  A hallmark of biological tissues, viewed as complex cellular materials, is
the active generation of mechanical stresses by cellular processes, such as
cell divisions. Each cellular event generates a force dipole that deforms the
surrounding tissue. Therefore, a quantitative description of these force
dipoles, and their consequences on tissue mechanics, is one of the central
problems in understanding the overall tissue mechanics. In this work we analyze
previously published experimental data on fruit fly \textit{D. melanogaster}
wing epithelia to quantitatively describe the deformation fields induced by a
cell-scale force dipole. We find that the measured deformation field can be
explained by a simple model of fly epithelium as a linearly elastic sheet. This
fact allows us to use measurements of the strain field around cellular events,
such as cell divisions, to infer the magnitude and dynamics of the mechanical
forces they generate. In particular, we find that cell divisions exert a
transient isotropic force dipole field, corresponding to the temporary
localisation of the cell nucleus to the tissue surface during the division, and
traceless-symmetric force dipole field that remains detectable from the tissue
strain field for up to about $3.5$ hours after the division. This is the
timescale on which elastic strains are erased by other mechanical processes and
therefore it corresponds to the tissue fluidization timescale. In summary, we
have developed a method to infer force dipoles induced by cell divisions, by
observing the strain field in the surrounding tissues. Using this method we
quantitatively characterize mechanical forces generated during a cell division,
and their effects on the tissue mechanics.
