Stored Elastic Bending Tension as a Mediator of Embryonic Body Folding

During development, amniote vertebrate embryos transform from a flat, multi-layered sheet into a three-dimensional cylindrical form, through ventral folding of the lateral sides of the sheet (the lateral plate, LP) and their fusion in the ventral midline. Although this basic aspect of vertebrate body plan formation has long been described, it is not understood at the mechanistic level. Each side of the LP is comprised of two tissue layers: a dorsal somatopleure (Sop) of pseudostratified coelomic epithelium, extracellular matrix (ECM) and ectoderm, and a ventral splanchnopleure (Spl) of similar construction except with endoderm instead of ectoderm. Using a chick embryo slice system we find that the flat stage is actually a poised balance of opposing elastic bending tensions, with dorsal bending tension in the Sop opposing ventral bending tension in the Spl. An intact extracellular matrix is required for generating the bending tensions, as localized enzymatic digestion of Sop or Spl ECM dissipates tension, while removal of the endodermal or ectodermal layers has no effect. As development proceeds, the Sop undergoes Epithelial-Mesenchymal Transition, ECM fragmentation and dissipation of dorsal bending tension, while the Spl ECM and ventral bending tension remain intact, thus changing the balance of bending forces in the LP to promote ventral folding. Consistent with these findings, interference with the elastic ECM component fibrillin in the Spl in vivo reduces stored bending tension and perturbs ventral body folding. A generalizable conceptual model is presented in which embryonic growth, in the context of specific embryonic geometrical constraints, leads to accumulation of bending tension in the LP ECM, which is harnessed to drive body folding.