Several experiments have demonstrated that Ca2+ is directly responsible for propagating the cortical granule reaction, and that these ions are stored within the egg itself. The drug A23187 is a calcium ionophore—a compound that allows the diffusion of ions such as Ca2+ across lipid membranes, permitting them to travel across otherwise impermeable barriers. Placing unfertilized sea urchin eggs into seawater containing A23187 initiates the cortical granule reaction and the elevation of the fertilization envelope. Moreover, this reaction occurs in the absence of any Ca2+ in the surrounding water; thus, the A23187 must be stimulating the release of Ca2+ that is already sequestered in organelles within the egg (Chambers et al. 1974; Steinhardt and Epel 1974).

In sea urchins and vertebrates (but not snails and worms), the Ca2+ responsible for the cortical granule reaction is stored in the endoplasmic reticulum of the egg (Eisen and Reynolds 1985; Terasaki and Sardet 1991). In sea urchins and frogs, this reticulum is pronounced in the cortex and surrounds the cortical granules (FIGURE 1; Gardiner and Grey 1983; Luttmer and Longo 1985). The cortical granules are themselves tethered to the cell membrane by a series of integral membrane proteins that facilitate calcium-mediated exocytosis (Conner et al. 1997; Conner and Wessel 1998). Thus, as soon as Ca2+ is released from the endoplasmic reticulum, the cortical granules fuse with the cell membrane above them. Once initiated, the release of Ca2+ is self-propagating. Free calcium is able to release sequestered calcium from its storage sites, thus causing a wave of Ca2+ release and cortical granule exocytosis.

(A) S. Luttmer and F. J. Longo. 1985 Dev Growth Diff 27: 349–359, courtesy of S. Luttmer; (B) S.M. McPherson et al. 1992. J Cell Bioll 116: 1111–1121, courtesy of F. J. Longo.

 

FIGURE 1Endoplasmic reticulum surrounding cortical granules in sea urchin eggs. (A) The endoplasmic reticulum has been stained to allow visualization by transmission electron microscopy. The cortical granule is seen to be surrounded by dark-stained endoplasmic reticulum. (B) An entire egg stained with fluorescent antibodies to calcium-dependent calcium release channels. The antibodies show these channels in the cortical endoplasmic reticulum.