Spicules - mineralised rods that constitute the sea urchin skeleton - form from a small number of primary mesenchymal cells (PMCs). During gastrulation,these PMCs locate along the ectodermal wall in a stereotypical pattern that determines skeletal morphology. As yet unknown guidance cues from the ectoderm are thought to control PMC positioning. VEGF/VEGFR signalling between the ectoderm and PMCs is now shown by Christian Gache's lab on to be the missing link that directs PMC migration and, thus, skeletal morphology. While VEGFR is expressed in PMCs, its ligand, VEGF, is expressed in the overlying ventrolateral ectoderm. Impaired VEGFR signalling perturbs PMC positioning(and spicules subsequently don't form), whereas VEGF overexpression results in skeletal abnormalities. Ectopically expressing VEGF in embryos in which endogenous VEGF expression is blocked restores spicule formation. These and other findings reveal that localized VEGF acts as both a guidance cue and differentiation signal, providing a crucial link between the positioning and differentiation of PMCs and embryonic skeletal morphogenesis.
