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Dynamic modes of morphogen transport

Morphogens are secreted signaling molecules that mediate tissue patterning and growth of embryonic tissues. They are secreted in a localized region and spread through the tissue to form a graded concentration profile. We present a cell-based model of morphogen spreading that combines secretion in a local source, extracellular diffusion and cellular trafficking. We introduce hydrodynamic modes of morphogen transport and characterize the dynamics of transport by dispersion relations of these dynamic eigenmodes. These dispersion relations specify the characteristic relaxation time of a mode as a function of its wavelength. In a simple model we distinguish two distinct dynamic modes characterized by different timescales. We find that the slower mode defines the effective diffusion and degradation as well as the shape of the concentration profile in steady state. Using our approach we discuss mechanisms of morphogen transport in the developing wing imaginal disc of the fruit fly \textit{Drosophila}, distinguishing three transport regimes: transport by extracellular diffusion, transport by transcytosis and a regime where both transport mechanisms are combined.