Atypical embryonic synapses fail to regenerate in adulthood.

Functional recovery following central nervous system (CNS) injury in adult animals may depend on the reestablishment of the precise pattern of connections made during development. When the nervous system is injured during embryonic development, functional recovery may involve the formation of atypical connections. Can such atypical synapses regenerate in adults, particularly in a nervous system known for its capacity for repair? When the S interneuron in one segmental ganglion of the leech Hirudo is killed during development, two neighboring S cells extend their axons into the ganglion and restore function by making electrical synapses with the usual synaptic targets of the killed S cell.