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The inner cell mass (ICM) forms within the blastocyst, prior to its implantation within the uterus. The ICM is a cellular mass on one side of the hollow interior of the round embryo, the outer layer of which is called the trophoblast. The ICM is the only pluripotent cell lineage in the blastocyst. The ICM gives rise to all embryonic tissues as well as some extraembryonic structures.
Embryonic stem cells are derived from the ICM and retain its cardinal features (pluripotency, high proliferation and most of the transcriptional regulatory networks).