Egg Activation

Abstract

Egg activation is the process of converting the female gamete, known as an oocyte or egg, into a cell that can support embryogenesis. In many organisms, this process is triggered by the fertilising sperm, but some organisms trigger egg activation by other mechanisms. Here we review the physiological state of the egg before activation, the mechanisms used to initiate egg activation, and the downstream cellular changes that must occur for a successful transition into a developing embryo. We will consider egg activation as it occurs in both invertebrate and vertebrate model systems, highlighting key similarities and differences between them. Key Concepts Animal eggs must undergo a process of ‘activation’ to allow them to initiate embryo development. Egg activation transitions a mature – but arrested – oocyte to a totipotent cell that can initiate mitosis and embryogenesis. Activation is triggered by a rise in calcium that sweeps across the egg in wave(s). The calcium wave is triggered by fertilisation in vertebrates, echinoderms, and molluscs and by sperm‐independent mechanical triggers in insects. The calcium wave is propagated through release of calcium from internal stores. The calcium rise triggers events including completion of meiosis, reorganisation of the sperm nucleus into a male pronucleus, translation of some stored maternal mRNAs and destruction of others in the egg, modulation of the phosphoproteome of the egg, and modification of the egg's membranes and external coverings to block fertilisation by additional sperm.