Measuring and Modeling Morphogenetic Stress in Developing Embryos

The biological tissues of a developing organism are built and reshaped by intra-embryonic forces. Such morphogenetic forces can be assessed and measured similarly to residual stresses. We will discuss in vivo measurements of morphogenetic stress using two quantitative laser-microsurgery techniques. The first uses a laser to drill a sub-cellular hole in a sheet of epithelial cells. The subsequent dynamic retraction of surrounding cells allows one to infer the local mechanical stress. The second uses a laser to isolate a single cell from the rest of a cell sheet. Isolation is accomplished on a microsecond time scale by holographically shaping a single laser pulse. The subsequent retraction (or expansion) of the isolated cell allows one to separate and quantify the effects of cell-internal and cell-external stresses in the determination of cell shape. Both types of experiment are strongly supported by cell-level finite element models. We will discuss application of these techniques and models to the time-dependent biomechanics of epithelial tissues during early fruit fly embryogenesis – specifically during the processes of germband retraction and dorsal closure.