  The connection between the properties of cell tissue and those of the single
constituent cells remains to be elucidated. At the purely mechanical level, the
degree of rigidity of different cellular components, such as the nucleus and
the cytoplasm, modulates the interplay between the cell inner processes and the
external environment, while simultaneously mediating the mechanical
interactions between neighboring cells. Being able to quantify the correlation
between single-cell and tissue properties would improve our mechanobiological
understanding of cell tissues. Here we develop a methodology to quantitatively
extract a set of structural and motility parameters from the analysis of
time-lapse movies of nuclei belonging to jammed and flocking cell monolayers.
We then study in detail the correlation between the dynamical state of the
tissue and the deformation of the nuclei. We observe that the nuclear
deformation rate linearly correlates with the local divergence of the velocity
field, which leads to a non-invasive estimate of the elastic modulus of the
nucleus relative to the one of the cytoplasm. We also find that nuclei
belonging to flocking monolayers, subjected to larger mechanical perturbations,
are about two times stiffer than nuclei belonging to dynamically arrested
monolayers, in agreement with atomic force microscopy results. Our results
demonstrate a non-invasive route to the determination of nuclear relative
stiffness for cells in a monolayer.
