A cyclical wave of hairy1 mRNA expression occurs during the formation of each somite in chick embryos.

The expression pattern of the hairy1 transcription factor correlates with the formation of each individual somite. At the top row, microscopy images show hairy1 mRNA expression in chicken embryos with 15, 16, and 17 somites (dark blue). The lower row shows drawings of those same areas that clarify how hairy1 expression moves as a wave during somite formation, and embryo maturation. Hairy1 mRNA, shown in blue, starts broadly in the posterior (bottom). With time, the band of expression progresses upward and narrows as it approaches the anterior end. Eventually, it forms a narrow stripe at the posterior end of the newly formed somite (SI or SII). This entire process takes approximately 90 minutes. Scale bar 200μm.