The changes in structural organization of actin in the sea urchin egg cortex in response to hydrostatic pressure.

Abstract

Hydrostatic pressure was used to study the structural organization of actin in the sea urchin egg cortex and the role of cortical actin in early development. Pressurization of A. punctulata eggs to 600 .psi. at the 1st cleavage division caused the regression of the cleavage furrow and the disappearance of actin filament bundles from the microvilli. Within 30 s to 1 min of decompression these bundles reformed and furrowing resumed. Pressurization of dividing eggs to 7500 .psi. caused the regression of the cleavage furrow and the complete loss of microvilli from the egg surface. Following release from this higher pressure, the eggs underwent extensive, uncoordinated surface contractions, but failed to cleave. The eggs gradually regained their spherical shape and cleaved directly into 4 cells at the 2nd cleavage division. Microvilli reformed on the egg surface over a period of time corresponding to that required for the recovery of normal egg shape and stability. During the initial stages of their regrowth the microvilli contained a network of actin filaments that began to trnasform into bundles when the microvilli had reached .apprx. 2/3 of their final length. Moderate levels of hydrostatic pressure cause the reversible disruption of cortical actin organization. This network of actin may stabilize the egg surface and partcipate in the formation of the contractile ring during cytokinesis. Actin filament bundles are not required for the regrowth of microvilli after their removal by pressurization. Preliminary experiments demonstrate that F-actin is not depolymerized in vitro by pressures up to 10,000 .psi. and suggest that pressure may act indirectly in vivo, either by changing the intracellular ionic environment or by altering the interaction of actin binding proteins with actin.