Ontogenesis of the laminar structure in areas 17 and 18 of the human visual cortex. A quantitative study.

A quantitative morphological study of the pre- and postnatal development in the primary (area 17) and secondary (area 18) visual cortical regions was performed on 108 human brains. The neuropil proportion and thickness were measured with an image analyzer for the different cortical layers and the resulting data were approximated with logistic growth functions. The different layers show a marked heterochrony both within and between the areas. The neuropil proportion of layer I is the compartment to develop first in both areas. It has the lowest growth velocity, followed by layer VI and layers V, IV, III and II. This maturational sequence reflects the sequence of appearance of immature neurons during the migration period of neocortical ontogenesis. The development of the neuropil proportion is highly synchronized between areas 17 and 18 during the prenatal period, but in the first postnatal weeks, area 17 grows more quickly than area 18. Later on, this relation is reversed and area 18 reaches adult values of neuropil proportions about three months earlier than area 17. The growth in thickness of all layers is completed later than the growth in neuropil proportion. The growth in layer thickness is completed in Area 18 about two months earlier than in area 17, although area 18 has a greater cortical thickness. The results are compared with data on growth in volume, dendritic arborization and the development of visual function.