In the Pax gene family, which encodes DNA-binding proteins, Pax 2 has been known to play important roles in the formation of the midbrain/hindbrain boundary, eye, inner ear and kidney in vertebrates (Bioessays 19 (1997) 755). In this article, we report a segmentally regulated pattern of Pax 2 expression during chicken somitogenesis. Pax 2 mRNA is localized in the rostral end of the unsegmented presomitic mesoderm (PSM), abutting anteriorly on a prospective segmentation border. This pattern repeats every segmentation cycle (90 min) observed in ovo and also in the hall embryo culture assay in which one hall of PSM along the midline is fixed immediately while the other half is cultured for a given period. We also determined the sequence of changes in Pax 2 expression during a segmentation cycle by comparing the pattern of Pax 2 with that of Lunatic-fringe (L-fringe), known to cycle periodically in posterior PSM. A systematic comparison of the expression patterns between Pax 2, L-fringe and EphA4 further highlighted a close relationship between EphA4 and Pax 2 during a segmentation cycle. Lastly, Pax 2 is not segmentally expressed in mouse PSM, suggestive of species (avian)-specific mechanisms underlying somitic segmentation.
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