Gap junctional communication in the early Xenopus embryo

In the Xenopus embryo, blastomeres are joined by gap junctions that allow t he movement of small molecules between neighboring cells. Previous studies using Lucifer yellow (LY) have reported asymmetries in the patterns of junc tional communication suggesting involvement in dorso-ventral patterning. To explore that relationship, we systematically compared the transfer of LY a nd neurobiotin in embryos containing 16-128 cells. In all cases, the juncti on-permeable tracer was coinjected with a fluorescent dextran that cannot p ass through gap junctions. Surprisingly, while LY appeared to transfer in w hole-mount embryos, in no case did we observe junctional transfer of LY in fixed and sectioned embryos. The lack of correspondence between data obtain ed from whole-mounts and from sections results from two synergistic effects . First, uninjected blastomeres in whole-mounts reflect and scatter light o riginating from the intensely fluorescent injected cell, creating a diffuse background interpretable as dye transfer. Second, the heavier pigmentation in ventral blastomeres masks this scattered signal, giving the impression of an asymmetry in communication. Thus, inspection of whole-mount embryos i s an unreliable method for the assessment of dye transfer between embryonic blastomeres, A rigorous and unambiguous demonstration of gap junctional in tercellular communication demands both the coinjection of permeant and impe rmeant tracers followed by the examination of sectioned specimens. Whereas LY transfer was never observed, neurobiotin was consistently transferred in both ventral and dorsal aspects of the embryo, with no apparent asymmetry. Ventralization of embryos by UV irradiation and dorsalization by Xwnt-8 di d not alter the patterns of communication. Thus, our results are not compat ible with current models for a role of gap junctional communication in dors o-ventral patterning.