Figure 1.

Heterogeneity of single cells within bulk populations. Cells isolated based on limited number of markers using FACS, by micropipette, optical tweezers or microfluidics, may contain multiple different cell types and cell states (left panel). If these cells are subjected to bulk analysis (squares), their heterogeneity and unique functions are masked by an average of gene expression signals. If studied individually (single cells), then their distinct activation responses to stimuli, such as cytokines, lipopolysaccharide or bacteria, can be uncovered (right panel). A specific stimulus can have variable effect on cells and leads to different outcome readout for the population in total. For instance, the rare cells may be early responders, which differentiate and activate other cells via paracrine signalling, or the cell types may respond differently, by cytokine production (cell type 2/round cells) or differentiation.