  Cell shape is linked to cell function. The significance of cell
morphodynamics, namely the temporal fluctuation of cell shape, is much less
understood. Here we study the morphodynamics of MDA-MB-231 cells in type I
collagen extracellular matrix (ECM). We systematically vary ECM physical
properties by tuning collagen concentrations, alignment, and gelation
temperatures. We find that morphodynamics of 3D migrating cells are externally
controlled by ECM mechanics and internally modulated by Rho-signaling. We
employ machine learning to classify cell shape into four different
morphological phenotypes, each corresponding to a distinct migration mode. As a
result, we map cell morphodynamics at mesoscale into the temporal evolution of
morphological phenotypes. We characterize the mesoscale dynamics including
occurrence probability, dwell time and transition matrix at varying ECM
conditions, which demonstrate the complex phenotype landscape and optimal
pathways for phenotype transitions. In light of the mesoscale dynamics, we show
that 3D cancer cell motility is a hidden Markov process whereby the step size
distributions of cell migration are coupled with simultaneous cell
morphodynamics. We also show that morphological phenotype transitions
facilitate cancer cells to navigate non-uniform ECM such as traversing the
interface between matrices of two distinct microstructures. In conclusion, we
demonstrate that 3D migrating cancer cells exhibit rich morphodynamics that is
regulated by ECM mechanics, Rho-signaling, and is closely related with cell
motility. Our results pave the way to the functional understanding and
mechanical programming of cell morphodynamics for normal and malignant cells.
