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replacement takes place is not well understood. The histone H3.3 has been
shown to localize preferentially to the paternal chromatin during the zygote
stage (
Loppin et al., 2005; Santenard et al., 2010; Torres-Padilla, Bannister,
Hurd, Kouzarides, & Zernicka-Goetz, 2006; van der Heijden et al., 2005
).
We have shown in the lab that the mutation of the lysine K27 to alanine of
H3.3 results in a missegregation of chromosomes as well as developmental
arrest and mislocalization of HP1. The same mutation in H3.1 does not seem
to have any effect on either HP1 localization or developmental progression
(
Santenard et al., 2010
). The same work suggested that incorporation of
H3.3 in the paternal pericentric heterochromatin is important for the initial
establishment of pericentromeric heterochromatin through its lysine 27. It
therefore seems that H3.3 has acquired a role in remodeling heterochroma-
tin after fertilization. This is in line with the preferential heterochromatic
localization of H3.3 during the first embryonic divisions
(
Akiyama,
Suzuki, Matsuda, & Aoki, 2011; Santenard et al., 2010
).
2.4. Nuclear architecture in the mammalian embryo
The most astonishing observation that can be made immediately after fertil-
ization is probably the difference in the size of the two pronuclei and the
distinctive organization of the heterochromatin in both of them, which is
radically different to that of somatic cells (
Fig. 1.3
).
The paternal pronucleus is almost 25% bigger than the maternal one.
This spatial organization reflects the decondensation of the paternal genome
that starts with fertilization. This process, which is accompanied by cycles of
recondensation and further decondensation (
Bouniol-Baly, Nguyen,
Besombes, & Debey, 1997; Martin et al., 2006; Probst et al., 2010;
Probst, Santos, Reik, Almouzni, & Dean, 2007
), might be important for
the subsequent establishment of the nuclear compartments and/or charac-
teristic embryonic nuclear architecture. No heterochromatic marks can
be detected on the paternal chromatin at this time, but H3K9me1 and
H3K27me1 have been observed on the paternal pronucleus as early as dur-
ing the first decondensation (
Santos et al., 2005
). This coincidence might
reflect some spatiotemporal requirement that needs to be put in place before
heterochromatic marking (e.g., H3K27me3) can be added a couple of hours
later. Indeed, the changes in decondensation coincide with the emergence
of the NLBs. These are ring-like structures around which the centromeric
and pericentromeric sequences localize (
Probst et al., 2007
). The organiza-
tion of the pericentromeric domains around NLBs is maintained at least until
