Mammalian oocytes undergo major changes in zinc content and localization in order to be fertilized, the most striking being the rapid exocytosis of over ten billion zinc ions, known as zinc sparks. Here we report that fertilization of amphibian Xenopus laevis eggs also initiates a zinc spark that progresses across the cell surface in coordination with dynamic calcium waves. This zinc exocytosis is accompanied by a newly recognized loss of intracellular manganese: synchrotron-based X-ray fluorescence and analytical electron microscopy reveal that zinc and manganese are sequestered in a system of cortical granules that are abundant at the animal pole. Through Electron-Nuclear Double-Resonance (ENDOR) studies, we rule out Mn 2+ complexation with phosphate or nitrogenous ligands in intact eggs but the data are consistent with a carboxylate coordination environment. Our observations suggest that zinc and manganese fluxes are a conserved feature of fertilization in vertebrates and that they function as part of a physiological block to polyspermy.

Quantitative high spatial resolution microanalysis of elemental distributions is a challenge in virtually all biological systems with issues ranging from detection sensitivity, radiation damage, through to quantification. This is particularly true when investigating distributions of metals which are found in virtually all cells and organisms. Fluctuations in the concentrations of these species (including Ca, Fe, Cu, Zn,) control a host of functional processes within the cells. Dynamic changes in essential metal ion concentrations play key regulatory roles in physiological events. Furthermore, surges in metal uptake from the extracellular environment can be pathogenic [1,2]. Numerous studies of element distributions have been conducted using synchrotron x-ray fluorescence (S-XRF), due in part to the sensitivity of xray excitation for low level concentrations and the ability to study whole cells [3]. Analytical electron microscopy (AEM) applied to organic/inorganic materials is not a new concept and has been applied for many decades using both x-ray and electron loss spectroscopy [4-7], however, advances in detector technology are facilitating new insights for today's research [8]. This evolution has additionally opened up new opportunities to study more challenging systems involving soft-matter and/or biological systems including experiments involving cryo-EM [9].