Oogenetic and zygotic gene expression directing early bovine embryogenesis: A review

Abstract

The oogenetic contribution to early bovine embryogenesis begins at least as early as the secondary follicle phase with the synthesis of rRNA and hnRNA. In contrast to rodents, bovine oocytes are still in the growth phase when follicles reach the antral stage and although intensive rRNA synthesis eventually ceases the synthesis of hnRNA is not completely inactivated. During meiotic maturation, a requirement for protein synthesis continues in the cow, as well as in the pig, and this denotes a major difference between the domestic animal and the amphibian and mouse. Maternal factors present in the egg as mRNA and protein also play an obligatory role in supporting development of the zygote after ZGA. Bovine oocytes characterized morphologically to have the greatest developmental competence following in vitro maturation, fertilization, and culture differ quantitatively and qualitatively in their mRNA content from either incompetent oocytes or blastocysts. Identification of qualitative differences in mRNA transcript content will enhance our understanding of the molecular genetic factors essential for maternally directed control of early mammalian development. This knowledge will enhance the efficacy of in vitro maturation protocols utilized in either an agricultural or clinical setting, and attempts to clone domestic animals by nuclear transfer. Lastly, despite the existence of species specific differences in the precise timing of the onset of zygotic transcription in mammals, commonalties exist including evidence for a minor degree of transcriptional activity preceding the major activation of the genome. Patterns of gene expression may also be conserved, at least between the cow and the mouse, including a transient expression pattern which may represent a hallmark of chromatin remodeling during ZGA. Characterization of zygotic mRNA expression patterns and their relation to development in vivo and in vitro will be essential for defining optimized culture conditions to sustain early embryonic development.