The Notch gene in Drosophila encodes a transmembrane protein with homology to EGF that appears to mediate cell-cell interactions necessary for proper epidermal vs. neural fate decisions. In this study, we examine Notch expression in detail throughout embryonic and imaginal development using confocal laser-scanning microscopy and specific mAb probes. We find that Notch is expressed in a tissue-specific manner as early as the cellular blastoderm stage, when cells of the presumptive mesoderm clearly express less Notch than adjacent ectodermal precursors. Notch is abundantly expressed during the initial determination of neuronal lineages, such as the embryonic neuroblasts and the precursors of sensory neurons in the imaginal disc epithelia, but expression quickly decreases during subsequent differentiation. These changing patterns of Notch expression do not correlate well with cell movements, and thus do not appear to support the notion that the major function of Notch is to maintain epithelial integrity via adhesive mechanisms. Our data suggest instead that Notch may act as a cell-surface receptor, perhaps functioning in the lateral inhibition mechanism that is necessary for proper spacing of neuronal precursors.

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