Insights into early molluscan neuronal development through studies of transmitter phenotypes in embryonic pond snails

Pond snails have long been the subject of intense scrutiny by researchers i nterested in general principles of development and also cellular and molecu lar neurobiology. Recent work has exploited both these fields of study by e xamining the ontogeny of the nervous system in these animals. Much of this work has focussed upon the development of specific transmitter phenotypes t o provide vignettes of neuronal subpopulations that can be traced from earl y embryonic life through to adulthood, While such studies have generally co nfirmed previous explanations of gangliogenesis in gastropods, they have al so indicated the presence of several neurons that appear earlier and in pos itions inconsistent with classical views of gastropods neurogenesis. The ea rliest of these cells contain FMRFamide-related peptides and have anteriorl y projections that mark the future locations of ganglia and interconnecting pathways that will comprise the postembryonic central nervous system. Thes e posterior, peptidergic cells, as well as certain, apical, monoaminergic n eurons, disappear and apparently die near the end of embryonic life. Finall y, populations of what appear to be peripheral sensory neurons begin to exp ress catecholamines by around midway through embryonic life. Like several o f the neurons expressing a variety of transmitters in the developing centra l ganglia, the catecholaminergic peripheral cells persist into postembryoni c life. Transmitter phenotypes, cell shapes and locations, and neuritic mor phologies all suggest that many of the neurons observed in early embryonic pond snails have recognizable homologues across the molluscs. Such observat ions have profoundly altered our views of neurogenesis in gastropods over t he last few years. They also suggest the promise for pond snails as fruitfu l models for studying the roles and mechanisms for pioneering fibres, cues triggering apoptosis, and contrasting origins and mechanisms employed for g enerating central vs. peripheral neurons within a single organism. (C) 2000 Wiley-Liss, Inc.