Recent findings with respect to epiblasts before and after implantation have produced proposals for classifying pluripotency into two distinct phases: "naive" and "primed".  The baseline stem cells commonly used in science that are referred as Embryonic stem cells (ESCs) are derived from a pre-implantation epiblast; such epiblast is able to generate the entire fetus, and one epiblast cell is able to contribute to all cell lineages if injected into another blastocyst. On the other hand, several marked differences can be observed between the pre- and post-implantation epiblasts, such as their difference in morphology, in which the epiblast after implantation changes its morphology into a cup-like shape called the "egg cylinder" as well as chromosomal alteration in which one of the X-chromosomes undergoes random inactivation in the early stage of the egg cylinder, known as X-inactivation .  During this development, the egg cylinder epiblast cells are systematically targeted by Fibroblast growth factors , Wnt signaling, and other inductive factors via the surrounding yolk sac and the trophoblast tissue,  such that they become instructively specific according to the spatial organization.  Another major difference that was observed, with respect to cell potency, is that post-implantation epiblast stem cells are unable to contribute to blastocyst chimeras ,  which distinguishes them from other known pluripotent stem cells. Cell lines derived from such post-implantation epiblasts are referred to as epiblast-derived stem cells which were first derived in laboratory in 2007; it should be noted, despite their nomenclature, that both ESCs and EpiSCs are derived from epiblasts, just at difference phases of development, and that pluripotency is still intact in the post-implantation epiblast, as demonstrated by the conserved expression of Nanog , Fut4 , and Oct-4 in EpiSCs,  until somitogenesis and can be reversed midway through induced expression of Oct-4 .