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

ABSTRACT During early embryo development, cell division is highly organized and synchronized. Understanding the mechanical properties of embryonic cells as a material is crucial for elucidating the physical mechanism underlying embryogenesis. Previous atomic force microscopy (AFM) studies on developing embryos 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) towards 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 . SIGNIFICANCE Understanding the rheological properties of embryonic cells is crucial to elucidate the origin and mechanism of the functional and morphological changes in cells during embryogenesis. AFM-based microrheology revealed that single cells of ascidian embryos in the animal hemisphere in the cleavage stages followed a single power-law rheology, which has no characteristic time scale. Furthermore, the rheological parameters, such as cell stiffness and fluidity, were collapsed onto a master curve with a negative correlation during the cell cycle. These rheological behaviors are similar to those observed in single cells in vitro, indicating that the embryonic cells tightly interacting with each other conserved the intrinsic rheological behaviors of isolated single cells.