Bulk membrane fluidity increases after fertilization or partial activation of sea urchin eggs.

Membrane fluidity increases within 10 min after fertilization of Strongylocentrotus purpuratus and Lytechinus pictus eggs as detected by a decrease in order parameter of the spin label fatty acid 5-doxylstearate. The magnitude of this change is proportional to the fraction of fertilized eggs present. The order parameter decreases when unfertilized eggs are partially activated by ammonia treatment but does not change when the extracellular coats are removed. The increase in membrane fluidity, therefore, appears to be a "late" event in the fertilization program. The increase in fluidity is confined to the polar head group region of the membrane bilayer. Spectral and autoradiographic analyses indicate that the spin label resides in lipid bilayers throughout the cell. Thus, these measurements apply to bulk cell membranes. Detectable changes in lipid composition do not occur shortly after fertilization.