In sea urchin embryogenesis, large blastomeres, called micromeres, and their progeny act as an important signalling centre and give rise to mesenchymal skeleton-forming cells. Ettensohn et al. have now identified a new and essential component of the gene network that controls micromere specification – Alx1, the first known invertebrate member of the Cart1/Alx3/Alx4 family of vertebrate paired-class homeodomain proteins that function in limb and craniofacial skeletal development. Morpholino knockdown and gene expression experiments show that Alx1 controls genes required for epithelial-mesenchymal transition and biomineralization. Importantly, these findings, on , hint at an evolutionary link between certain features of skeletal development in vertebrates and sea urchins, and indicate that the ancestral deuterostome from which they derive might have had a mesenchymal cell lineage that engaged in biomineralization, in which an Alx1-like protein functioned.
