Application and measurement of tissue-scale tension in avian epithelia <i>in vivo</i> to study multiscale mechanics and inter-germ layer coupling

As cross-disciplinary approaches drawing from mechanics have increasingly influenced our understanding of morphogenesis, tools to measure and perturb physical aspects of embryogenesis have expanded as well. However, it remains a challenge to measure mechanical properties and apply exogenous tissue-scale forces in vivo, particularly for epithelia. Exploiting size and accessibility of the chick embryo, we describe a technique to apply and measure exogenous forces in the order of ∼1-100 µN to the endoderm. To demonstrate the utility of this approach, we performed several proof-of-concept experiments, revealing fundamental, yet unexpected, mechanical behaviors in the early embryo. This included heterogeneous single-cell mechanotypes within the endoderm, a complex non-cell autonomous mechanical role for actin, and tight mechanical coupling across germ layers. To illustrate the broader utility of this method, we expanded this approach to the ectoderm as well, where the mechanical behavior of neural plate cells was distinct from that of the endoderm. These findings provide basic insights into the mechanics of embryonic epithelia in vivo in the early avian embryo, and provide a useful tool for future investigations of how morphogenesis is influenced by mechanical factors.