Local intracellular stiffening of ascidian embryo in cleavage developmental stage observed by atomic force microscopy

During the cleavage stage in ascidian embryos, cells undergo repeated and regular cell division in the animal hemisphere. We investigated how the intracellular stiffness, that is, the apparent Young's modulus of cells in the animal hemisphere changes in the cleavage stages from the 16-cell to 76-cell stages using atomic force microscopy (AFM). The AFM force mapping experiments showed that a pair of dot-like stiffening clearly appeared in the intracellular region, which involves the cortical and cytoplasmic regions indented by the AFM probe, in a period of the interphase after cell division. The dot-like structures disappeared before beginning to increase the whole cell stiffening towards the proceeding mitotic phase. We found that the positioning of a pair of dots was located so as to intersect the daughter cells formed in the proceeding cell division. These results suggest that the local intracellular stiffening observed in the interphase may play an important role in controlling the orientation of the mitotic spindle that determines the direction of cell division.