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DEVELOPMENTAL BIOLOGY
Gradients of signalling molecules dictate where specific cell types form in developing tissues, but how these gradients are set up is much debated. A model proposed 40 years ago by Francis Crick may provide an answer.
How do the thousands of different cell types in an animal arise time and again at particular locations during embryonic development? The answer lies partly in the distribution of signalling molecules called morphogens1, which are released from local sources and form concentration gradients in target tissues. Cells that are close to the source of the morphogen are exposed to high signal concentrations and activate developmental programs that differ from those in cells that are farther away and exposed to lower levels of morphogen. This powerful strategy means that the same signalling molecule can be used in the formation of different cell types. But how are morphogen gradients established? On page 533 of this issue, Yu et al.2 describe one mechanism. They propose that, during the development of the zebrafish embryo, the morphogen fibroblast growth factor 8 (FGF8) spreads rapidly by diffusion from a local source and is then taken up by target tissues. This implies that the combination of free random motion and cellular uptake generates a signalling gradient that endows cells with different developmental fates.
Yu and colleagues' findings2 support a model proposed almost 40 years ago by Francis Crick, dubbed the source-sink model. Crick put forward a mechanism3 to explain how morphogen gradients could be set up in a developing tissue. He calculated that a stable gradient can be generated by the local production of a signal at one end of a tissue (the source), its spread into surrounding cells, and its local removal at the other end (the sink). Crick argued as part of the source-sink model that the spreading of...