The Effect of High Hydrostatic Pressure on the Mechanical Properties of the Surface of the Sea-Urchin Egg

There is evidence that high hydrostatic pressure can solate and weaken gel-like structures within cells. It was thus of interest to determine its effect on the mechanical properties of the cell surface since it has been suggested that there is a gel-like cortex beneath the plasma membrane which makes a significant contribution to its mechanical properties. The mechanical property measured was the resistance to deformation when the eggs were compressed mechanically. The eggs of Arbacia were compressed by placing a small plate made either from a fragment of coverslip or Melinex plastic sheet on a small group of eggs (20-100) so that the eggs supported the plate. The deformation of the eggs was determined by measuring their increased diameter. The mechanical properties of the cell membrane of unfertilized eggs appear to be unaffected by high hydrostatic pressure. When compressed fertilized eggs, from which the fertilization membrane had been, removed by urea treatment, were subjected to pressure of about 7x104 kN m-2 (10000 lb/in.2) there was a significant increase in cell diameter. This increase was such that it appeared as if the resistance to deformation of the membrane were about quartered. Surprisingly after a few minutes the diameter decreased again indicating a contraction at the membrane even while under high hydrostatic pressure. Release of the hydrostatic pressure resulted in a further reduction in diameter and thus contraction at the surface, the final diameter sometimes being less than that before the hydrostatic pressure was applied.