MORPHOGENESIS OF CALCITIC SPONGE SPICULES - A ROLE FOR SPECIALIZED PROTEINS INTERACTING WITH GROWING CRYSTALS

Crystals formed in biological tissues often adopt remarkable morpholog ies that are thought to be determined mainly by the shapes of the conf ined spaces in which they grow. Another possible way of controlling cr ystal shape, demonstrated only in vitro, is by means of specialized pr oteins preferentially interacting with certain crystal faces. In so do ing, they reduce the rate of growth in these directions and consequent ly change the overall crystal shape. In an X-ray diffraction study of the distribution of defects within the lattice of calcite crystals pro duced by certain sponges, we show that a remarkable correlation exists between the defect patterns or crystal texture and the macroscopic mo rphology of the spicules. This was observed in two cases in which prot eins are present within the spicule crystal, but not in a third case w here such intracrystalline proteins are absent. Furthermore, one of th e spicules exhibited marked differences in texture even within familie s of structurally identical crystal planes, demonstrating that the org anisms exert exquisite control over the microenvironment in which crys tals grow. We conclude that highly controlled intercalation of special ized proteins inside the crystals is an additional means by which orga nisms control spicule growth.