During cytokinesis, actin, myosin, formins and associated proteins, self-assemble into the equatorial acto-myosin contractile ring. The constriction of the ring separates the cytoplasm of dividing cells into two. While the list of molecular players is almost complete, the molecular and collective mechanisms driving the assembly and constriction of the ring remain unclear. We will collaborate with experimentalists to develop computational tools for the analysis of images of dividing fission yeast cells expressing fluorescent markers for actin (GFP-CHD) and myosin (Rlc1p-RFP), which reveal crucial information on the pattern of contractile ring formation. We will then use the information to develop and test mathematical and numerical models of the mechanics and dynamics of contractile ring assembly and to motivate further experiments. In fission yeast, the contractile ring assembles through the actin-dependent condensation of a broad band of membrane-bound "nodes" containing myosin Myo2p, formin Cdc12p, and other proteins. The condensation mechanism is highly dynamic involving continuous actin polymerization and disassembly and intermittent node motions. Cdc12p presumably nucleates actin filaments which establish transient connections between nodes that are pulled together and form a ring through Myo2p motor activity. We will test this hypothesis by developing novel computational methods to segment actin filaments in 3D static images and to track actin filaments and bundles in time-lapse movies. This will enable us to analyze the topology and dynamics of condensing actomyosin networks and systematically quantify the locations of vertices, filament lengths, rates of polymerization, disassembly, and bundling. By correlating these dynamics to the locations of Myo2p nodes we will rigorously establish the relationship between nodes and sites of actin remodeling. Successful tracking of actin motions will further allow us to extract biophysical parameter values such as filament diffusion coefficients, cytoplasmic viscosity, and forces. We will model the role of physical constraints on the mechanism of establishing connections between nodes and the effect of force on formin-mediated actin elongation. We will test global models of ring assembly and develop statistical analysis and visualization methods for systematic comparison of model predictions to experiment. Cytokinesis, the final step of cell division, is driven by the constriction of an equatorial contractile ring consisting of actin and associated proteins. Despite the biomedical importance of deciphering the mechanisms and controls of cell division, the precise mechanistic steps of contractile ring assembly and constriction remain unclear. We will resolve quantitative details of the assembly of the contractile ring by analyzing fluorescence microscopy images of dividing cells and by developing numerical and mathematical mechanistic models.