Biology Reference
In-Depth Information
Minor satellite
Major satellite
NLB
NLB
NLB
Sperm
head
Zygote PN1
Zygote PN2
Zygote PN3-5
Early 2-cell
Late 2-cell
4-cell
MII oocyte
NLB
NLB
NLB
Figure 1.3 Nuclear architecture changes during preimplantation development in the
mouse. The nuclear organization changes dynamically after fertilization. The
nucleolar-like bodies (NLBs) are established after fertilization at the pronuclear stage
(PN) 1 in the zygote. At the PN2 stage, the minor and major satellites (MMS) reorganize
to form a ring-like structure around the NLBs. Between the PN3 and PN5 an increase in
the number of NLBs occurs, but the position of the MMS remains the same. At the early
2-cell stage (E2C), the nuclear organization starts to change and by the end of the 2-cell
stage (L2C) many MMS start to form new chromocenters independently of the NLBs
structure. By the 4-cell stage, most of the NLBs have disappeared and instead the major-
ity of MMS are clustered into “somatic”-like chromocenters. Note that nuclei are not at
the same scale.
the 4-cell stage in some nuclei (
Martin et al., 2006; Probst et al., 2010, 2007
).
As development progresses, the NLBs are replaced with chromocenters,
which are the clusters of centromeric and pericentromeric regions that appear
typically brightly stained by 4
0
,6-diamidino-2-phenylindole (DAPI) in most
somatic cells. The chromocenters are a defining feature of somatic nuclei orga-
nization. The disappearance of any reminiscent NLBs in the mouse embryo
seems to coincide with the 8-cell stage, which is the stage when cells under-
take their first differentiation step (polarization and compaction). It is interest-
ing to note that at this stage, the levels of H3K9me3 seem to be lowest
(
Fig. 1.2
), and H4K20me3 is also absent from embryonic chromatin, sug-
gesting that a loss of heterochromatin might be important for this structural
