  In this paper we propose an existence and uniqueness theory for the solutions
of a system of non-linear hyperbolic conservation laws, structured in age and
maturity variables, representing a tissue environment. In particular we are
interested in the investigation of the role of stem cells in its homeostasis.
The main result presented in this paper is the consistence of the stability
results arising from the analisys of the model we designed with the
experimental observations on which several branches of medicine are currently
attempting to trade on their research activity.
