Patterns of junctional communication in the early amphibian embryo

It has long been recognized that cells in early embryos can communicate with each other via a direct cell-to-cell pathway, probably mediated by gap junctions. Low electrical resistance pathways, detected electrophysiologically, have been identified in all species examined so far. However, studies in various embryos on the transfer of molecules larger than small ions (for example, fluorescent dyes in the molecular weight range 350-500) have given conflicting results. In all these studies the ability to transfer dyes from cell to cell was determined without reference to the position of the injected cell in the embryo. In the experiments reported here, cell-cell transfer of the fluorescent dye, Lucifer yellow (molecular weight (Mr) 450) was re-examined in the early Xenopus laevis embryo by injecting the dye into identified cells, as the position of the injected cell within the embryo may be important. At the 32-cell stage, we found that dye transfer often occurred between animal pole blastomeres which were not sisters, as well as between sister cells, and also that Lucifer yellow was indeed transferred via gap junctions. The cell-cell transfer was not uniform within the animal pole; transfer was maximal near the dorsal side and minimal at the ventral side. This pattern may reflect differences in permeability or numbers of gap junctions across the embryo, and could be related to early events in development.