Gap junction assembly in the preimplantation mouse conceptus is independent of microtubules, microfilaments, cell flattening, and cytokinesis.
Gap junctions first appear during compaction in the eight-cell stage of mouse development. Their assembly can be initiated in the near absence of transcription and protein synthesis from the four-cell stage, indicating the existence of preformed precursors. We have investigated the temporal control of this event, focusing on the possible involvement of the cytoskeleton, cell flattening, and cytokinesis. Embryos in various cleavage stages were treated with cytochalasins, to disrupt microfilaments and block cell flattening, cytokinesis, or both, or nocodazole, to promote microtubule depolymerization. To assess their capacity to initiate gap junction assembly after such treatments, the embryos were then aggregated with communication-competent, compacted embryos that had been labeled with carboxyfluorescein diacetate. Passage of the fluorescent dye, carboxyfluorescein, from labeled to unlabeled embryo was taken as evidence that interembryonic junction formation had occurred. The capacity to assemble gap junctions was acquired at the normal time by embryos prevented by cytochalasin treatment from undergoing cell flattening or any cytokinesis from fertilization onward. Likewise, treatment with nocodazole beginning in the four-cell or early eight-cell stage did not interfere with gap junction assembly. Neither drug affected the inability of four-cell embryos to assemble gap junctions prematurely. We conclude that intact microfilament or microtubule networks are not required for gap junction assembly in this system, nor do they restrain junctional precursors from assembling prematurely. Furthermore, the timing of gap junction assembly is not linked to cell flattening, cytokinesis, or cell number.