The Projections of Early Enteric Neurons Are Influenced by the Direction of Neural Crest Cell Migration

The enteric nervous system arises from the neural crest. In embryonic mice, vagal neural crest cells enter the developing foregut at approximately embryonic day 9.5 (E9.5) and then migrate rostrocaudally to colonize the entire gastrointestinal tract by E14.5. This study showed that a subpopulation of vagal crest-derived cells, very close to the migratory wavefront, starts to differentiate into neurons early, as shown by the expression of neuron-specific proteins and the absence of Sox10. Many of the early differentiating neurons transiently exhibited tyrosine hydroxylase (TH) immunoreactivity. The TH cells were demonstrated to be the progenitors of nitric oxide synthase (NOS) neurons. Immunohistochemistry, lesions, and DiI tracing were used to examine the projections of developing enteric neurons. The axons of first neurons in the gut (the TH-NOS neurons) projected in the same direction (caudally), and traversed the same pathways through the mesenchyme, as the migrating, undifferentiated, vagal crest-derived cells. To examine if the direction of migration and direction of axon projection are linked, coculture experiments were set up in which vagal crest-derived cells migrated either rostrocaudally (as they do in vivo), or caudorostrally (which they do not normally do), to colonize explants of embryonic aneural hindgut. The direction in which neurons projected was correlated with the direction of cell migration, but migration direction appears to be not the only mechanism influencing axon projection. Peristaltic reflexes involve both orally (rostrally) projecting neurons and anally (caudally) projecting neurons. Because few rostrally projecting neurons could be detected before birth, the full circuitry for peristaltic reflexes appears to develop after birth.