| Human embryonic stem cells (hESCs) have the capacity to generate all different cell types of the body in vitro. Due to this ability, these cells are an important tool in understanding the mechanisms that guide fate decisions in early human development. This ability also gives them utility in the development of therapeutic cell and tissue replacement strategies, as well as allows researchers to generate the large amounts of cells needed for drug screens and for the understanding of the cellular phenotypes of human disease. Current protocols to accomplish differentiation into specific cell types are often inefficient at generating pure homogenous populations of a specific lineage. Recent advances in differentiation technology have made it clear that many cues are responsible for driving certain cell fates in hESCs. These cues include chemical factors, such as growth factors and cytokines, as well as mechanical factors, such as changes in the rigidity of cells, and their cytoskeleton. Using BMP4-driven ectoderm differentiation as a model, we have seen that certain cells within a hESC colony have a higher propensity to differentiate in response to this chemical cue. These cells with higher differentiation potential are found at the edge of the undifferentiated colony, where there is a distinct mechanical environment compared to the interior of the colony. This mechanical environment includes highly organized actin fibers, which strongly co-localize with myosin activity. Additionally, cells at the edge of undifferentiated colonies apply stronger traction forces to their underlying substrate than interior cells do. By manipulating the colony edge and the mechanical state of these colonies, we hypothesized that we could improve upon the differentiation efficiency of protocols based on chemical addition alone. Many methods were tried to accomplish this, and plating smaller colonies, and thus increasing the amount of edge per undifferentiated colony saw the most obvious effect. Our results suggest that there is an important differentiation factor at the colony edge, which is established by both the cell-cell interactions within the colony and the cell-ECM interactions with the colony substrate. This helps to illuminate the important role that these adhesions, and their effect on cellular mechanics, play in cell fate decisions. |
