The medical and bioindustrial applications of pluripotent stem cells rely on our
understanding of their biology. Pluripotent stem cell lines derived from embryos in
different stages depend on distinct signalling pathways. Embryonic stem cells (ESCs),
derived from the inner cell mass (ICM) of preimplantation embryos, are dependent on
LIF/STAT3 signalling, while epiblast stem cells (EpiSCs), established from the
postimplantation embryos, require Activin A/Smad2/3 signalling. Recent studies
revealed the presence of intermediate pluripotent stem cell populations. Their growth
factor responsiveness, gene expression pattern and associated chromatic signatures, are
compatible with the state intermediate between ESCs and EpiSCs. However, it remains
unknown whether such cell populations represent a stable clonally intermediate cell
state. In this thesis, I describe the discovery and characterization of novel stem cell lines
displaying gene expression pattern intermediate between ESCs and EpiSCs. These cells
respond to LIF/STAT3 as well as Activin/Smad2/3 signalling at single cell level. They
can integrate into the ICM and generate chimeric embryos. In keeping with a more
advanced differentiation stage than that of ESCs, the LIF/Activin dual responsive stem
cells showed accelerated temporal gene expression kinetics during in vitro
differentiation in embryo bodies. I found that these properties are shared by some
induced pluripotent stem cell (iPSC) lines. The notion of an intermediate state was
consolidated by a genome-wide microarray profiling. The hierarchical clustering
analysis grouped LIF/Activin dual responsive stem cells together into a cluster
intermediate between ESCs and EpiSCs. These findings advanced our understanding of
the regulation of pluripotency. A better understanding of distinct differentiation state of
pluripotent stem cells and their signalling responsiveness is crucial for developing
tailored strategies for lineage/cell type differentiation.