Figure 1.

Stem cells in early development and in vitro. Early development in eutherians begins following fertilization (fusion of the male and female gamete). The single-cell zygote soon forms with the potential to generate a complete organism. This ability is referred to as totipotency and is lost following development past the two-cell stage (in mouse). Formation of the blastocyst coincides with the onset of pluripotency within the inner cell mass (ICM). These cells are innately capable of differentiation to all three germ lineages. Isolation and culture of cells from the ICM produces what are known as “pluripotent stem cells” (PSCs). These in vitro models of the pluripotent ICM vary in the precise developmental stage that they resemble along the naive-primed axis, which has been shown to be highly culture-condition-dependent. Cultures of PSCs contain rare populations of two-cell-like cells that mirror the transcriptome of the in vivo two-cell-stage embryo. However, as of yet, no conditions have been defined for the maintenance of these cells in culture. (ESCs) embryonic stem cells, (EpiSCs) epiblast stem cells.