Intracrystalline macromolecules are involved in the morphogenesis of calcitic sponge spicules.

Control over the shapes of biologically formed crystals is generally not well understood. We have studied the morphogenesis of the different-shaped calcareous sponge spicules using high-resolution synchrotron X-ray diffraction. We show that a remarkable correlation exists between the distribution of defects within spicule crystals at the nanometer level and their macroscopic morphologies at the millimeter level. These defects are produced by controlled intercalation of specialized macromolecules into the crystals. We also show that such intracrystalline macromolecules are involved in the regulation of the shapes of synthetic crystals grown de novo from solution, and epitaxially overgrown on the spicule surfaces. We conclude that intracrystalline macromolecules play an important role in modulation of the morphologies of the forming biogenic crystals. Possible mechanisms that may account for the observed growth patterns are supported by fluorescence labeling experiments in vivo.