  We consider a reaction-diffusion system of densities of two types of
particles, introduced by Edouard Hannezo et al. in the context of branching
morphogenesis. It is a simple model for a growth process: active, branching
particles form the growing boundary layer of an otherwise static tissue,
represented by inactive particles (Cell, 171(1):242-255.e27, 2017). The active
particles diffuse, branch and become irreversibly inactive upon collision with
a particle of arbitrary type. In absence of active particles, this system is in
a steady state, without any a priori restriction on the amount of remaining
inactive particles. Thus, while related to the well-studied FKPP-equation, this
system features a game-changing continuum of steady state solutions, where each
corresponds to a possible outcome of the growth process. However, simulations
indicate that this system self-organizes: traveling fronts with fixed shape
arise under a wide range of initial data. In the present work, we describe all
positive and bounded traveling wave solutions, and obtain necessary and
sufficient conditions for their existence. We find a surprisingly simple
symmetry in the pairs of steady states which are joined via heteroclinic wave
orbits. Our approach is constructive: we first prove the existence of almost
constant solutions and then extend our results via a continuity argument along
the continuum of limiting points.
