Local presentation of substrate molecules directs axon specification by cultured hippocampal neurons

Axon specification is a crucial, early step in neuronal development, but li ttle is known about how this event is controlled in vivo. To test the hypot hesis that local presentation of growth-promoting molecules can direct axon specification, we cultured hippocampal neurons on substrates patterned wit h stripes of poly-L-lysine and either laminin (LN) or the neuron-glia cell adhesion molecule (NgCAM). Although undifferentiated neurites contacted bot h substrates equally, axons formed preferentially on LN or NgCAM. Time-laps e studies revealed that changes in the growth pattern of a cell indicative of axon specification began almost immediately after the growth cone of one of the neurites of the cell contacted LN or NgCAM. When cells were plated on alternating stripes of LN and NgCAM, cells with their somata on LN usual ly formed axons on NgCAM, whereas those with somata on NgCAM preferentially formed axons on LN. This suggests that the change from one axon-promoting substrate to another also provides a signal sufficient to specify the axon. These results demonstrate that contact with preferred substrate molecules can govern which neurite becomes the axon and thus direct the development o f neuronal polarity.