leech neurons in culture have provided novel insights into the steps in the formation of neurite outgrowth patterns, target recognition and synapse formation. identified adult neurons from the central nervous system of the leech can be removed individually and plated in culture under well-controlled conditions, where they retain their characteristic physiological properties, grow neurites and form specific chemical or electrical synapses. different identified neurons develop distinctive outgrowth patterns that depend on their identities and on the molecular composition of the substrate. on native substrates, the patterns displayed by these neurons reproduce characteristics from the adult or the developing neurons. in addition, the substrate may induce selective directed growth between pairs of neurons that normally make contact in the ganglion. upon contact, pairs of cultured leech neurons form chemical or electrical synapses, or both types depending on the neuronal identities. anterograde and retrograde signals during membrane contact and synapse formation modify the distribution of synaptic terminals, calcium currents, and responses to 5-hydroxytryptamine.