Distributions of PAX6 and PAX7 proteins suggest their involvement in both early and late phases of chick brain development.

We describe here the isolation of chicken PAX6 and PAX7 as members of the PAX gene family expressed in late stages of chick nervous system development. By generating monoclonal antibodies against these PAX molecules, we analyzed their protein distributions in the CNS, neural crest cells and muscle precursor cells. Our detailed description of protein distributions revealed several new features of PAX expression patterns. In particular, PAX6 and PAX7 are expressed in restricted regions in the CNS during early stages. In contrast, at later stages the expression becomes localized to subsets of postmitotic cells in further restricted regions. Such a transition from region-specific expression to cell-specific expression suggests two different roles of these PAX molecules in development: the regionalization and subdivision of the nervous system during early stages, and the differentiation of specific cell populations during late stages.