Neurons of the central nervous system are natural migrants, as most of them originate far from the place where they will eventually perform their normal function. Indeed, the large majority of neurons are generated from precursor cells that line the walls of the ventricular system, from where they migrate until they settle at their final position. Thus, after the genesis of specific cell types through an exquisite and controlled process of patterning and regionalization, neurons of the brain are set to migrate. In some cases, new neurons migrate for relatively short distances to settle, for instance, in the ventral horn of the spinal cord, where they become somatic motor neurons. In other cases, neurons migrate for incredibly long distances, sometimes up to thousands of times their own size, to settle in remote regions of the brain, as in the case of the interneurons of the cerebral cortex or the olfactory bulb. Thus, independent of the neuronal type, location, or function, neuronal migration is always a fundamental step in brain development.
The complexity of the brain in vertebrates is proportional, to a large extent, to the elaboration of the mechanisms controlling neuronal migration. This is particularly evident in the mammalian forebrain and, more specifically, in the telencephalon, where the development of the isocortex has been accompanied by an enormous increase in the distance covered by migrating neurons from the ventricular zone to their final destination. This is in sharp contrast with the situation found in amphibians, for example, in which neurons barely migrate away from the place they originate. Thus, as a mechanism that shapes the development of the brain, changes in neuronal migration have greatly contributed to its diversification during evolution.
In this article, we review concepts on neuronal migration through evolution, with a focus on the central nervous system (CNS). Whenever possible, we will refer to the development of the cerebral cortex as a model system for studying the cellular and molecular mechanisms controlling neuronal migration. Of note, although the general principles that control migration in the peripheral nervous system are essentially identical to those in the CNS, this subject is beyond the scope of this article. To learn more about this, the reader is referred to reviews focusing on the mechanisms controlling neural crest migration (Robinson, et al., 1997; Locascio and Nieto, 2001; Kalcheim and Burstyn-Cohen, 2005).
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