Figure 4

sap18 mRNA expression pattern in chick embryos. Chick embryos staged HH10, HH12+ and HH14 were processed for in situ hybridization using a probe for chick sap18. Transverse sections at the indicated AP levels on stage HH10 embryo are shown. At early stages, sap18 is expressed at the level of the primitive streak in prospective neural cells (section 6, pnc), is absent from the neural plate (np) facing the PSM (section 5, psm) and is upregulated throughout all the neural tube positioned rostrally to the PSM territory (sections 1–3, nt). In older embryos, sap18 is expressed along the entire neural tube AP axis (stages HH12+ and HH14). In all embryos, sap18 expression is absent from most of the PSM tissue (brackets) being upregulated only in the rostral part of the PSM, the region that will give rise to the prospective somites (whole embryos and section 4, ps) and in already formed somites (whole embryos and section 3, s).