The goal of this research is to re-explore the developmental origin of neurons, the cells of the central nervous system. The universally held view is that all neurons arise from the ectoderm, one of the three primary germ layers during embryogenesis. This conclusion, based on numerous fate mapping and cell lineage studies, is undoubtably true for the majority of neurons, but is it true for all neurons? My recent work on the Midline Mesodermal Cells (MMCs) in fruit fly Drosophila melanogaster raises the interesting hypothesis that cells with a neuronal function need not always be ectodermally derived. This hypothesis will be critically examined in the proposed using the MMCs, a set of 20 cells in flies which are unequivocally mesodermal in origin yet exhibit a number of neuronal properties. They have a prominent cell body, two primary processes projecting out a specific nerve to a specific skeletal muscle, express several neural-specific markers, have voltage dependent channels, receive synaptic-like inputs, and affect several target tissues. The two aims in this study address the remaining criteria for neuronal status, namely whether the MMCs express and release neurotransmitter-like molecules. Successful results will provide the first clear cut example of neurons arising from a non-ectodermal lineage.
