The Two-Cell Embryo—Two Sides of the Same Coin?

Abstract

Every month, numerous papers appear in the popular scientific journals, including Biology of Reproduction, describing the expression of candidate genes, or the global gene expression pattern, in mammalian embryos. Many of these studies are very interesting and cleverly designed, and all aim to give us better insight into the regulatory mechanisms underlying embryo development. Most studies, however, are by their nature invasive, and the findings therefore can be difficult to extrapolate beyond the developmental stages examined. Furthermore, partly because of the challenges of extracting sufficient mRNA for transcript analysis, such studies are normally based on pools of embryos; this can potentially mask differences between treatment groups. In this month’s Biology of Reproduction, Held and colleagues [1] describe a study that examined the relationship between the transcriptome of a single blastomere from a twocell bovine embryo and the developmental competence of the sister blastomere with the aim of identifying potentially important candidate genes as markers of developmental capacity. A few years ago, the same group published a novel model in which a biopsy sample was taken from an in vitroproduced blastocyst, following which the blastocyst was transferred to a recipient [2]. Based on pregnancy outcome, the biopsy samples were pooled for gene expression analysis, thus identifying genes associated with development to term and early pregnancy loss. This model was later replicated by the same group using in vivo-derived bovine embryos [3] and by other investigators using human embryos [4]. These studies [1] clearly indicate that developmental competence of bovine blastocysts is associated with distinct gene expression signatures. Interestingly, other recent studies have shown that following blastocyst transfer at Day 7, different types of embryos (in vivo-derived, in vitro fertilization (IVF)-derived, cloned) elicit different responses from the endometrium around the time of initiation of implantation, and these responses are associated with different developmental outcomes [5, 6]. In other words, to some extent, the embryo is master of its own destiny. The studies described in the current paper [1] address a similar question but go further back along the developmental trajectory. The authors search for an answer by modifying the biopsy technique referred to above; separating sister blastomeres of bovine two-cell embryos allows for the correlation of the transcriptome in one blastomere with the developmental capacity of the other. Developmental fates of sister blastomeres were highly correlated (if one blastomere developed to the blastocyst stage, the same applied to the other in 94% of cases), suggesting that this is a reasonable strategy. A large number of differentially expressed genes were observed between blastomeres whose sister blastomeres reached the blastocyst stage and those which did not cleave after separation. Likewise, the transcription differed between those that formed blastocysts and those that stopped developing before embryonic genome activation (8to 16-cell stage). Downstream analysis revealed clustering of genes associated with cell response to oxidative stress. The expression of a selected group of candidate genes was then confirmed by comparing expression in early cleaving and late cleaving two-cell embryos and in embryos cultured under high and low oxygen conditions. Why are such studies important? In cattle, most embryonic loss occurs very early during embryogenesis, typically around Day 16 post conception, before maternal recognition of pregnancy [7]. Fertilization rates in vivo are normally high (;90%). Recent studies in high-producing dairy cows suggest that a significant proportion of loss may occur by as early as Day 7 (the blastocyst stage); approximately 50% of embryos flushed from single-ovulating cows on Days 6–7 were viable [8]. In vitro, despite .80% of oocytes typically cleaving to the two-cell stage after IVF, only approximately 30% develop to the blastocyst stage. This has been convincingly demonstrated to be primarily the result of compromised oocyte quality rather than suboptimal culture conditions, as oocytes matured in vivo develop to the blastocyst stage in vitro at high rates after IVF [9]. Thus, understanding what regulates developmental competence may contribute to our understanding of embryonic mortality and survival. This has potential implications for improving conception rates in dairy cows, for genomic selection of embryos in breeding programs, and having ramifications for human assisted reproduction, where embryos are typically transferred at early cleavage stages.