Limb formation is currently explained by a long-standing model, which states that signals from tissue in the limb bud called the apical ectodermal ridge (AER) instruct cells of the upper arm to form first, followed by cells of the forearm, and finally cells of the hand. But two papers in August 1 Nature challenge this view and present evidence that all cell types are produced at the same time in the limb bud, and then, under the control of the AER, the different regions of cells expand at different times to form the complete limb.

Andrew Dudley and colleagues at the Harvard Medical School excised the AER from chicken wing buds at various developmental stages, collected the buds 6–8 h after AER removal and assayed for apoptotic cells. They observed that soon after AER removal some cells die and cell proliferation is dramatically reduced, but the domain of cell loss...

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