Anatomy

Vesicle

Organic chemistry

Biomineralization

Seawater

Membrane

Biology

Sponge spicule

Environmental chemistry

Biophysics

Paleontology

Mineralogy

Marine invertebrates

Chemistry

Ecology

Mineral

Sea urchin

Calcite

Calcium carbonate

Calcium

Cell biology

Biochemistry

Spicule

Proceedings of the National Academy of Sciences

Significance Organisms form mineralized skeletons, many of which are composed of calcium salts. Marine organisms extract calcium ions from sea water. One of the main unresolved issues is how organisms concentrate calcium by more than three orders of magnitude to achieve mineral deposition in their skeleton. Here we determine the calcium state in each of the calcium-containing vesicles inside the spicule-building cells of sea urchin larvae. We show that within one cell there is a wide range of concentrations and states from solution to solid. We hypothesize that calcium concentration increases gradually in each vesicle, starting from seawater levels and until mineral is deposited. This model might well be relevant to other phyla, thus advancing the understanding of biomineralization processes.

Cellular pathways of calcium transport and concentration toward mineral formation in sea urchin larvae