As with many phenomena in neurobiology, in retrospect we now realize that Ramon y Cajal made the initial observations recognizing these regressive events more than a hundred years ago when he followed the development of avian Purkinje and mammalian granule cells. The cellular and molecular mechanisms of neuronal remodeling are only starting to be elucidated. In insects, however, pruning takes place during metamorphosis when large-scale rearrangements occur within the entire nervous system. In humans, more than half of the neural connections formed during embryonic development are eliminated within the first two years of life and then further remodeled during puberty. The elimination of these exuberant connections is largely timed to postnatal development in vertebrates but the precise timing varies among organisms. These regressive events eliminate cells, synapses and long stretches of axons in a precise and timely fashion during development and are essential to sculpt the mature nervous system of both vertebrates and invertebrates. Mounting evidence from multiple systems suggest that an exuberant network of neurons and connections is generated during the early stages of development and later remodeled by a wide variety of cellular strategies, as part of the normal course of network establishment and refinement and that this is not an anecdotal phenomenon. Despite its beauty and simplicity, it has become evident in recent years that the chemoaffinity theory provides only a partial answer to how the wiring of the nervous system is established in vivo during development. The discovery of the key families of cues that govern axon guidance, such as netrins, slits, semaphorins and ephrins, has provided the kind of molecular proof that the chemoaffinity hypothesis lacked for many years. Thus, according to Sperry, each axon “knows” where to go during the initial wiring of the nervous system or even when the circuit has to reassemble itself after injury. In its core, Sperry’s hypothesis postulated that each growing axon has a unique molecular identity that determines its attraction or repulsion from distinct local cues. Since it was proposed in the middle of the 20 th century, Roger Sperry’s chemoaffinity hypothesis has been validated in many neuronal systems across various model organisms.
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