| In both insects and mammals, olfactory sensory neurons in the antennae or nasal epithelium each express a single type of olfactory receptor protein that binds to odorants, determining the neuron's molecular receptive range. All sensory neurons that express a given olfactory receptor send their axons to converge in one or two substructures, termed glomeruli, in the primary olfactory center, the antennal lobe or olfactory bulb. From there, relay interneurons transmit olfactory information to higher brain centers.; My thesis work addresses the following questions: How is olfactory information represented in higher brain centers? How do specific connections form during development in order to create the neural circuitry required for the detection and discrimination of odors? And how is the reorganization of these connections controlled in order to subserve distinct functions over the lifetime of the organism?; Drosophila melanogaster is an attractive model for olfactory system studies because of its highly stereotyped antennal lobe, consisting of about 45 uniquely identifiable glomeruli, its amenability to molecular genetic techniques, and its history as a model organism for olfactory conditioning. My colleagues and I employed a Drosophila-specific genetic technique called MARCM to label the daughters of dividing neural progenitor cells with a membrane-bound Green Fluorescent Protein molecule, dissected whole brains during various stages of development, imaged the labeled neurons using confocal microscopy, and analyzed the resulting images by eye and with both commercial and original software. Based on the resulting data, we were able to make the following novel conclusions:; First, projection neurons predictably innervate particular glomeruli in the antennal lobe based on their lineage and birth order, indicating that they are pre-specified to carry particular categories of olfactory information before they encounter the axons of sensory neuron partners. Second, projection neurons whose dendrites innervate particular glomeruli exhibit characteristic axon branching patterns in at least one higher brain center, the lateral horn, suggesting that they form stereotyped connections with third order neurons to relay olfactory information. Finally, embryonic-born projection neurons are morphologically differentiated in both larval and adult stages and reorganize their processes during metamorphosis in response to ecdysone and TGFbeta signaling. |
