Genetic networks of human pre-implantation development. Genetic networks of human pre-implantation development. Maternal transcripts inherited from the oocyte are degraded through subsequent rounds of cell division. Human genome activation principally occurs between the 4- and 8-cell stages, and perhaps as early as the 2-cell stage. It is unclear when genes associated with the restriction of the TE or ICM cell lineage are expressed in human embryos, but data suggest that these lineage-associated genes are expressed in human embryos later than in mice at around the early blastocyst stage. Human embryos can be cultured in vitro for 7-8 days post-fertilization and, in vivo, human embryos implant around day 7. The derivation of epiblast stem-like cells from human pre-implantation embryos suggests that human embryos might be capable of reaching a more mature stage in vitro than mouse pre-implantation embryos, which can be cultured in vitro up to 4 days post-fertilization. The dashed arrows indicate the possibility of earlier minor gene activation and lineage-associated gene expression. Epi, epiblast; TE, trophectoderm; PE, primitive endoderm. Kathy K. Niakan et al. Development 2012;139:829-841 © 2012.