In many embryonic tissues, graded signals (morphogens) provide the positional information that governs the pattern of cellular differentiation. It is widely thought that cells interpret morphogen gradients by producing corresponding levels of intracellular signalling activity, which regulate differential gene expression. But could other, distinct, mechanisms be used to interpret some morphogens? On , James Briscoe and co-workers investigate the morphogen activity of bone morphogenetic protein (BMP) signalling in the chick dorsal neural tube. BMPs are thought to provide the positional information that specifies the spatial pattern of the dorsal interneurons. In neural tube explants, the researchers report, the duration of exposure to Bmp4 generates distinct levels of intracellular signalling and induces specific dorsal identities. Moreover, they find that a dynamic gradient of BMP activity progressively specifies more dorsal identities in the neural tube in vivo. Given these results, the researchers propose a model for morphogen interpretation in which the temporally dynamic control of signalling is required to specify the spatial pattern of cellular differentiation.
