Chromatic neuronal jamming in a primitive brain

Jamming models developed in inanimate matter have been widely used to describe cell packing in tissues 1 – 7 , but predominantly neglect cell diversity, despite its prevalence in biology. Most tissues, animal brains in particular, comprise a mix of many cell types, with mounting evidence suggesting that neurons can recognize their respective types as they organize in space 8 – 11 . How cell diversity revises the current jamming paradigm is unknown. Here, in the brain of the flatworm planarian Schmidtea mediterranea , which exhibits remarkable tissue plasticity within a simple, quantifiable nervous system 12 – 16 , we identify a distinct packing state, ‘chromatic’ jamming. Combining experiments with computational modelling, we show that neurons of distinct types form independent percolating networks barring any physical contact. This jammed state emerges as cell packing configurations become constrained by cell type-specific interactions and therefore may extend to describe cell packing in similarly complex tissues composed of multiple cell types.