Cellular differentiation results from the asymmetrical self-renewal of cells whereby one daughter cell retains the characteristics of the progenitor lineage while the other becomes committed to differentiation into a new lineage. Each daughter cell acquires a new epigenetic landscape, but the mechanism for this reprogramming is currently ill-defined.
Differentiation of some adult stem cells is associated with non-random DNA template segregation (NRTS) at mitosis. Analysis of NRTS is achieved by labelling the newly synthesized DNA strand in a given S-phase and tracking its (and its non-labelled template strand) allocation into daughter cells. In this study, we examined the fate of sister chromatids across each of the first four cell-cycles of the mouse embryo.
Embryos were incubated in KSOM + amino acids containing 1 µM bromodeoxyuridine (BrdU) for the duration of S-phase of a given cell cycle. They were then cultured through two cell-cycles and the allocation of template DNA between daughter cells assessed by immunolocalization of BrdU.
Labelling of blastomeres in the 1-cell, 2-cell and 8-cell embryo resulted in random allocation of template DNA in subsequent daughter cells. By contrast, labelling during the 4-cell S-phase resulted in non-random allocation of template DNA to the apolar daughter cells following the asymmetrical division of 8-cell embryos. Furthermore, tracking these cells through to blastocysts showed a significant accumulation of template DNA in the inner cell mass compared to the trophectoderm. Analysis of DNA methylation showed that there was a strong negative association between the location of template DNA and global immuno-detectable DNA methylation.
This study points for the first time to NRTS being involved in the asymmetric cell division leading to the first round of differentiation in the early embryo and is associated with global modification of the epigenetic landscape in each of the resultant cell lineages.