Development of the human cerebral cortex: Boulder Committee revisited

Key Points Much more is now known about human cortical development than was known in 1970, when the Boulder Committee attempted to standardize the heterogeneous and confusing nomenclature that was used in the field. In light of this, a revised standardization is needed. A transient layer with a diverse population of neurons forms between the neuroepithelium and the pial surface of the dorsal telencephalon before the appearance of the cortical plate (CP). The term preplate, which is already widely used, should be adopted for this layer. The subventricular zone appears as a distinctive proliferative layer before the emergence of the CP, earlier than previously recognized. There is no distinct cell-sparse layer under the pial surface before the CP forms. Thus, the term marginal zone should be used only after the appearance of the CP, to refer to the residual superficial part of the preplate, which becomes the layer 1 of the mature cortex. The term intermediate zone (IZ) has been used in various ways. In future it should be reserved for the heterogeneous compartment that lies between the proliferative layers and the postmigratory cells above. The IZ contains radially and tangentially migrating cells and a thickening layer of extrinsic axons that eventually constitutes the white matter. The subplate (SP) is a distinct and functionally important transient layer, located directly below the cortical plate, which was not recognized by the Boulder Committee. In rodents and carnivores most SP neurons are born before the first CP cells. In humans, preplate cells also contribute to the SP, but its substantial thickening at later stages probably involves the addition of later-born neurons. In 1970 the Boulder Committee met to standardize the nomenclature used to describe the developing human cortex. Bystron and colleagues describe how new insights since that time have led to the need to revise this nomenclature, and provide their recommendations. In 1970 the Boulder Committee described the basic principles of the development of the CNS, derived from observations on the human embryonic cerebrum. Since then, numerous studies have significantly advanced our knowledge of the timing, sequence and complexity of developmental events, and revealed important inter-species differences. We review current data on the development of the human cerebral cortex and update the classical model of how the structure that makes us human is formed.