The role of calcium oscillations during preimplantation embryo development
During fertilisation the sperm causes a series of Ca2+ i oscillations in the egg that lead to events such as cortical granule exocytosis, pronuclei formation, exit from meiosis and entry into mitosis. These events are collectively termed "egg activation" and are essential for initiating development in the egg. Injection of PLC£ into mouse eggs has previously been shown to cause Ca2+ i oscillations, egg activation and development to the blastocyst stage. In this Thesis I show that injection of PLC£ cRNA also causes Ca2+ i oscillations in pig eggs and in human eggs which had failed to fertilise after assisted reproduction technologies (ART). The human eggs were shown to undergo further development up to the blastocyst stage. The Ca2+ i oscillations during egg activation are known to lead the immediate events of egg activation but the role of these Ca2+ i oscillations on later preimplantation development in the mouse has not been established. The work presented in this Thesis addresses this issue by comparing differences in mouse embryo development after parthenogenetic activation using stimuli that induce a Ca2+ i increase (e.g. Sr2"1" and ethanol) with those that do not cause any Ca2+ i change (e.g. cycloheximide). The data shows that parthenogenetic activation in the absence of a Ca2+ increase leads to embryo loss around the 8-cell stage, and that any blastocysts that form in absence of a Ca2+ i increase have a reduced inner cell mass and an increased incidence of apoptosis. Ca2+ i oscillations during activation do not appear to have an effect on the timing or amount of overall global gene expression during embryonic genome activation (EGA). However, when I used microarray analysis I found that a significant number of genes were differentially expressed in 8-cell stage embryos that were activated with a Ca2+ i increases compared to those that activated in the absence of a Ca2+ i change. These results provide evidence that Ca2+ i increases at egg activation have an important role in regulating events during the later stages of preimplantation development.