Abstract
Preimplantation embryo development involves four stages: fertilization, cell cleavage, morula and blastocyst formation. During these stages, maternal and zygotic epigenetic factors play crucial roles. The gene expression profile is changed dramatically, chromatin is modified and core histone elements undergo significant changes. Each preimplantation embryo stage has its own characteristic epigenetic profile, consistent with the acquisition of the capacity to support development. Moreover, histone modifications such as methylation and acetylation as well as other epigenetic events can act as regulatory switches of gene transcription. Because the epigenetic profile is largely related to differentiation, epigenetic dysfunction can give rise to developmental abnormalities. Thus, epigenetic profiling of the embryo is of pivotal importance clinically. Given the importance of these aspects, this review will mainly focus on the epigenetic profile during preimplantation embryo development, as well as interactions between epigenetic and genetic regulation in these early developmental stages.
Highlights
Starting from fertilization and ending with implantation, preimplantation embryo development can be divided into several well-orchestrated stages: fertilization, cell cleavage, morula and blastocyst formation
Microarray analysis has broadened our knowledge of gene expression profiles in the preimplantation embryo
Certain gene expression profiles help to unravel the mystery of these developmental stages, which further promote research on the epigenome
Summary
Starting from fertilization and ending with implantation, preimplantation embryo development can be divided into several well-orchestrated stages: fertilization, cell cleavage, morula and blastocyst formation. Similar to DNA methylation, histone modification changes dynamically during preimplantation development in stage- and cell type-specific manners, which are required for the precise regulation of gene expression. Xchromosome inactivation (XCI) and the Kcnq imprinted domain X chromosome activity changes dynamically during preimplantation development and its inactivation is a combination of epigenetic events including DNA methylation, histone modifications and RNA-mediated silencing (Fig. 2). Imprinting is initiated by a ncRNA Kcnq1ot from the 2cell stage during preimplantation development and is maintained by altering the status of histone modifications including H3Ac, H3K4me, H3K9me and H3K27me, resulting in allele-specific (paternal) and region-specific (placenta/embryo) gene silencing [77]. Single epigenetic events (DNA methylation, histone modification, RNAi, etc.) are able to play important roles as switches in the regulation of gene expression, but they always interact to accomplish their responsibilities.
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