Abstract
During the period of oocyte growth, chromatin undergoes global rearrangements at both morphological and molecular levels. An intriguing feature of oogenesis in some mammalian species is the formation of a heterochromatin ring-shaped structure, called the karyosphere or surrounded “nucleolus”, which is associated with the periphery of the nucleolus-like bodies (NLBs). Morphologically similar heterochromatin structures also form around the nucleolus-precursor bodies (NPBs) in zygotes and persist for several first cleavage divisions in blastomeres. Despite recent progress in our understanding the regulation of gene silencing/expression during early mammalian development, as well as the molecular mechanisms that underlie chromatin condensation and heterochromatin structure, the biological significance of the karyosphere and its counterparts in early embryos is still elusive. We pay attention to both the changes of heterochromatin morphology and to the molecular mechanisms that can affect the configuration and functional activity of chromatin. We briefly discuss how DNA methylation, post-translational histone modifications, alternative histone variants, and some chromatin-associated non-histone proteins may be involved in the formation of peculiar heterochromatin structures intimately associated with NLBs and NPBs, the unique nuclear bodies of oocytes and early embryos.
Highlights
The development of germ cells, the formation of a zygote and the subsequent cleavage of the embryo are amazing phenomena in nature
We tried to group data on the dynamics of chromatin configuration and its regulatory mechanisms according to the main stages of development, but not according to molecular processes as is typical for reviews on this topic. We found this approach more appropriate to describe the molecular mechanisms that can determine the morphodynamics of heterochromatin, the distribution pattern of which is specific for a certain stage of pro-embryonic and early embryonic development
We briefly analyzed key epigenetic factors that could determine the morphodynamics of most prominent DAPI-positive structures—the karyosphere and the karyosphere-like rings—in mammalian oocytes, zygotes, and early embryos
Summary
The development of germ cells, the formation of a zygote and the subsequent cleavage of the embryo are amazing phenomena in nature. Similar data on chromatin rearrangements in mammalian zygotes and early embryos are less numerous and poorly systematized. It is clear that the patterns of chromatin distribution in the pronuclei of zygotes and in the nucleus of embryos at the initial stages of cleavage are significantly different from those in the nucleus of differentiated somatic cells. Rapid development of modern methods especially using improved techniques of sequencing and chromatin mapping [5,6,7,8,9,10] has expanded our knowledge about the mechanisms of chromatin rearrangements during mammalian development and shed light on the features of gene expression and the specific dynamics of the epigenetic landscape of oocytes and early embryos
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