Delineating the mechanisms behind fate decisions of human embryonic stem cells

Human embryonic stem (ES) cells are derived from the inner cell mass of blastocyst stage embryos. Once explanted and cultured in vitro under appropriate conditions, human ES cells retain pluripotency (i.e. capacity to differentiate into all somatic cell types) and acquire the ability to self-renew indefinitely. These two properties of human ES cells make them an invaluable resource for developmental biology, cell replacement therapies, drug development and toxicology screening. In order to exploit these unique cells and to better understand human development, it is imperative to understand the mechanisms behind the specification of somatic cell types. Much work has been conducted on monolayer formats to delineate signalling and gene expression networks responsible for lineage specific differentiation, however less focus has been on the use of embryoid bodies as a more representative model of in vivo differentiation. In this study we develop a differentiation assay to better recapitulate embryonic development, which we also show as a useful model for predictive toxicology. Within the assay, however, we found persistently heterogeneous differentiation. To better understand how hESCs make lineage decisions, we went back to interrogate heterogeneity within the stem cell compartment, and show that discreet, but functional heterogeneity biases cells to particular fates. Finally, we shed light onto a potential mechanism through which heterogeneity arises, which could offer a platform for future work to homogenise stem cells thus resulting in uniform, controlled differentiation