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Abstract
Although a growing number of studies investigates functional genome organization in
somatic cell nuclei, it is largely unknown how mammalian genome organization is established
during embryogenesis. To address this question, we investigated chromo center formation
and the peculiar arrangements of chromosome domains in early mouse embryos. At the
one-cell stage, we observed characteristic arrangements of chromosomes and chromo
center components. Subsequently, starting with the burst of zygotic genome transcription
major rearrangements led to the establishment of somatic type chromo centers with
a defined spatio-temporal organization. These processes appeared to be completed at
the blastocyst stage with the onset of cell differentiation. During the same developmental
period, a fraction of pericentric heterochromatin that was late replicating in the
first cycle underwent switches in replication timing, spatial organization and epigenetic
marks. Cloning experiments revealed that the genome organization typical for more
advanced stages was quickly reverted into the one-cell stage-specific form after nuclear
transfer, supporting the idea that reprogramming associated genome remodeling in normal
and cloned embryos is determined by cytoplasmic factors. Together, the results suggest
that distinct but characteristic forms of nuclear genome organization are required
for genome reprogramming in early embryos and for proper regulation of differential
gene expression patterns at later stages.