls embryos ended abruptly at the ileocecal valve. Colonization of the proximal and mid-colon proceeded during subsequent stages of development in ls/ls embryos, but at a retarded rate and in an irregular pattern. The terminal gut was never colonized in these mutant mice. These observations suggest that congenital aganglionosis coli which develops in ls/ls embryos results from a defect of non-neuroblastic mesenchyme that regionally impairs neuroblast migration.