Features of the Development of the Human Cerebral Cortex during the Second Trimester of Gestation

The aim of the present work was to study patterns of the postmigration development of neurons accompanying stratification of the dorsal precentral and superior temporal areas of the neocortex of the fetal human brain. The brains of 10 fetuses at 20–26 weeks of gestation were studied; tissue blocks were embedded in paraffin and serial sections were cut, each tenth section being stained by the Nissl method. The remaining sections were immunostained with antibodies to MAP2, layer-specific proteins SATB2, FOXP1, and CTIP2, as well as reelin and N200. In four cases, additional blocks were used – these were not embedded in paraffin but were processed by the Clarity or MALDI IMS methods. From week 20 to 26 of intrauterine development, neurons immunopositive for layer-specific transcription factors in the cortical plate of the cortical areas studied were confined to different stages of the cortical plate: SATB2 + with the upper and FOXP1 + and CTIP2 + with the lower. Quantitative assessment of the levels of maximal SATB2 + , FOXP1 + , and CTIP + neuron density identified embryonic layers eII, eIII, eV, and eVI in the subplate. During the period of interest, the numbers of MAP2-immunopositive neurons in cortical plate of both areas increased. The first cluster of MAP2 + pyramidal cells was seen in 20-week fetuses in layer eV, while a cluster appeared in layer eIII by week 26. Analysis of the results obtained from immunostaining of large blocks of human fetal cerebral cortex provided evidence of a complex spatial organization of the marginal zone, with reelin-positive Cajals–Retzius cells at the surface and a fibrous plexus in the depth of the marginal zone. Overall, the data provide evidence that two anatomically distant areas of the cortex – the precentral and superior temporal – develop synchronously during the first half of the fetal period.