Do they acquire neuron-like morphologies in your cultures as well? What signalling mechanisms are taking place when they're seeded at lower cell density?
Unlike you are worrying about the signaling pathway, there are established systems of proliferation and differentiation culture which have inductive signals in the respective systems. They are mimicked of signaling pathways found in vivo neural cell differentiation within the embryo or in vitro embryonic stem cells or equivalents. If you make a similar niche as in those of any material-based scaffolds or surfaces.