Oocyte Activation Deficiency and Advances to Overcome

Abstract

This chapter defines oocyte activation and the main molecular events related to this crucial phenomenon which takes place upon sperm-oocyte fusion. Oocytes are arrested at metaphase-II until fertilization and are alleviated from arrest by their interaction with a sperm-borne oocyte activation factor (SOAF). This factor triggers a series of calcium (Ca2+) oscillations which release the oocyte from metaphase-II and initiates the first sequence of embryogenesis. While different theories and protein candidates have been proposed to explain the mechanism by which the SOAF acts, mounting evidence indicates that a sperm-soluble factor, phospholipase C zeta (PLCζ), is the protein which evokes Ca2+ oscillations in the ooplasm. Here, we review the cascade of downstream events triggered by PLCζ and refer to the vital role of Ca2+ homeostasis during oocyte activation. Failure of the oocyte to activate is known to underlie certain types of human infertility, but the diagnosis of this condition is complex, and treatment options are limited at present to the use of artificial oocyte activators, which operate via electrical, mechanical and chemical means. As chemical oocyte activators are the most commonly used at present, we briefly review the available clinical data for these agents, both in terms of their efficiency and safety, and discuss how alternative, more endogenous, methods could alleviate growing global concerns surrounding the use of artificial oocyte activating agents.