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In many tissues, the precise control of epithelial packing geometry is essential for proper function. The surface of the fruit fly wing is decorated with an hexagonally packed array of hairs that are aligned along the proximal distal axis of the wing. Their packing reflects the underlying hexagonal arrangement of the wing epithelial cells that construct them. Wing epithelial cells are irregularly packed throughout most of larval and pupal development. Less than half the cells are hexagonal, and five-sided cells outnumber 7-sided cells, resulting in an average neighbor number of less than 6. Shortly before hairs form, these cells remodel their intercellular contacts and regularize their packing; the fraction of hexagonal cells increase to 72% and the variability of cell perimeters and individual cell contact lengths decreases dramatically. Junction remodeling depends on rapid endocytosis and recycling of Cadherin and other junctional components, and is facilitated by the action of a group of proteins called the Planar Cell Polarity (PCP) proteins. PCP proteins localize non-uniformly at intercellular junctions, and become aligned along the proximal distal axis of each cell as the wing epithelium becomes hexagonal. We are currently interested in what factors determine the energy of different packing geometries and how they might change as the epithelium becomes hexagonal.
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