Mechanisms and Dynamics of Heterochromatin Formation During Mammalian Development: Closed Paths and Open Questions - Current Topics in Developmental Biology - page 7

chromosomes. H3K9me3 can be clearly seen in two-cell nuclei, but it marks
asymmetrically half of each nucleus, which corresponds to the maternal
chromatin that remains physically segregated from the paternal chromatin
within the nucleus ( Liu, Kim, & Aoki, 2004; Santenard et al., 2010 )
( Fig. 1.1 ). The levels of H4K20me3 are significantly reduced at the 2-cell
stage to almost undetectable levels ( Kourmouli et al., 2004; van der
Heijden et al., 2005; Wongtawan, Taylor, Lawson, Wilmut, & Pennings,
2011 ), and H3K64me3 is no longer detected at the late 2-cell stage ( Daujat
et al., 2009 ). The 4-cell stage is thus characterized by the absence of
H3K64me3, H4K20me3, and a marked decrease in H3K9me3 levels.
H4K20me3 is not reestablished until after implantation, but the detailed spa-
tiotemporal pattern of de novo methylation of H4K20me3 has not been stud-
ied. Global levels of H3K9me3 on the other hand continue to be reduced
during the 8-cell stage judged from immunofluorescence analysis, and it
would appear that H3K9me3 levels start to increase at the 16-cell stage and
through the blastocyst stage, and are maintained thereafter ( Puschendorf
et al., 2008 and our unpublished observations). It should be noted though that
most of these analyses are based on immunofluorescence approaches that have
not been thoroughly or precisely quantified. It is worth mentioning that
reacquisition of H3K9me3 globally seems to coincide with the time when
the blastomeres undergo what can be described as the first differentiation step

Euchromatin

H3K4me3
Histone acetylation

DNA methylation

Facultative heterochromatin

H4K20me2/3

H3K64me3

H3K27me3

H3K9me2/3

HP1 a / b

Figure 1.1 Histone posttranslational modifications and the two chromatin states.
Euchromatin is globally marked by H3K4me3 and acetylation of histone tails. However,
facultative heterochromatin is marked by H3K27me2/3 and DNA methylation. The latter
together with H3K64me3, H3K9me2/3, and H4K20me2/3 are typical of constitutive
heterochromatin.