17 March 2017
Department of Biology
New York University
The Drosophila visual system is composed of the retina and the optic lobes, lamina, medulla, and lobula complex. These structures receive retinotopic inputs from photoreceptors specialized in motion vision (lamina), or color and polarized light vision (medulla). At least 100 types of neurons in the optic lobes process these inputs for extracting visual information.
The medulla contains 40,000 neurons of more than 80 cell types that are organized in 800 columns corresponding to the 800 unit eyes in the retina (ommatidia). How is this variety of neurons generated and how is retinotopy established? Medulla neurons are born from 800 neural stem cells that sequentially express five transcription factors in a temporal manner, similar to the sequence of transcription factors observed in embryonic neural stem cells. Different neurons emerge in each temporal window, therefore generating a series of 800 neurons of each type: These 'uni-columnar neurons' are generated throughout the neuroepithelium and have a 1:1 stoichiometry with the photoreceptors that innervate the medulla. We will describe the mechanisms controlling the transition from one neural stem cell stage to the next.
How are the less numerous 'multi-columnar' neurons that have larger receptor fields and are present at a ~1:10 stoichiometry with photoreceptors generated? We will show that these subtypes emerge from the same neural stem cells that also produce uni-columnar neurons, but differ in different regions of the medulla neuroepithelium, which is highly patterned with each region contributing to producing different multi-columnar neurons. In spite of their restricted origins, these neurons still contribute to the entire retinotopic map through migration of their cell bodies.
Therefore, the generation of 80 cell types involves the integration of temporal and spatial patterning that preserves retinotopy of neurons present at different stoichiometry.
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