Lineage in the cerebral cortex: when is a clone not a clone?

Of supreme importance for higher functions, the neocortex is the most prominent and evolutionarily recent structure in the mammalian brain. The multi-layered sheet of cortical neurons develops from the most anterior vesicle of the neural tube, whose wall thickens ante elaborates into several concentric layers or laminae (six in most areas of the mammalian cortex). Histogenesis involves the radial movement of cells outwards from the zone of proliferation at the innermost (ventricular) surface, with neurons migrating along the processes of radial glial cells that span the thickness of the cortex ~. Deeper layers are formed first, with more superficial layers following in sequence from the inside to the outside. Perpendicular to these laminae, the cortex is organized into radial functional units 100-500 ~ wide; cells lying at various depths within a single cortical column share afferent inputs and display similar electrophysiological responses 2. Cortical areas are comprised of many such columns, and show morphological diversity and functional specialization; major cortical divisions include the motor cortex, the somatosensory cortex and the visual cortex. A central issue is the possible role of cell lineage in generating the complex architecture and cell types specific to each of these areas. Two main questions arise: 'are clones of cells committed to a particular phenotype?' and 'is there a relationship between lineage boundaries and cytoarchitectonic units?'