Simulating the Development of the Superficial Patch System

Injections of neural tracers into various mammalian neocortical areas reveal a common patchy motif of clustered axonal projections (Rockland et al., 1982, Muir et al., 2010). We have used the Java-based simulation framework CX3D (Zubler & Douglas, 2009) to investigate how this patchy connectivity could arise in a single layer (II/III) of neocortex. CX3D allows the simulation of cellular growth in a physical 3D environment that respects forces and diffusion of cellularly secreted substances which can act as guidance cues for axonal growth. Individual neurons in the cortical layer II/III expressed the activator-inhibitor components of a Gierer-Meinhardt reaction-diffusion system (Gierer & Meinhardt, 1972). The steady-state reaction-diffusion pattern across the neurons is approximately hexagonal, and for appropriate parameter settings we could reproduce the surprising linear relationship observed between average patch diameter and inter-patch spacing (Binzegger et al., 2007, Muir et al., 2011). The growth cones at the tips of extending axons used the morphogens secreted by intra-patch neurons as guidance cues to direct their growth and invoke arborization, so yielding a general patchy arborization across layer II/III. We conclude that a simple Gierer-Meinhardt system installed in the neurons of the developing neocortex is sufficient to explain the experimentally observed patchy connectivity.