Challenges associated with in vitro control of stem cell fate decisions
Advances in bioengineering approaches now allow sophisticated manipulation of live cells in vitro for a range of applications. For these applications, stem cells are often the cell source of choice as they offer several advantages [7,8]. First, stem cells can be expanded to supply a large number of cells with varying levels of self-renewal and differentiation potential. For instance, there are now well-established protocols for stem cells such as MSCs that enable precise expansion or differentiation into multiple cell lineages . Second, the use of stem cells facilitates temporal control of cellular activities as they can be maintained as undifferentiated cells until appropriate extrinsic cues are presented. Third, advances in stem cell technology permit the use of patient-derived somatic cells as induced pluripotent stem cells, opening up new possibilities for patient-specific applications that may significantly enhance therapeutic outcome.
However, using stem cells is often challenging due to difficulties associated with precisely controlling stem cell fate decisions in vitro [2,3]. Despite technological advances in engineering stem cell fate decisions, underlying mechanisms of stem cell renewal and differentiation remain unclear as the critical factors required to maintain many types of stem and progenitor cells in vitro have yet to be uncovered . This is particularly challenging with the use of adult stem cells as they quickly lose their stem potential in vitro [2,3]. The heterogeneity of stem and progenitor cell populations poses another challenge [2,3]. Because most stem cell populations are not homogeneous and therefore contain cells with varying functional capacity (e.g., progenitor cells, differentiating cells, terminally differentiated cells, other types of cells), stem cell populations need to be screened routinely at the single-cell level to exclude any unwanted cell populations . Currently available tools (e.g., fluorescence labeling, clonal expansion assays, repopulation assays, in vivo models) rarely allow in situ screening of individual live cells for dynamic monitoring of stem cell populations because they are almost always end-point analyses of ensemble populations of cells that are expensive, labor intensive, and time delayed [2,3]. For more detailed information on stem cell engineering and currently available tools of stem cell characterization, please refer to recent reviews [2,3]. Naturally, the lack of suitable analytical approaches for quantitation of stem cell fate decisions in vitro also hampers efforts to identify factors that are essential for stem cell maintenance. These shortcomings in stem cell fate control efforts in vitro call for a new analytical approach that could spatiotemporally resolve single stem cell activities in real time.