On his website he writes: «Embryonic stem (ES) cells are pluripotent stem cells derived from inner cell mass of mammalian blastocysts. (faith.org.uk)
We show that the fraction of cells present within this state is influenced by factors that both promote and suppress primitive endoderm differentiation, but conditions that support ES cell self - renewal prevent their progression into differentiation and support an equilibrium between this state and at least one other that resembles the Nanog positive inner cell mass of the mammalian blastocysts. (journals.plos.org)
Gardner DK, Stilley KS, Lane M. High protein diet inhibits inner cell mass formation and increases apoptosis in mouse blastocysts developed in vivo by increasing ammonium in the female tract. (adelaide.edu.au)