Origins of the insect enteric nervous system: differentiation of the enteric ganglia from a neurogenic epithelium

The enteric nervous system (ENS) of the moth Manduca sexta is organized into two distinct cellular domains: an anterior domain that includes several small ganglia on the surface of the foregut, and a more posterior domain consisting of a branching nerve plexus (the enteric plexus) that spans the foregut-midgut boundary. Previously, we showed that the neurons of the posterior domain, the enteric plexus, are generated from a large placode that invaginates from the caudal lip of the foregut; subsequently, the cells become distributed throughout the enteric plexus by a sequence of active migration. We now demonstrate that the neurons of the anterior domain, the cells of the enteric ganglia, arise via a distinct developmental sequence. Shortly after the foregut has begun to form, three neurogenic zones differentiate within the foregut epithelium and give rise to chains of cells that emerge onto the foregut surface. The three zones are not sites of active mitosis, as indicated by the absence of labelling with a thymidine analogue and by clonal analyses using intracellularly injected dyes. Rather, the zones serve as loci through which epithelial cells are recruited into a sequence of delamination and neuronal differentiation. As they emerge from the epithelium, the cells briefly become mitotically active, each cell dividing once or twice. In this manner, they resemble the midline precursor class of neural progenitors in the insect central nervous system more than neuroblast stem cells. The progeny of these zone-derived precursors then gradually coalesce into the ganglia and nerves of the anterior ENS. Although this reorganization results in some variability in the precise configuration of neurons within the ganglia, the overall morphology of the ganglia is highly stereotyped, consisting of cortical layers of cells that surround a ventral neuropil. In addition, a number of the neurons within the frontal and hypocerebral ganglia express identifiable phenotypes in a manner that is similar to many cells of the insect central nervous system. These observations indicate that the differentiation of the enteric ganglia in Manduca involves an unusual combination of features seen during the formation of other regions of the nervous system and, as such, constitutes a distinct program of neurogenesis.