A coupled system of nonlinear mixed-type equations modeling early stages of angiogenesis is analyzed in a bounded domain. The system consists of stochastic differential equations describing the movement of the positions of the tip and stalk endothelial cells, due to chemotaxis, durotaxis, and random motion; ordinary differential equations for the volume fractions of the extracellular fluid, basement membrane, and fibrin matrix; and reaction-diffusion equations for the concentrations of several proteins involved in the angiogenesis process. The drift terms of the stochastic differential equations involve the gradients of the volume fractions and the concentrations, and the diffusivities in the reaction-diffusion equations depend nonlocally on the volume fractions, making the system highly nonlinear. The existence of a unique solution to this system is proved by using fixed-point arguments and Hölder regularity theory. Numerical experiments in two space dimensions illustrate the onset of
formation of vessels.