Electron probe x-ray microanalysis studies on the ionic environment of nuclei and the maintenance of chromatin...

We have demonstrated the application of electron probe X-ray microanalysis to studies aimed at determining the role of the ionic environment in the maintenance of a stable chromatin structure during interphase. Our results indicate that X-ray microanalysis can be used to accurately measure the concentration of ions, such as Na+ and K+ within subcellular compartments. We also present evidence from our X-ray microanalysis studies to show that only a portion of the K+ in the interphase nucleus is in a free ionic state and that the rest of the intranuclear K+ is adsorbed to intranuclear macromolecules. The stable structural state of interphase chromatin was correlated with the maintenance of a normal range in the chemical activities of monovalent ions in the oocyte nucleus. Finally, we demonstrated that the chemical activity of the monovalent ions probably increases beyond this normal interphase range as the chromatin in the nucleus condenses into discrete chromosomes during the first meiotic division in the amphibian oocyte. The above observations support the hypothesis of Kellermayer et al. (1984) that the maintenance of chromatin structure in the nucleus of living cells requires a balance between the presence or absence of mobile nuclear proteins and the chemical activity of monovalent cations. It also suggests that an elevation of the chemical activity of the monovalent cations above a normal range will result in the destabilization of interphase chromatin structure. It is hypothesized that an elevation of monovalent cations may be directly involved in the structural and functional changes in chromatin which are known to take place at the time of meiosis and mitosis.