A role for hairy1 in regulating chick limb bud growth.

Limb growth in higher vertebrate embryos is initially due to the outgrowth of limb buds and later continues as a result of elongation of the skeletal elements. The distal limb mesenchyme is crucial for limb bud outgrowth. Members of the Hairy/Enhancer of Split family of DNA binding transcriptional repressors can be effectors of Notch signaling and often act to maintain cell populations in an undifferentiated, proliferating state, properties predicted for the distal limb mesenchyme. We find that a member of this family, c-hairy1, is expressed in this region and that two alternatively spliced isoforms, c-hairy1A and c-hairy1B, of this gene are produced, predicting proteins that differ in their basic, DNA binding, domains. Viral misexpression of c-hairy1A causes a reduction in size of the limb and shortened skeletal elements, without affecting the chondrocyte differentiation program. Misexpression of c-hairy1B leads to a significantly lesser shortening of the bones, implying functional differences between the two isoforms. We conclude that c-hairy1 regulates the size of the limb, suggesting a role for Notch signaling in the distal mesenchyme.