AN IDENTIFIED SET OF VISCERAL MUSCLE BANDS IS ESSENTIAL FOR THE GUIDANCE OF MIGRATORY NEURONS IN THE ENTERIC NERVOUS-SYSTEM OF MANDUCA-SEXTA

During the formation of the enteric nervous system (ENS) in Manduca, a population of similar to 300 enteric neurons (the EP cells) become di stributed along the foregut and midgut by migrating onto specific sets of visceral muscle bands. Only after their migration is complete do t he neurons express a variety of position-specific phenotypes, includin g a peptidergic phenotype that is usually restricted to a subset of EP cells on the midgut. To investigate whether direct interactions betwe en the EP cells and these pathways are necessary for either neuronal m igration or differentiation, we have investigated the developmental or igins and functional role of the muscle bands in embryonic culture. Us ing scanning electron microscopy, immunohistochemistry, and mitotic la beling with bromodeoxyuridine, we found that the eight major muscle ba nds: of the midgut form by the coalescence of longitudinal muscle fibe rs on the midgut surface, apparently in response to regional cues asso ciated with the underlying epithelium. These bands then serve as migra tory pathways for the EP cells, which travel rapidly along the bands ( but not onto adjacent interband musculature) and then complete their d ifferentiation. Dye labeling of individual EP cells revealed that prio r to migration onset, each neuron extended widely distributed filopodi a onto both the band and interband regions of the midgut surface. As t he muscle bands coalesced, however, the leading process of each EP cel l became increasingly confined to a specific hand, onto which it subse quently migrated. In a series of surgical manipulations of both the mu scle bands and the migratory neurons, we demonstrated that these pathw ays are both necessary and sufficient to support the migratory behavio r of the EP cells. Surgical interventions that prevented the neurons f rom contacting the muscle bands inhibited migration, while contact bet ween isolated EP cells and a muscle band supported both their migratio n and differentiation. However, the acquisition of mature phenotypes b y the EP cells was not strictly dependent on the migration of these ne urons to their expected positions. In particular, the onset of neurope ptide expression could be detected in at least some of the neurons who se migration onto the midgut had been blocked. Thus, in the embryonic ENS, the migration and delayed differentiation of the EP cells represe nt precisely coordinated aspects of development that are nevertheless regulated in an independent manner. (C) 1996 Academic Press, Inc.