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.

Keywords: oocyte; embryo; fertilisation; egg activation; calcium; meiosis; cortical granule; phospholipase C; IP3

Figure 1. Sperm‐induced calcium oscillations in the mouse egg. Changes in intracellular calcium were monitored with the fluorescent calcium indicator fura 2.
Figure 2. The calcium wave during Drosophila egg activation. The figure shows stills from a movie of a mature Drosophila oocyte as it is undergoing egg activation in vitro. The oocyte contains the calcium indicator GCaMP3, which was deposited into it by a female who expressed it in her germline. Initially the oocyte swells. Then, calcium levels (false‐coloured here; scale at right) increase at the oocyte poles (particularly the posterior pole, which is the one that shows the rise in vivo). A wave of increased calcium then sweeps across the egg, propagated although the action of IP3‐mediated calcium release from stores. The centre of the egg has lower calcium levels because it is the site of the yolk. Scale bar, 200 µm. Reproduced from Kaneuchi et al. 2015 © PNAS.
Figure 3. Schematic diagram showing conserved major regulators of meiosis II arrest and resumption in vertebrates. Some details differ among vertebrates. For example, *Mos is required for both the establishment and maintenance of the metaphase II arrest in Xenopus, but only for the maintenance of the arrest in mouse. **p90RSK is dispensable for the metaphase II arrest in mouse. Also, in mouse, WEE1B, CDC25A and CDC25B (not shown) regulate the stability of CDK1; this has not been tested in Xenopus.
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Further Reading

Kashir J, Deguchi R, Jones C, Coward K and Stricker SA (2013) Comparative biology of sperm factors and fertilization‐induced calcium signals across the animal kingdom. Molecular Reproduction and Development 80: 787–815.

Krauchunas AR and Wolfner MF (2013) Molecular changes during egg activation. Current Topics in Developmental Biology 102: 267–292.

Miao YL and Williams CJ (2012) Calcium signaling in mammalian egg activation and embryo development: the influence of subcellular localization. Molecular Reproduction and Development 79: 742–756.

Swann K and Lai FA (2016) Egg activation at fertilization by a soluble sperm protein. Physiological Reviews 96: 127–149.

Walser CB and Lipshitz HD (2011) Transcript clearance during the maternal‐to‐zygotic transition. Current Opinion in Genetics & Development 21: 431–443.

Von Stetina JR and Orr‐Weaver TL (2011) Developmental control of oocyte maturation and egg activation in metazoan models. Cold Spring Harbor Perspectives in Biology 3: a005553.

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Zhang, Zijing, Wolfner, Mariana F, and Williams, Carmen J(Jan 2018) Egg Activation. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003300.pub2]