Deconstructing human embryonic stem cell cultures: niche regulation of self-renewal and pluripotency.

The factors and signaling pathways controlling pluripotent human cell properties, both embryonic and induced, have not been fully investigated. Failure to account for functional heterogeneity within human embryonic stem cell (hESC) cultures has led to inconclusive results in previous work examining extrinsic influences governing hESC fate (self renewal vs. differentiation vs. death). Here, we attempt to reconcile these inconsistencies with recent reports demonstrating that an autologously produced in vitro niche regulates hESCs. Moreover, we focus on the reciprocal paracrine signals within the in vitro hESC niche allowing for the maintenance and/or expansion of the hESC colony-initiating cell (CIC). Based on this, it is clear that separation of hESC-CICs, apart from their differentiated derivatives, will be essential in future studies involving their molecular regulation. Understanding how extrinsic factors control hESC self-renewal and differentiation will allow us to culture and differentiate these pluripotent cells with higher efficiency. This knowledge will be essential for clinical applications using human pluripotent cells in regenerative medicine.