Abstract
Understanding how cell fates are determined from graded extracellular signals is a major issue in signal transduction. Briscoe et al. studied the induction of neuronal cell fates in chicken embryos manipulated to ectopically express several homeodomain factors regulated by the graded signal Sonic Hedgehog (Shh). The homeodomain proteins are divided into two classes: class I, whose expression is repressed by Shh, and class II, whose expression is induced by Shh. The authors suggest that events for specifying the pattern of neuronal cell fates in the ventral neural tube occur in the following sequence: (1) graded Shh signaling leads to activation or repression of the homeodomain proteins in the progenitor cells, (2) specific members of class I and class II proteins cross-repress each other's expression to establish and stabilize the progenitor domains, (3) the particular combination of class I and class II proteins expressed directs the type of neuron the cell will become.Briscoe, J., Pierani, A., Jessell, T.M., and Ericson, J. (2000) A homeodomain protein code specifies progenitor cell identity and neuronal fate in the ventral neural tube. Cell 101: 435-445. [Online Journal]
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