Development of embryonic cells containing serotonin, catecholamines, and FMRFamide-related peptides in Aplysia californica

This study demonstrates the presence of a relatively extensive but previous ly unrecognized nervous system in embryonic stages of the opisthobranch mol lusc Aplysia californica. During the trochophore stage, two pairs of cells were observed to be reactive to antibodies raised against the neuropeptides FMRFamide and EFLRIamide. These cells were located in the posterior region of the embryo, and their anterior projections terminated under the apical tuft. As the embryos developed into veliger stages, serotonin-like immunore active (LIR) cells appeared in the apical organ and were later observed to innervate the velum. Also, aldehyde-induced fluorescence indicative of cate cholamines was present in cells in the foot, oral, and possibly apical regi ons during late embryonic veliger stages. Just before the embryo hatches as a free-swimming veliger, additional FMRFamide-LIR and catecholamine-contai ning cells appeared in regions that correspond to the ganglia of what will become the adult central nervous system (CNS). Neurons and connectives that will contribute to the adult CNS appear to develop along the pathways that are pioneered by the earliest posterior FMRFamide-LIR cells. These observa tions are consistent with the hypothesis that, besides their presumed roles in the control of embryonic behaviors, some elements may also guide the de velopment of the CNS. Embryonic nervous systems that develop prior to and o utside of the adult CNS have also been reported in pulmonate and prosobranc h species of molluscs. Therefore, the demonstration of early developing neu rons and their transmitter phenotypes in A. californica presents new opport unities for a better understanding of the ontogeny and phylogeny of both be havioral and neuronal function in this important model species.