Synaptic connections made by axons regenerating in the central nervous system of adult mammals

The restoration of connections in the injured central nervous system (CNS) of adult mammals is hindered by the failure of axons to grow back to their natural fields of innervation. Following transection of the optic nerve of adult rodents, the guided regeneration of retinal ganglion cell (RGC) axons along a transplanted segment of peripheral nerve (PN) has shown that these neurones retain their capacities to form well-differentiated synapses in both normal and abnormal targets. The main aim of this review is to describe the anatomical and functional characteristics of some of these connections and to suggest that their terminal distribution and morphology may be the result of a persistence in these targets of molecular determinants that influence normal connectivity in the intact animal.