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Change in K + concentration induces key molecular genetic transformations: gene expression [1], DNA recombination/degradation [2], and (de)polymerization of actin [3]. The oscillation of the intracellular potassium level of early mammalian embryos was predicted from the cyclicity of the 240 pS K + channel whose activity is synchronized with the phases of the cell cycle [4][5]. Therefore, the purposes of this work were the following: (1) to study the potassium content and distribution in the cytoplasm of the mouse zygote at different stages of its development and (2) on the basis of the experimental data, elaborate the mathematical model of K + diffusion during the first cell cycle. The obtained results allowed us to put forward the hypothesis that the mechanism of the approach of pronuclei during the G 1 /S transition is mediated by the polymerization of actin-like proteins in the cell.It is known that G-actin exposed to physiologically high K + concentration is polymerized into the F form after binding the cation [6,7]. The storage of potassium, if it is present, can significantly change the electrolyte diffusion rate in the cytoplasm. This rate is suggested to be comparable to the rate in a water solution. This also indicates the appearance of a gradient of the element concentration between the periphery and the central region of the mouse zygote. The K + concentration should be changed with time according to the activity of the 240 pS K + channel. The electron probe microanalysis of potassium compartmentalization in the embryo cytoplasm was employed to test our hypothesis.This method is based on recording the intensity of the K α line of X-radiation excited in the zygote structure by accelerated electrons of the probe, which is formed in the electron microscope. The technique of embryo section preparation and the parameters of electron probe microanalysis were discussed earlier [8,9].The monitoring of embryo morphology and measurement of cytoplasmic potassium concentration in it was performed in 2-µ m slices using a JSM U3 electron microscope (JEOL, Japan). The results of the quantitative electron probe analysis are shown in the table.Analysis of potassium distribution in the cytoplasm of mouse zygote. After the fertilization, the potassium concentration changes during the zygote development (table). It is noteworthy that a significant difference between peripheral cytoplasm of embryo zygote and its central part was observed during the transition between G 1 /S and G 2 /M phases. These data suggest that the parameters of K + diffusion in the internal medium of the zygote do not correspond to those in a water solution.The analysis of the spatiotemporal characteristics of the distribution of cytoplasmic potassium in the zygote during different phases of its development (table) allows us to formulate the boundary and initial conditions for the calculation of the diffusion coefficient. At the initial moment, potassium concentration in the cytoplasm is distributed uniformly and is similar to that in the oocyte duri...

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