Cell-generated mechanical forces play a critical role during
tissue morphogenesis and organ formation in the embryo.
Little is known about how these forces shape embryonic
organs, mainly because it has not been possible to measure
cellular forces within developing three-dimensional (3D)
tissues in vivo. We present a method to quantify cell-generated
mechanical stresses exerted locally within living embryonic
tissues, using fluorescent, cell-sized oil microdroplets with
defined mechanical properties and coated with adhesion
receptor ligands. After a droplet is introduced between cells
in a tissue, local stresses are determined from droplet shape
deformations, measured using fluorescence microscopy and
computerized image analysis. Using this method, we quantified
the anisotropic stresses generated by mammary epithelial cells
cultured within 3D aggregates, and we confirmed that these
stresses (3.4 nN mm−2) are dependent on myosin II activity
and are more than twofold larger than stresses generated by
cells of embryonic tooth mesenchyme, either within cultured
aggregates or in developing whole mouse mandibles.