1. Developmental Genetics, Biotechnology Center, TUD,
2. Biophysics, Biotechnology Center, TUD, and,
3. Center for Regenerative Therapies, TUD, Tatzberg 47-49, 01307 Dresden, Germany
4. These authors contributed equally to this work.
5. Present address: Institute of Toxicology and Genetics, Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany.

Correspondence to: Petra Schwille^2,3Michael Brand^1,3 Correspondence and requests for materials should be addressed to P.S. (Email: petra.schwille@biotec.tu-dresden.de) or M.Br. (Email: michael.brand@biotec.tu-dresden.de).

It is widely accepted that tissue differentiation and morphogenesis in multicellular organisms are regulated by tightly controlled concentration gradients of morphogens^{1, 2}. How exactly these gradients are formed, however, remains unclear^{3, 4, 5, 6, 7, 8, 9, 10, 11, 12}. Here we show that Fgf8 morphogen gradients in living zebrafish embryos are established and maintained by two essential factors: fast, free diffusion of single molecules away from the source through extracellular space, and a sink function of the receiving cells, regulated by receptor-mediated endocytosis. Evidence is provided by directly examining single molecules of Fgf8 in living tissue by fluorescence correlation spectroscopy, quantifying their local mobility and concentration with high precision. By changing the degree of uptake of Fgf8 into its target cells, we are able to alter the shape of the Fgf8 gradient. Our results demonstrate that a freely diffusing morphogen can set up concentration gradients in a complex multicellular tissue by a simple source-sink mechanism.

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