A blastocyst is a tiny, early stage of a developing human or animal embryo. It's a cluster of cells that forms about 5-7 days after fertilization. Full definition
Currently, the NIH defines acceptable hESCs as «cells that are derived from the inner cell mass of blastocyst stage human embryos.» (scienceprogress.org)
The existence of ES cell precursors to this lineage is supported by the observed heterogeneous expression of other PrEn genes, Lefty1, Cerl, and Gata6 in the ICM of blastocyst stage embryos, the stage from which ES cells are derived [17], [44]--[46]. (journals.plos.org)
The first reports of the generation of human induced pluripotent stem cells (hiPSCs) from fibroblasts [1, 2] were greeted with great enthusiasm over their ability to mimic the properties and function of embryonic stem cells without entailing the ethical controversies and resourcing issues associated with the use of human blastocysts in derivation. (stemcellres.biomedcentral.com)