Synchronous development of pyramidal neurons across the human frontal cortex during the perinatal period

The principal neurons in associative areas of the human frontal cortex show intensive dendritic outgrowth and elongation during the perinatal period. In this study, we quantitatively analyzed the dendritic tree development and spine formation on the rapid Golgi impregnated layer III pyramidal neurons in prospective motor, Broca´s, premotor and prefrontal areas ranging from the 32nd postconceptional week until the 3rd postnatal month in human. At the 32nd postconceptional week the pyramidal neurons already show typical morphological features: the triangular cell body shape, as well as the prominent apical dendrite reaching the marginal zone with the bifurcating terminal tuft. The total number of basal and oblique dendrites is already reached at that stage. After basic morphology is established the two sharply segregated stages will follow: the first stage is characterized mainly by the outgrowth of new segments and the second by dendritic elongation. On average, neurons in primary regions have the same level of maturation as neurons in high order associative areas. Within the areas analyzed, large differences in the level of maturation among neurons were observed with some of the neurons already at 60% of adult values. Our data do not support the traditional view of hierarchical development from primary to high order areas, but rather suggest cross-areal asynchronous maturation of the neurons inside the cortico-cortical network in the early period of infancy. Such maturation pattern may represent a neurobiological basis in the sequential development of the initial cognitive functions that are already present in newborns and young infants.