However, there is no direct evidence for a sperm surface receptor on the plasma membrane of mammalian eggs capable of signaling Ca2+ release. The second hypothesis is that after gamete fusion the sperm introduces a cytosolic factor into the egg that triggers [Ca2+]i oscillations by interacting with a presently unknown target(s). Important supporting evidence for this hypothesis comes from the fact that injection of sperm cytosolic factors into eggs can induce [Ca2+]i oscillations similar to those observed during fertilization and can initiate par-thenogenetic development up to the blastocyst stage. Similarly, direct injection of a whole sperm into the cytosol of mammalian eggs, which avoids any contact between the sperm and egg plasma membranes, initiates Ca2+ responses that resemble those observed during fertilization.
Although a sperm-derived PLC and several other proteins located in potentially different sperm compartments have been proposed to be the active component(s) of sperm factor (SF), the exact nature and location of SF are still unknown.
The signal transduction pathway by which injection of SF induces [Ca2+]i oscillations in mammalian eggs has been extensively investigated. Recently, results from several laboratories indicate that, similar to fertilization, [Ca2+]i oscillations induced by SF can be blocked either by injection of heparin, a nonspecific competitive inhibitor of the IP3 receptor (IP3R), or by injection of a specific antibody against the IP3R.