After cells in the developing embryo divide, microtubules at the dividing poles of each cell pull actin proteins into a ring. Those rings expand across the cells and, with the help of the motor protein myosin, reshape each cell into a pyramid shape. The changing geometry of the cells allows the blastocyst cavity to form. The cavity becomes sealed as each cell’s ring touches another ring, and as myosin pushes the edges, or junctions, of the rings together, with the help of cell adhesion molecules, zippering the junctions shut. If this process does not happen, embryos may fail to develop normally.
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