Alterations of expression of developmentally important genes in preimplantation bovine embryos by in vitro culture conditions: implications for subsequent development.

Recent advances in molecular technology and in vitro production of bovine embryos have enabled studies of gene transcription in preimplantation embryos. On the basis of knowledge of the sequence of the selected gene, various modifications of Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) technology have been employed. Several lines of evidence in mouse and cattle indicate that expression patterns of genes from in vitro-produced embryos are not necessarily representative of those of in vivo embryos. An important gene that has been found to be expressed by in vivo-derived bovine blastocysts, but not in their in vitro-produced counterparts, is the Connexin43 gene that is crucial for maintenance of compaction. The bovine leukemia inhibitory factor (bLIF) and LIF-receptor-beta (LR-beta) genes were expressed by in vitro-produced embryos, but not in their in vivo counterparts. The heat shock protein gene 70.1 (Hsp 70.1) was upregulated by blastocysts produced in vitro compared to in vivo embryos, while the glucose transporter-1 mRNA (Glut-1) was downregulated by morulae produced in vitro as compared to in vivo-derived morulae. Furthermore, mRNA expression levels of a set of "marker genes" were shown to be affected by the presence or absence of serum in the culture medium. Most embryos grown under serum-free conditions had higher mRNA abundances than those cultured in serum-enriched medium. It is hypothesized that persistent alterations of the normal gene expression patterns may be responsible for the large offspring syndrome that is observed in approximately one third of the calves resulting from the transfer of in vitro-produced embryos. A primary candidate for such deviations may be an altered methylation pattern that can either lead to silencing or induction of a specific gene. Messenger RNA phenotyping of genes essential in early development provides a useful tool to assess the normality of the produced embryos and a tool to optimize in vitro culture conditions for bovine embryos.