Calcium-induced decrease in membrane fluidity of sea urchin egg cortex after fertilization.

Fertilization of the sea urchin egg is a dramatic example of cell activation resulting from the interaction of an external stimulus, the spermatozoon, with the cell surface. Growing and quiescent cells may have different membrane states. Here we report membrne fluidity measurements on a surface membrane fraction, the cortex, isolated from unfertilized and fertilized eggs. The fluidity of the fertilized egg cortex, measured by electron spin resonance (ESR) spectroscopy using 5-doxylstearate as a probe, is less than that of the unfertilized cortex. In the intact egg the intracellular CA2+ to the cortex fraction isolated from unfertilized eggs triggers a fluidity decrease in vitro. The fluidity decrease seems to represent a Ca2+-induced change in membrane structure rather than a direct interaction of Ca2+ with phospholipid headgroups.