Biology Reference
In-Depth Information
change in the conformation of pericentromeric chromatin. This is even more
important if we take into account the reports, indicating that the establishment
of NLBs-like structure after somatic nuclear transfer might increase the proba-
bility of success in reprogramming the somatic nucleus (
Maalouf et al.,
2009; Martin et al., 2006
). Functionally, whether this particular spatial nuclear
positioning of the heterochromatic and/or its evolution toward a chromo-
center configuration is important for development has not been addressed.
From all the above, it is clear that global heterochromatin dynamics seem
to be particularly relevant during the earliest stages of development. They
most likely impact not only on inheritance but also on the establishment
of the embryonic epigenome and the embryo's subsequent differentiation
capacities. The dramatically different configuration of embryonic hetero-
chromatin compared to somatic cells is probably linked functionally to
the plasticity of the early embryo and to the reprogramming process that
occurs during this period. Our understanding of the molecular mechanisms
that govern
de novo
establishment of heterochromatin domains after fertili-
zation is, however, still very poor. It is therefore essential to uncover the
mechanisms driving heterochromatin formation in mammals in order to
fully understand the regulation of epigenetic reprogramming and establish-
ment of pluripotency and plasticity.
Because the paternal genome, due to its packing into protamines, has to
acquire a nucleosomal configuration and all the subsequent chromatin sig-
natures for the first time after fertilization, the early mouse embryo consti-
tutes a unique system to address the mechanisms of heterochromatin
formation in mammals. However, it is not known how chromatin domains
and their epigenetic signatures are established
de novo
in the zygote nor it is
the extent to which these domains contribute to the regulation of cell
potency. Most of our knowledge on heterochromatin formation and in par-
ticular on its establishment comes from work in
Schizosaccharomyces pombe
and
Arabidopsis
(
Grewal & Elgin, 2007; Martienssen, Kloc, Slotkin, &
Tanurdzic, 2008
), but little is known on how heterochromatin is initially
established in mammalian cells. This is mainly because in most cells hetero-
chromatin only needs to be maintained as opposed to established
de novo
.
In the next section, we will first discuss extensively the main known
mechanisms of establishment and inheritance of heterochromatin in other
model systems, in particular, in fission yeast. We will then devote a section
to draw parallels and open questions in the mammalian embryo and will dis-
cuss how other heterochromatic regions such as retrotransposons could be
regulated during early mammalian embryogenesis.
