Dye-coupling in the early squid embryo.

Functional cell-to-cell communicaton pathways were studied in pre-organogenetic squid embryos using the fluorescent dyes Lucifer Yellow-CH (LY) and Fluorescein-isothiocyanate Dextran (FITC-D). LY (M.Wt.450) micro-injected into ectoderm or mes-endoderm cells spread extensively over the embryo, flowing laterally along each germ layer, crossing germ layer boundaries and also the cell-to-yolk syncytium boundary. In contrast, when FITC-D (M.Wt.9000 or 17,500) was micro-injected into ectoderm or mes-endoderm cells, spread was limited to a maximum of 7 cells within the respective germ layer.FITC-D did not cross germ-layers or flow into the yolk syncytium. Our results suggest there are two classes of cell-to-cell communication pathways in the germ-layer stage squid embryo; a ubiquitously distributed junction that is permeable to small molecules and a larger junction that allows the transport of both small molecules and macromolecules upto 17.500 M.Wt. The former pathway, which may correspond to the gap junction, is operative throughout the mitotic cycle, while the latter, possibly a cytoplasmic bridge, is functional while cells are in interphase, but does not allow the transport of large molecules during mitotic activity.