Cells in a developing embryo make and break gap-junction connections in specific and interesting patterns, suggesting that gap junctions play an important role in the signaling processes that occur between these cells. At the eight-cell stage, mouse embryos undergo compaction, changing from a clump of loosely associated cells to a tightly sealed ball (see the figure Part A). You wish to know whether gap junctions are present before or after this change in adhesion.
Using very fine glass micropipettes, you can measure electrical events and at the same time microinject the enzyme horseradish peroxidase (HRP), 40,000 daltons, or the fluorescent dye fluorescein, 330 daltons. Fluorescein glows bright green under ultraviolet illumination, and HRP can be detected by fixing the cells and incubating them with appropriate substrates. You obtain different results at both the two-cell and eight-cell stages, depending on whether the injections are made immediately after cell division or later (Part B). Immediately after cell division, cytoplasmic bridges linger for a while before cytokinesis is completed.