The specificity of gap junctional channels in early fish embryos and its significance for pattern formation in development.

The data obtained support the hypothesis that cells of developing embryos use cell-to-cell communication via gap junctions (GJs) for the transmission of signals participating in the pattern formation of gene expression. It was revealed that a ring area of the early mesoderm gene activation in loach (Misgurnus fossilis) embryos, which lies in the proximity of the yolk cell (YC), is simultaneously a region of the increased dye transfer between the blastomeres and the YC for fluorescein (F) and Lucifer Yellow (LY). In search for the reason of such a heterogeneity of dye transfer, it was found that the permeability of GJs between the blastomeres increased from the early through mid-blastula stage. In parallel, the adhesiveness of the blastomeres also increased, that, apparently, directs the GJ uncoupling in the central region of the blastoderm basal layer. More important is that the changes in the permeability of the GJs between the YC and the blastomeres located in the presumptive mesoderm area were accompanied by changes in GJ selectivity. By comparing the kinetics of the transfer of the fluorescent substances of opposite charges (LY and F as anions and 4', 6-diamidino-2-phenylindole (DAPI) as a cation), we found that all GJs at the mid-blastula stage and GJs of the blastomeres located in the central area of the basal layer at the early gastrula stage were permeable for each dye, but the permeability for the cationic dye was slightly lower than for the anionic ones. At the early gastrula stage, GJs of a germ ring area (prospective mesoderm region) acquire much higher anionic selectivity; they become more permeable for LY and F but virtually impermeable for DAPI. These results suggest that embryonic cells can regulate the selectivity of GJ channels and control the transport of the secondary messengers based on their electrochemical properties.