Structural Plasticity in Adult Nervous System: An Historic Perspective

In the adult central nervous system, neurons are endowed with a remarkable power: a continuous structural remodeling of their connections owing to minuscule changes of their axonal terminal branches and synaptic boutons, simultaneously with the growing and pruning of their distal dendritic branches and spines. In this review, we summarize the history of the concept of structural plasticity from its beginnings in the nineteenth century to its consolidation in our days, with the establishment of a clear relationship between the structural plasticity of neural circuits and memory and learning processes. After a short review of the morphological changes observed at all levels of highly activated neurons (dendritic spines, distal dendritic branches, axonal initial segments) in adult healthy animals, we cover a more classical topic by reporting the structural changes consecutive to neuronal lesions, particularly after axotomy or deafferentation. Finally, we discuss work done with grafts of embryonic central nervous tissue in adult mice with genetic or experimental brain lesions in order to determine the capability of the embryonic cells to substitute missing neurons in the adult animals. We present in particular the “adaptive rejuvenation,” i.e., the plasticity induced in the host adult cells by the grafted immature neural cells to allow their interaction and subsequent integration in the adult circuit. Our conclusion is the remarkable capacity for structural plasticity of adult neural cells, which still needs the full understanding of their genetic regulation.