Intracellular organelles in the saga of Ca2+ homeostasis: different molecules for different purposes?

An increase in the concentration of cytosolic free Ca2+ is a key component regulating different cellular processes ranging from egg fertilization, active secretion and movement, to cell differentiation and death. The multitude of phenomena modulated by Ca2+, however, do not simply rely on increases/decreases in its concentration, but also on specific timing, shape and sub-cellular localization of its signals that, combined together, provide a huge versatility in Ca2+ signaling. Intracellular organelles and their Ca2+ handling machineries exert key roles in this complex and precise mechanism, and this review will try to depict a map of Ca2+ routes inside cells, highlighting the uniqueness of the different Ca2+ toolkit components and the complexity of the interactions between them.