Fig. 3. A model for somitogenesis. (A) Double in situ hybridization of a 2-day-old chicken embryo with Raldh2 (retinaldehyde dehydrogenase 2) and Fgf8 (fibroblast growth factor 8) probes. Anterior is towards the top. These genes participate in the establishment of mutually inhibitory, antagonistic gradients of retinoic acid (RA) and fibroblast growth factor (FGF) signaling. (B) Molecular mechanisms leading to a segmental pattern. Segment patterning genes are periodically activated by the segmentation clock, whose main regulators are the Notch and Wnt signaling pathways. The spatial activation of the segment patterning genes is defined by the RA and FGF antagonistic gradients: RA positively regulates their transcription, whereas FGF signaling represses RA activity and inhibits presomitic mesoderm maturation.