Skeletogenesis begins in the early sea urchin blastula (9–10 hours post fertilization) when the primary mesenchyme cells (PMCs), the sole descendants of the large micromere daughter cells, undergo an epithelial–mesenchymal transition (EMT) and break away from the apical layer, thus entering the blastocoel, forming a cell cluster at the vegetal pole. It is a key interaction between the two principal populations of mesodermal cells in the sea urchin embryo, PMCs and secondary mesenchyme cells (SMCs), that regulates SMC fates and the process of skeletogenesis. In a wild type embryo, skeletal elements are exclusively produced by PMCs. Due to their nature in giving rise to the larval skeleton, they are sometimes called the skeletogenic mesenchyme. Certain SMCs have a skeletogenic potential, however, signals transmitted by the PMCs suppress this potential in the SMCs and direct these cells into alternative developmental pathways.
