Mapping single-cell rheology of ascidian embryos in the cleavage stages using AFM.

During early embryo development, cell division is highly organized and synchronized. Understanding the mechanical properties of embryonic cells as a material is crucial in elucidating the physical mechanism underlying embryogenesis. Previous studies on developing embryos using atomic force microscopy (AFM) revealed that single cells of ascidian embryos in the cleavage stage stiffened and softened during cell division. However, how embryonic cells, as a compliant material, exhibit viscoelastic properties during the cell cycle remains poorly characterized. In this study, we investigated the rheological properties of embryonic cells in the animal hemisphere in the cleavage stages using stress-relaxation AFM and approach-retraction force curve AFM techniques. The AFM measurements revealed that developing single cells followed a power-law rheology observed in single-cell rheology in vitro. The embryonic cells increased the modulus (stiffness) and decreased the fluidity (the power-law exponent) toward cell division. We found three rheological states in developing embryos during the cell cycle. The correlation between the cell modulus and the fluidity during the cell cycle was collapsed onto a master curve with a negative correlation, indicating that embryonic cells tightly interacting with the neighboring cells conserved the universality of rheological behavior observed in single cells in vitro.