A milestone is marked in our understanding of the brain with the recent acceptance, contrary to early dogma, that the adult nervous system can generate new neurons. One could wonder how this dogma originally came about, particularly because all organisms have some cells that continue to divide, adding to the size of the organism and repairing damage. All mammals have replicating cells in many organs and in some cases, notably the blood, skin, and gut, stem cells have been shown to exist throughout life, contributing to rapid cell replacement. Furthermore, insects, fish, and amphibia can replicate neural cells throughout life. An exception to this rule of self-repair and continued growth was thought to be the mammalian brain and spinal cord. In fact, because we knew that microglia, astrocytes, and oligodendrocytes all normally divide in the adult and respond to injury by dividing, it was only neurons that were considered to be refractory to replication. Now we know that this long accepted limitation is not completely true, because there are two rather discrete areas of the brain, the dentate gyrus of the hippocampal formation and the subventricular zone and its projection through the rostral migratory stream to the olfactory bulb, which can generate new neurons.

Although the discovery that there are limited areas of the brain where neurons do regenerate is important, it also deepens our curiosity as to why it is that most neurons in fact cannot replicate. One reason why neuronal replication in the adult was considered unlikely was the complexity of most neurons; with their highly branched dendrites and polysynaptic axonal combinations, they were considered to be terminally differentiated and unable to re-enter the cell cycle and divide, strictly from a mechanistic view. Another problem was conceptual. If neurons were able to divide, how would the newly created cells with their new dendrites, axons, and synapses, functionally integrate into the brain without disrupting existing circuits? With the dominant theories of brain function being based on a computer analogy with fixed circuits, it did not seem at all logical that adult brain circuitry could be continually adding new components.