Extending the analogy between intracellular motion in mammalian cells and glassy dynamics
The physics of how molecules, organelles, and foreign objects move within living cells has been extensively studied in organisms ranging from bacteria to human cells. In mammalian cells, in particular, cellular vesicles move across the cell using motor proteins that carry the vesicle down the cytoskeleton to their destination. We have recently noted several similarities between the motion of such vesicles and that in disordered, "glassy", systems, but it remains unclear whether that is a general observation or something specific to certain vesicles in one particular cell type. Here we follow the motion of mitochondria, the organelles responsible for cell energy production, in several mammalian cell types over timescales ranging from 50 ms up to 70 s. Qualitative observations show that single mitochondria remain stalled, remaining within a spatially limited region, for extended periods of time, before moving longer distances relatively quickly. Analysing this motion quantitatively, we observe a displacement distribution that is roughly Gaussian for shorter distances ($\lesssim$ 0.05 $μ$m) but which exhibits exponentially decaying tails at longer distances (up to 0.40 $μ$m). We show that this behaviour is well-described by a model originally developed to describe the motion in glassy systems. These observations are extended to in total 3 different objects (mitochondria, lysosomes and nano-sized beads enclosed in vesicles), 3 different mammalian cell types, from 2 different organisms (human and mouse). We provide further evidence that supports glass-like characteristics of the motion by showing a difference between the time it takes to move a longer distance for the first time and subsequent times, as well as a weak ergodicity breaking of the motion. Overall, we demonstrate the ubiquity of glass-like motion in mammalian cells, providing a different perspective on intracellular motion.