Activation of Mammalian Oocytes

Abstract

Mammalian oocytes are ovulated arrested at the metaphasic stage of the second meiotic division (MII), from which they initiate the embryonic development if fertilized in a timely manner. Prior to the MII arrest, the oocytes undergo maturation, during which they progress from the G2/M stage of the first meiotic division to MII. During this process, the oocytes undergo significant reorganization and redistribution of organelles and acquire a full complement of signaling molecules. These changes render the mammalian oocytes competent to exit meiosis and initiate embryonic development. Exit from the MII arrest is accomplished by fertilization, and is commonly referred to as “oocyte activation.” Oocyte activation comprises a sequence of cellular changes, all of which have to be completed to assure the development to the term. Oocyte activation is triggered by Ca 2+ release, and in mammalian oocytes, multiple [Ca 2+ ] i oscillations are required to achieve full activation. This chapter focuses on the Ca 2+ requirements of mammalian oocytes necessary to initiate activation and development, and compares the mechanisms of action of the various parthenogenetic procedures. The probable pathway(s) by which the sperm may initiate Ca 2+ release and the mechanism(s) that may control the persistence and termination of oscillations are also reviewed. The cellular and molecular events required for a pronuclear assembly as well as the likely differences in the assembly and composition of nuclear envelope membranes formed following the transfer of a somatic nucleus and their impact on the embryo development are discussed. Finally, recent evidence suggesting a role for [Ca 2+ ] i oscillations as an apoptotic-inducing agent, rather than an activating agent, is examined.