When neurons adhere (independently) to a 2-d substrate, they often migrate in a similar direction. One mechanism for this is the entanglement of their outgrowing neurites, via cell adhesion molecules, in a process sometimes called fasciculation.
Initially, these fasciculated neurites should be in tensile equilibrium. However, during development and migration the cell with the stronger tensile force will tend to pull the other neurite (and thus its associated neuron) closer.
Over time, this tendency would cause neurons to cluster together. An interesting new study from Sun et al demonstrates this clustering effect nicely.
When cells are plated at a relatively low density, they tend to form multiple clusters. For example, see the schematic below showing the migration of hippocampal neurons plated on a circular substrate over 24 hours:
When the cells are relatively more dense, they will typically form one big mega-cluster. As an example, see this set of time-lapse images of hippocampal neurons grown over 12 days in vitro (DIV):
Their model predicts that a genetic or biochemical intervention which inhibits neurite fasciculation would reduce the clustering of neurons in this sort of system.
Sun Y, Huang Z, Yang K, Liu W, Xie Y, et al. (2011) Self-Organizing Circuit Assembly through Spatiotemporally Coordinated Neuronal Migration within Geometric Constraints. PLoS ONE 6(11): e28156. doi:10.1371/journal.pone.0028156