In the sea urchin embryo, primary mesenchyme cells (PMCs) are committed early in development to direct skeletogenesis, provided that a permissive signal is conveyed from adjacent ectoderm cells. We showed that inhibition of extracellular matrix (ECM)-ectoderm cells interaction, by monoclonal antibodies (mAb) to Pl-nectin, causes an impairment of skeletogenesis and reduced expression of Pl-SM30, a spicule-specific matrix protein. When PMCs are experimentally removed, some secondary mesenchyme cells (SMCs) switch to skeletogenic fate. Here, for the first time we studied SMC transfating in PMC-less embryos of Paracentrotus lividus. We observed the appearance of skeletogenic cells within 10 h of PMCs removal, as shown by binding of wheat germ agglutinin (WGA) to cell surface molecules unique to PMCs. Interestingly, the number of WGA-positive cells, expressing also msp130, another PMC-specific marker, doubled with respect to that of PMCs present in normal embryos, though the number of SM30-expressing cells remained constant. In addition, we investigated the ability of SMCs to direct skeletogenesis in embryos exposed to mAbs to Pl-nectin after removal of PMCs. We found that, although phenotypic SMC transfating occurred, spicule development, as well as Pl-SM30-expression was strongly inhibited. These results demonstrate that ectoderm inductive signals are necessary for transfated SMCs to express genes needed for skeletogenesis.