Cell determination and differentiation of identified serotonin-immunoreactive neurons in the grasshopper embryo.

We have begun to investigate the factors that underlie neurotransmitter determination in the central nervous system of the grasshopper embryo. The most prominent serotonin-immunoreactive neurons in the segmental ganglia are three clonally related interneurons, cells S1, S2, and S3. S1 and S2 are sibling neurons and are the first two born in the family of neurons that is produced by neuroblast 7-3; cell S3 derives from one of the second-born pair in the same family. S1 is serotonin-immunoreactive in all thoracic and abdominal segments, S2 in all but two segments (T3 and A1), and S3 in only the prothorax (T1). These segment-specific differences are not due to differential cell death but rather can be ascribed to biochemical differences between lineally homologous neurons. Furthermore, these homologous neurons also display interesting segment-specific differences in their morphology. Laser ablation of neuroblast 7-3 before it begins its series of programmed cell divisions results in the absence of its normal serotonin-immunoreactive neuronal progeny when screened at mature stages. These experiments support the hypothesis that transmitter determination is at least in part regulated by cell lineage in the grasshopper embryo and that some clonally related neurons appear to share a common transmitter determination by virtue of their common lineage. The results further suggest that cell lineage regulation of transmitter determination may vary according to segment or may be modified by as yet unidentified segment-specific factors.