Studies on the cellular pathway involved in assembly of the embryonic sea urchin spicule

Micromeres from the 16-cell stage sea urchin embryo were isolated and cultured in vitro in seawater containing 3% horse serum. Under these conditions these cells differentiate into spicule-forming, primary mesenchyme cells. To obtain insight into the route traveled by Ca2+ to form the pseudocrystalline spicule composed of CaCO3 and matrix proteins, studies with various inhibitors were undertaken. Experiments with members of several different classes of Ca2+ channel blockers established that the Ca2+ utilized for spiculogenesis must be taken up by the cells. Moreover, studies using two agents that disrupt the endomembrane system, monensin and brefeldin A, showed that both blocked spicule formation. Based on these experiments, we conclude that extracellular Ca2+ must enter the primary mesenchyme cells prior to being deposited extracellularly as CaCO3 and that this ion and/or the matrix proteins found in the spicule are routed through the secretory pathway that has been established to exist in a wide variety of other cell types.