Models of cell proliferation with different states of activity

The study of a biological system made of several populations of cells and different constituents is performed. Especially, we refer to the growth of a tumoural mass in the avascular state. A general scheme of the corresponding mathematical problem is obtained and some of the most representative models in literature are discussed. We propose a new approach to the problem, based either on nonlocal interactions among the constituents or on the existence of a chemical. potential driving motion of intercellular fluid.