A systems view of cellular heterogeneity: Unlocking the "wheel of fate".

Systems biology offers a view of the cell as an input-output device: a biochemical network (or cellular "processor") that interprets cues from the microenvironment to drive cell fate. Advancements in single-cell technologies are unlocking the cellular black box, revealing heterogeneity in seemingly homogeneous cell populations. But are these differences technical variability or biology? In this review, we explore this question through a systems biology lens, offering a framework for conceptualizing heterogeneity from the cell's perspective and summarizing systems and synthetic biology tools for capturing heterogeneity. While cellular inputs shape the probability of attaining particular fates, each cell spins a stochastic "wheel of fate." Applying this framework, we explore heterogeneity in two case studies: human pluripotent stem cell (hPSC) culture and beta cell differentiation. Looking forward, we discuss how a systems approach to heterogeneity may enable more predictable outcomes in stem cell research, with broad implications for developmental biology and regenerative medicine.