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The absence of a Ca²⁺ signal during mouse egg activation can affect parthenogenetic preimplantation development, gene expression patterns, and blastocyst quality

Department of Anatomy and Developmental Biology, University College London, Gower Street, London WC1E 6BT, UK, ¹ Department of Physiology, University College London, Gower Street, London WC1E 6BT, UK, ² Developmental Genomics and Aging Section, Laboratory of Genetics, National Institute on Aging, National Institute of Health, 333 Cassell Drive, Suite 3000, Baltimore, MD 21224, USA and ³ Department of Obstetrics and Gynaecology, School of Medicine, Heath Park, Cardiff University, Cardiff CF14 4XN, UK

Correspondence should be addressed to K Swann; Email: swannk1{at}cf.ac.uk

A series of Ca²⁺ oscillations during mammalian fertilization is necessary and sufficient to stimulate meiotic resumption and pronuclear formation. It is not known how effectively development continues in the absence of the initial Ca²⁺ signal. We have triggered parthenogenetic egg activation with cycloheximide that causes no Ca²⁺ increase, with ethanol that causes a single large Ca²⁺ increase, or with Sr²⁺ that causes Ca²⁺ oscillations. Eggs were co-treated with cytochalasin D to make them diploid and they formed pronuclei and two-cell embryos at high rates with each activation treatment. However, far fewer of the embryos that were activated by cycloheximide reached the blastocyst stagecompared tothose activated by Sr²⁺ orethanol. Any cycloheximide-activated embryos that reached the blastocyst stage had a smaller inner cell mass number and a greater rate of apoptosis than Sr²⁺-activated embryos. The poor development of cycloheximide-activated embryos was due to the lack of Ca²⁺ increase because they developed to blastocyst stages at high rates when co-treated with Sr²⁺ or ethanol. Embryos activated by either Sr²⁺ or cycloheximide showed similar signs of initial embryonic genome activation (EGA) when measured using a reporter gene. However, microarray analysis of gene expression at the eight-cell stage showed that activation by Sr²⁺ leads to a distinct pattern of gene expression from that seen with embryos activated by cycloheximide. These data suggest that activation of mouse eggs in the absence of a Ca²⁺ signal does not affect initial parthenogenetic events, but can influence later gene expression and development.

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