Genetic control of cortical development.

evolution and the neural substrate of human mental abilities. Molecular biological techniques have opened the possibility of investigating how specific regulator y genes and morpho-regulatory molecules mediate formation of this complex structure. During evolution, the cerebral cortex has undergone disproportionate growth relative to the rest of the brain [reviewed by Northcutt and Kaas (Northcutt and Kaas, 1995)]. This increase predominately affected its surface area, rather than its thickness, and was accompanied by the addition and elaboration of functional subdivisions. Furthermore, the [deletion] region that has increased in size the most, the neocortex, acquired a characteristic laminar structure that is not readily apparent in the cortex of non-mammalian vertebrates. These changes in the size, complexity and histological organization of the cerebral cortex presumably ref lect the elaboration of the cortical circuitry which ultimately endowed the human brain with its extraordinary capacity for reasoning and language. The cortex, or pallium, is comprised of several subdivisions. In mammals, these functionally distinct regions include the hippocampus, parahippocampal areas, neocortex, entorhinal cortex, olfactory cortex, claustrum and olfactory bulb. Each has characteristic histological features, wiring diagrams and functionally distinct subdomains. Based on cytoarchitectonic criteria, the human cerebral cortex was subdivided into more than 50 areas (Brodmann, 1909). More recent methods are revealing even greater complexity in the categories, number and functional significance of cortical parcellation (Felleman and Van Essen, 1991). The remarkable conservation of the pattern of areal divisions within a given species suggests the existence of a highly conserved and rigidly regulated regional specification program that controls their development (Rakic, 1988). The evidence presented in this issue of Cerebral Cortex is in accord with the view that neocortical regional specification depends on interactions between intrinsic properties of cortical cells with input from subcortical structures. Cortical development has been scrutinized by developmental neurobiologists during this century. Their studies provide an essential physiological, anatomical and cellular foundation for elucidating the molecular mechanisms that regulate its assembly. The discovery of genes that regulate brain development is now revolutionizing our ability to understand the intricacies of neuro-embryology, and to identify how neuropathological processes derail cortical development. Genetic studies of invertebrate development in f lies and nematodes have had a fundamental role in expediting the progress. The identification of genes that regulate regional specification (e.g. gap and homeotic genes), segmentation (e.g. segment polarity genes), neurogenesis (e.g. neurogenic and proneural genes) and programmed cell death (apoptotic genes) provided key insights into …

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