The extent to which the early mouse embryo is able to synthesize the components required for the formation of RNA from naturally occurring exogenous energy sources which support development in vitro remains to be determined. Recent reports have described the synthesis of RNA and DNA in pre-implantation mouse embryos when cultured or incubated in vitro with labelled nucleosides (Ellem & Gwatkin, 1968; Pikó, 1970), but an exogenous supply of these nucleic acid precursors is not necessary for early development (Thomson TenBroeck, 1968). The present study was, therefore, undertaken to discover whether pyruvate and glucose, as exogenous energy sources, act as precursors for the synthesis of soluble-RNA (s-RNA) during this time. Moreover, since mouse embryos require the presence of a bicarbonate buffer system for their development in vitro, and since the embryos gain a substantial amount of carbon through the fixation of CO2 (Wales, Quinn & Murdoch, 1969; Graves & Biggers, 1970) incorporation of 14C from bicarbonate into the s-RNA of the embryos has also been examined.
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