HOW DO SEA-URCHINS INVAGINATE - USING BIOMECHANICS TO DISTINGUISH BETWEEN MECHANISMS OF PRIMARY INVAGINATION

The forces that drive sea urchin primary invagination remain mysteriou s, To solve this mystery we have developed a set of finite element sim ulations that test five hypothesized mechanisms, Our models show that each of these mechanisms can generate an invagination; however, the me chanical properties of an epithelial sheet required for proper invagin ation are different for each mechanism, For example, we find that the gel swelling hypothesis of Lane et al. (Lane, M. C., Koehl, M. A. R., Wilt, F. and Keiler, R. (1993) Development 117, 1049-1060) requires th e embryo to possess a mechanically stiff apical extracellular matrix a nd highly deformable cells, whereas a hypothesis based on apical const riction of the epithelial cells requires more compliant extracellular matrix. For each mechanism, we have mapped out a range of embryo desig ns that work. Additionally, the simulations predict specific cell shap e changes accompanying each mechanism. This allows us to design experi ments that can distinguish between different mechanisms, all of which can, in principle, drive primary invagination.