Heterogeneity in the formation of primary and secondary visual fields during human prenatal development.

The human neocortex has a huge surface area with unique cytoarchitectonics, most of which is concealed in sulci. Some cytoarchitectonic fields are associated with macroscopic landmarks. In particular, the primary visual field 17 is associated with the calcarine sulcus. During the prenatal development of the human brain, neocortical gyri and sulci undergo changes and modifications after primary formation. To explore the morphogenetic processes in visual fields during the formation of the primary (provisional) and secondary (permanent) sulci, the occipital lobe of the human fetal brain was studied using immunohistochemical methods. The distribution of various glial and neuronal markers (S-100, β-III-tubulin, NeuN, reelin) in the calcarine sulcus and parietooccipital sulcus was compared. The heterogeneity in the formation of primary and secondary visual fields was demonstrated. The study revealed that the development of the primary visual field 17, linked with the calcarine sulcus, preceded the development of a shared anlage of fields 18 and 19 linked with the parietooccipital sulcus. The functional differentiation of the primary visual field begins during the period of thalamic afferent ingrowth. This process coincides with the temporal smoothing of the calcarine sulcus, indicating a simultaneous progression of functional specialization and structural modifications. At the late fetal period, cortical plate of gyri and sulci banks showed higher NeuN-labeling than inside the sulcus in the same cytoarchitectonic field.