Oolemma receptors and oocyte activation.

At fertilization the sperm triggers a series of intracellular calcium oscillations that are pivotal to oocyte activation and development. Although the biological significance of the characteristic intracellular calcium (Ca(2+)(i)) oscillations is not fully understood, calcium ions are known to be involved in cortical granule release and in controlling cell cycle progression. Two different hypotheses attempt to explain how sperm initiate (Ca(2+)(i)) oscillations in mammalian oocytes. One hypothesis is that spermatozoa interact with a receptor located in the plasma membrane of the oocyte, which results in induction of pathways leading to activation. This receptor is coupled to a GTP-binding protein or to have tyrosine kinase activity and have the ability to induce activation of phospholipase C (PLC). In turn, PLC stimulates the hydrolysis of phosphatidyl inositol (4,5)-bisphosphate (PIP2) to produce diacylglycerol (DAG) and 1,4,5 inositol trisphosphate (IP3), a common Ca(2+) releasing compound. Most studies used to develop the mammalian model of oocyte activation have been performed in the mouse. There is a paucity of information from other mammalian models. The predominant mouse model of oocyte activation is that there is a soluble factor (PLC-zeta) delivered to the cytosol after fertilization that induces oocyte activation. However, as data in other mammals is collected, substantial evidence is beginning to support the existence of other more complex oocyte activation pathways in both murine and non-murine systems. Indeed, activation may involve redundant processes, each of which acting alone may be able to induce aspects of oocyte activation. Recent findings demonstrate the involvement of receptors that are known to associate in large, multimeric complexes. This fact leads one to speculate that the process of oocyte activation by the sperm cell is a highly complex and elaborate process that likely involves many more players than perhaps was initially expected.