The aim of our visit was to examine the role of epidermal growth factor (EGF) activation of the EGF receptor (EGFR) in stimulating Ca ion fluxes in mouse oocytes and embryos. We know that the EGFR is expressed in both the oocyte and its attendant cumulus cells in a stage-dependent manner. Likewise, the EGFR is also expressed in embryos. EGF has been shown to be an important factor that promotes in vitro development of oocytes and embryos. Additionally, the EGFR is known in tissue culture cells to activate the release of Ca from intracellular stores via a phospholipase C-dependent pathway. Our specific focus is on putative coupling between this EGFR-mediated Ca release and potential stimulated efflux of Ca. Preliminary data showed that in all cumulus-oocyte complexes (COCs) tested (N=3), a strong EGF-stimulated Ca efflux was found. Surprisingly, we found that oscillations of Ca efflux with a period of 2-4 min were seen in COCs, suggesting that a dominant frequency generator regulates these oscillations in these gap junction-coupled cell complexes. Significantly, cumulus cell-free oocytes showed only an EGF-stimulated increase in basal Ca efflux with no concomitant oscillations. Future work will attempt to locate the site of the dominant oscillator, to combine the self-referencing Ca probe with fura-2 intracellular Ca ion measurement in the same cell mass, and to extend our observations to embryos in the 1, 2, 4 and 8-cell stages.
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