One cell-cycle early regulation of cell division axis by cortically localized factors mediates oriented cell division critical for embryo morphology

Abstract Cortically localized factors play major roles in regulating cell division orientation during oriented cell division. Posterior End Mark (PEM), a protein localized to the posterior vegetal structure called the centrosome-attracting body (CAB) and responsible for the anterior-posterior asymmetry of the 8-cell stage ascidian embryo, was thought to regulate the orientation of the cell division by attracting the centrosome toward the CAB at the 4-cell stage. However, this was not supported by analyses of the four-dimensional dynamics of the centrosome and the CAB. Here, we observed the positions of the centrosomes and the CAB by confocal microscopy and found that PEM attracts the centrosome to the CAB at the 2-cell stage, but not at the 4-cell stage. In the results, PEM regulated cell division orientations in anterior cells, which lack the CAB/PEM at the 4-cell stage, and the position of the CAB in the posterior cells. We propose a new model on how the 8-cell stage asymmetry is formed where the cell division orientation at the 4-cell stage results from the PEM-dependent shift of the division plane relative to the body axis at the 2-cell stage.