Abstract
Sea urchin larvae have an endoskeleton consisting of two calcitic spicules. The primary mesenchyme cells (PMCs) are the cells that are responsible for spicule formation. PMCs endocytose sea water from the larval internal body cavity into a network of vacuoles and vesicles, where calcium ions are concentrated until they precipitate in the form of amorphous calcium carbonate (ACC). The mineral is subsequently transferred to the syncytium, where the spicule forms. Using cryo-soft X-ray microscopy we imaged intracellular calcium-containing particles in the PMCs and acquired Ca-L2,3 X-ray absorption near-edge spectra of these Ca-rich particles. Using the prepeak/main peak (L2'/ L2) intensity ratio, which reflects the atomic order in the first Ca coordination shell, we determined the state of the calcium ions in each particle. The concentration of Ca in each of the particles was also determined by the integrated area in the main Ca absorption peak. We observed about 700 Ca-rich particles with order parameters, L2'/ L2, ranging from solution to hydrated and anhydrous ACC, and with concentrations ranging between 1 and 15 M. We conclude that in each cell the calcium ions exist in a continuum of states. This implies that most, but not all, water is expelled from the particles. This cellular process of calcium concentration may represent a widespread pathway in mineralizing organisms.
Highlights
Calcium ions play a critical role in many cellular processes
The labelled cell suspension in sea water was seeded onto gold finder-TEM-grids and the grids were vitrified by plunge freezing
The short-range order of the oxygen atoms around the calcium ion show that approximately half of the Ca-particles have L2’/L2 values smaller than those of hydrated amorphous calcium carbonate (ACC), indicating an even less ordered state
Summary
Calcium ions play a critical role in many cellular processes. Calcium ions are messengers for a wide range of cellular activities, including fertilization, cell differentiation, secretion, muscle contraction and programmed cell death [1, 2]. The sequestered ions can reach the mineralization site as solutes, but can concentrate intracellularly inside vesicles, where they precipitate to form highly disordered mineral phases [10,11,12,13,14,15]. In the latter case, specialized cells take up the ions through ion pumps, ion channels, or by endocytosis of extracellular fluid, and process the calcium ions until export to the final mineralization location [16,17,18,19]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
