The interplay of physical and molecular determinants in limb and cardiac cushion morphogenesis

In an effort to investigate the interplay between molecular determinants and biomechanical properties during early embryonic development we have investigated two separate but similar morphogenetic processes: limb morphogenesis and cardiac cushion morphogenesis. Accurate interpretation and modeling of the physical mechanisms with which embryonic morphogenetic processes occur requires quantitative measurements of the biomechanical properties of the living embryonic tissues involved. It has previously been suggested that embryonic tissues may be described as liquids and that the biomechanical properties of individual tissue masses/populations contribute to the forces necessary for shape changes during early organogenesis. In order to quantitatively investigate the interplay between physical and molecular determinants during early vertebrate embryonic development we proposed that intact living fragments of embryonic mesenchymal tissue behave as liquids and that the biomechanical properties of living spherical explants of mesenchyme may be quantitatively measured using novel techniques and standard liquid theory. Formation of the limbs in avian embryos occurs by changes in the properties of the somatopleural mesoderm in the limb fields whereby the limb buds are caused to bulge outwardly from the body. Signaling by FGF8 produced by the newly formed apical ectodermal ridge (AER) initiates this morphogenetic process. Employing formal similarities between embryonic tissues and immiscible liquids we have measured the apparent surface tensions of 4 day embryonic chicken wing and leg bud mesenchymal tissue, and of adjacent flank mesoderm. We found that the two types of limb tissues were