Meeting Abstract
The response of hatched sand dollar blastulae to salinity variation presents a simple case study of how biomechanics can influence the sensitivity of morphogenesis to environmental variation. The formation of a hollow blastocoel cavity, surrounded by a cell layer, is an important developmental event in diverse taxa. Salinity variation occurs in many habitats, and drives changes in cell size. Mechanical coupling between cellular and non-cellular parts of the embryo will control how such cell size changes affect embryo shape and proportions. I compared effects of salinity on the blastocoel to predictions for hypothesized mechanisms of blastocoel expansion and size control. The blastocoel-to-cell volume ratio decreased with salinity-driven cell swelling. This indicates that non-cellular components of the embryo are mechanically important. However, it is inconsistent with models in which the matrix layer surrounding the embryo (the hyaline layer) resists expansion with a yield stress (i.e. plastic deformation models). Hatched blastulae resisted isotropic compression by high molecular weight osmolytes, indicating that the blastocoel can exert pressure to support the cellular and hyaline layers. All observations were consistent with models in which blastocoel forces are balanced by elastic hyaline stiffness and/or cell stiffness. These models predict that salinity-driven cell swelling will alter blastocoel pressure. Because increased blastocoel pressure can inhibit invagination of the endoderm during gastrulation, I hypothesize that the physics of blastocoel expansion modulates effects of salinity on the sensitivity to gastrulation defects. Because successful gastrulation is necessary for survival of subsequent developmental stages, this might influence selection on the mechanism of blastula expansion.
