In mammals, this is caused by the introduction of phospholipase C isoform zeta (PLCζ) from the sperm cytoplasm.
Izumo is the sperm cell signal, that will trigger the egg receptor Juno.
The inositol signaling system has been implicated as the pathway involved with egg activation.
IP3 and DAG are produced from the cleavage of PIP2 by phospholipase C. However, another hypothesis is that a soluble 'sperm factor' diffuses from the sperm into the egg cytosol upon sperm-oocyte fusion.
A 2002 study demonstrated that mammalian sperm contain PLC zeta which can start the signaling cascade.
The slow block is through a biochemical mechanism triggered by a wave of calcium increase.
Once PLCζ is introduced into the oocyte by the sperm cell, it cleaves phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2) into diacyl glycerol (DAG) and inositol 1,4,5-trisphosphate (IP3).
[5] Male and female pronuclei move to the centre of the egg and membranes break down.
Oxidative stress theory is a hypothesis that it is evolutionarily favourable for mitochondria from the father to be destroyed, as it there is a greater possibility that the mitochondrial DNA has become mutated or damaged.
[5] Oocyte activation may be artificially facilitated by calcium ionophores, something that is speculated to be useful in case of fertilization failure, such as still occurs in 1–5% of intracytoplasmic sperm injection (ICSI) cycles.