Constructing the human cerebral cortex during infancy and childhood: Types and numbers of cortical columns and numbers of neurons in such columns at different age‐points

Abstract

This study examines JL Corel's data on neuron numbers in 35 human cortical areas for eight age points from 0 (birth) to 72 months, to analyze cortical columns, the presumed functional units of the cortex. For each cortical area at each age point, cortical surface divided by the square root of the area's neuron number gives cross‐sectional areas with radii ranging from 180 um at birth to 250 pm at 72 months. For the prefrontal cortex at birth and 48 months, these radii are approximately 2.10 and 1.19 times the longest radial basal dendrites, suggesting similar dimensions between these two measures of column radius. The logarithm of neuron number per cortical area and age point was examined in relation to the Weber‐Fechner law governing the relationship between stimulus intensity and perception. A mechanism for this law consistent with the cortical model of Douglas et al. illustrates the importance of local circuit neurons. The cross‐sectional areas of hexagonal columns for prefrontal cortex, using as radius, the longest radial extent of layer 5 pyramidal neuron basal dendrites, ranging from 0.013 mm‐ at birth to 0.064 mm² at 48 months, suggests that functional cortical columns increase cross‐sectional area during development. These cross‐sectional areas are55–100‐fold larger at birth, and229–277‐fold larger at 48 months, than those computed from somal width in prefrontal, layer 5 pyramidal neurons. Comparison of radial extent of pyramidal basal dendrites to their soma‐to‐soma distances shows that layer 3 pyramidal basal dendrites reach 1.5 and 4.0 other pyramidal neurons at 15 and 60 months, respectively, while layer 5, extra‐large pyramidal basal dendrites reach 1.14 and 1.72 other such neurons at birth and 48 months, respectively. If such a relationship holds for other cortical areas, then the Conel data can be used to estimate basal dendrite extent, for which there currently is a paucity of data.