Partially Fertilized Sea Urchin Eggs: An Electrophysiological and Morphological Study.: (Sea urchin eggs/partial...

Propagation of the cortical reaction in sea urchin eggs may be interrupted by a mild heat shock. In such partially fertilized eggs three distinct cortical zones may be distinguished. First, an activated area where cortical granule exocytosis is complete, the fertilization membrane is elevated, and there is a cortical meshwork of polymerized actin. Second, at the antipode an area where the cortical granules are intact, actin is not polymerized, and the surface structure in general resembles that of the virgin egg. Between the two there is a transitional zone, some 10 to 20 μm wide, where a fraction of cortical granules have exocytosed, giving rise to isolated "blebs" of elevated fertilization membrane. Partially fertilized eggs have resting potentials ranging from -20 to -80 mV, and upon re-insemination give rise to step depolarizations indicating that spermatozoa may interact and possibly fuse with the "unactivated surface".