The discovery of this cell type allows us to clearly define their viable ontogenetic sisters, these being the cells of the ICM. Single-cell analysis (on multiple independent datasets), supported by embryo visualisation, uncovers a common previously uncharacterised class of cells lacking commitment markers that segregates after embryonic gene activation (EGA) and shortly after undergo apoptosis.
Here, we provide a multi-method analysis of the early human embryo to resolve these issues. Perhaps most importantly, the inner cell mass (ICM), from which the foetus is derived and hence of interest in reproductive health and regenerative medicine, has proven hard to define. On a gross level, there is evidence for apoptosis, but the nature of the affected cell types is unknown. There remains much that we do not understand about the earliest stages of human development.
