Spatial and temporal distribution of Oct-4 and acetylated H4K5 in rabbit embryos

Rabbit is a unique species to study human embryology; however, there are limited reports on the key transcription factors and epigenetic events of rabbit embryos. This study examined the Oct-4 and acetylated H4K5 (H4K5ac) patterns in rabbit embryos using immunochemistry staining. The average intensity of the Oct-4 signal in the nuclei of the whole embryo spiked upon fertilization, then decreased until the 8-cell stage and increased afterwards until the compact morula (CM) stage. It decreased thereafter from the CM stage to the early blastocyst (EB) stage, with a minimum at the expanded blastocyst (EXPB) stage and came back to a level similar to that of the CM-stage embryos in the hatching blastocysts (HB). The Oct-4 signal was observed in both the inner cell mass (ICM) and the trophectoderm (TE) cells of blastocysts. The average H4K5ac signal intensity of the whole embryo increased upon fertilization, started to decrease at the 4-cell stage, reached a minimum at the 8-cell stage, increased again at the EXPB stage and peaked at the HB stage. While TE cells maintained similar levels of H4K5ac throughout the blastocyst stages, ICM cells of HB showed higher levels of H4K5ac than those of EB and EXPB. Understanding key genetic and epigenetic events during early embryo development will help to identify factors contributing to embryo losses and consequently improve embryo survival rates. As a preferred laboratory species for many human disease studies such as atherosclerosis, rabbit is also a pioneer species in the development of several embryo biotechnologies, such as IVF, transgenesis, animal cloning, embryo cryopreservation and embryonic stem cells. However, there are limited reports on key transcription factors and epigenetic events of rabbit embryos. In the present study, we documented the temporal and spatial distribution of Oct-4 protein and H4K5 acetylation during early embryo development using the immunostaining approach. We also compared the patterns of these two important biomarkers between the inner cell mass (ICM) and the trophectoderm (TE) cells in blastocyst-stage embryos. Our findings suggest that a combination of Oct-4, H4K5ac and possibly other biomarkers such as Cdx-2 is needed to accurately identify different lineages of cells in morula and blastocyst stage rabbit embryos. Importantly, we revealed a novel wave of Oct-4 intensity change in the ICM cells of rabbit blastocysts. The signal was high at the early blastocyst stage, reached a minimum at the expanded blastocyst stage and returned to a high level at the hatching blastocyst stage. We hypothesize that the signal may have reflected the regulation of Oct-4 through enhancer switching and therefore may be related to cell lineage formation in rabbit embryos. These findings enrich our understanding on key genetic and epigenetic programming events during early embryo development in rabbits.