Expression of proto-oncogenes in mouse eggs and preimplantation embryos

The expression of several protooncogenes has been investigated in mouse eggs and preimplantation embryos using reverse transcription coupled to amplification of cDNAs by the polymerase chain reaction (RT-PCR). The genes chosen for analysis included both cytoplasmic (c-raf-1, rasH, rasK, and rasN) and nuclear (c-fos and c-myc) proto-oncogenes encoding proteins involved in the transduction of signals from protein-tyrosine kinase growth factor receptors. Transcripts of the cytoplasmic proto-oncogenes were detected both as maternal and embryonic mRNAs at levels (ca. 1,000 copies per egg or embryo) approximately comparable to their levels of transcription in somatic cells. Transcripts of c-fos and c-myc were also detected in both eggs and embryos, although at more variable levels: Maternal transcripts were present at very low levels (ca. 1-10 copies per egg) in growing oocytes and ovulated eggs; embryonic transcription of c-myc increased, reaching mRNA levels of approximately 100-1,000 copies per embryo in four-cell embryos, morula, and blastocysts; in contrast the transcription of c-fos remained at low, barely detectable levels throughout preimplantation development. Although the significance of the low levels of c-fos mRNA is unclear, these results indicate that preimplantation embryos possess the basic intracellular signaling apparatus required to respond to polypeptide growth factors.