Phosphorylation of histone H3 at Ser10 (H3S10P) has been linked to a variety of 
cellular processes, such as chromosome condensation and gene 
activation/silencing. Remarkably, in mammalian somatic cells, H3S10P initiates 
in the pericentromeric heterochromatin during the late G2 phase, and 
phosphorylation spreads throughout the chromosomes arms in prophase, being 
maintained until the onset of anaphase when it gets dephosphorylated. 
Considerable studies have been carried out about H3S10P in different organisms; 
however, there is little information about this histone modification in 
mammalian embryos. We hypothesized that this epigenetic modification could also 
be a marker of pericentromeric heterochromatin in preimplantation embryos. We 
therefore followed the H3S10P distribution pattern in the G1/S and G2 phases 
through the entire preimplantation development in in vivo mouse embryos. We paid 
special attention to its localization relative to another pericentromeric 
heterochromatin marker, HP1β and performed immunoFISH using specific 
pericentromeric heterochromatin probes. Our results indicate that H3S10P 
presents a remarkable distribution pattern in preimplantation mouse embryos 
until the 4-cell stage and is a better marker of pericentromeric heterochromatin 
than HP1β. After the 8-cell stage, H3S10P kinetic is more similar to the somatic 
one, initiating during G2 in chromocenters and disappearing upon telophase. 
Based on these findings, we believe that H3S10P is a good marker of 
pericentromeric heterochromatin, especially in the late 1- and 2-cell stages as 
it labels both parental genomes and that it can be used to further investigate 
epigenetic regulation and heterochromatin mechanisms in early preimplantation 
embryos.