Reproductive System
At the time of fertilization the sperm activates the egg and induces embryonic development by triggering an elevation in the egg’s intracellular free Ca2+ concentration. In mammals the initial Ca2+ rise is followed by a series of repetitive Ca2+ transient
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Store-operated Ca in Egg Activation
Entry in Germ Cells: Role
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Zoltan Machaty, Chunmin Wang, and Kiho Lee
29.1
Introduction
The female gamete is produced by meiosis which consists of two rounds of nuclear divisions (Whitaker 1996). During the final stage of their production immature oocytes undergo maturation and mature oocytes (known as eggs) are released from the ovary. At this point their development is stopped at a species-specific stage of the cell cycle; ovulated mammalian eggs are arrested at the metaphase stage of the second meiotic division. Under physiological conditions it is the fertilizing sperm that re-starts the cell cycle machinery; by fusing to the egg it stimulates a series of events in the egg cytoplasm that are collectively known as activation. A number of evidences indicate that the key trigger for egg activation is a Ca2+ signal: an elevation in the egg’s intracellular Ca2+ level is responsible for all additional downstream events associated with fertilization. Results from sea urchins, fishes and frogs indicate that in these animals the sperm generates a single Ca2+ transient that leads to egg activation. In others including mammals, the sperm-induced Ca2+ signal takes the form of a long-lasting oscillation in the intracellular free Ca2+ (reviewed by Stricker 1999). It was also observed that the external Ca2+ concentration has a profound effect on the sperm-induced Ca2+ signal (Igusa and Miyazaki 1983).
Z. Machaty (*) • C. Wang Department of Animal Sciences, Lilly Hall of Life Sciences, Purdue University, West Lafayette, IN, USA e-mail: [email protected] K. Lee Animal Science Research Center, University of Missouri – Columbia, Columbia, MO, USA K. Groschner et al. (eds.), Store-operated Ca2+ entry (SOCE) pathways, DOI 10.1007/978-3-7091-0962-5_29, # Springer-Verlag/Wien 2012
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Z. Machaty et al.
Relevance of the Sperm-induced Ca2+ Oscillation
Depending on the species, the events of egg activation include a transient elevation in the intracellular free Ca2+ concentration of the egg, the release of the content of the cortical granules that results in a block to polyspermy, resumption of meiosis, changes in intracellular pH, recruitment of maternal mRNAs, formation of the maternal and paternal pronuclei, initiation of DNA synthesis and cleavage (Schultz and Kopf 1995). The oscillating signal seems to be important to ensure complete activation of the egg. By applying repetitive electroporation to induce different numbers of Ca2+ transients in eggs it was demonstrated that each Ca2+ rise pushes progressively forward the activation process (Ducibella et al. 2002). Cortical granule exocytosis takes place in a stepwise manner with each consequential Ca2+ rise triggering the release of the content of additional granules. Furthermore, early events such as resumption of meiosis are stimulated by a single elevation in the intracellular Ca2+ level while others require more transients. The meiotic arrest in mammalian eggs is maintained by the M-phase promoting factor (MPF) compo
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