Measurements of mechanical properties of the blastula wall reveal which hypothesized mechanisms of primary invagination are physically plausible in the sea urchin Strongylocentrotus purpuratus

Computer simulations showed that the elastic modulus of the cell layer rela tive to the elastic modulus of the extracellular layers predicted the effec tiveness of different force-generating mechanisms for sea urchin primary in vagination [L. A. Davidson, M. A. R. Koehl, R. Keller, and G. P. Oster (199 5) Development 121, 2005-2018]. Here, we measured the composite elastic mod ulus of the cellular and extracellular matrix layers in the blastula wall o f Strongylocentrotus purpuratus embryos at the mesenchyme blastula stage. C ombined, these two layers exhibit a viscoelastic response with an initial s tiffness ranging from 600 to 2300 Pa. To identify the cellular structures r esponsible for this stiffness we disrupted these structures and correlated the resulting lesions to changes in the elastic modulus. We treated embryos with cytochalasin D to disrupt the actin-based cytoskeleton, nocodazole to disrupt the microtubule-based cytoskeleton, and a gentle glycine extractio n to disrupt the apical extracellular matrix (ECM). Embryos treated less th an 60 min in cytochalasin D showed no change in their time-dependent elasti c modulus even though F-actin was severely disrupted. Similarly, nocodazole had no effect on the elastic modulus even as the microtubules were severel y disrupted. However, glycine extraction resulted in a 40 to 50% decrease i n the elastic modulus along with a dramatic reduction in the hyalin protein at the apical ECM, thus implicating the apical ECM as a major mechanical c omponent of the blastula wall. This finding bears on the mechanical plausib ility of several models for primary invagination. (C) 1999 Academic Press.