Early elements in gastropod neurogenesis.

The first elements of the nervous system of pond snails appear in the very early veliger stage of development at much earlier times than any previously described neurons. The first three cells are reactive to antibodies raised against both the neuropeptide FMRFamide and tubulin and their somata are located posteriorly within the embryo, not in anterior regions, as would be consistent with current concepts of gastropod neurogenesis. Furthermore, the extensive, anteriorly directed fibers from these cells appear to form a scaffold upon which the central ganglia and interconnecting pathways later develop. These findings challenge current thoughts on the origins of early embryonic neurons and on possible inductive cues and mechanisms of axonal navigation important in the development of the molluscan nervous system.