[Neocortex in the human embryo and fetus. Electron microscopic and Golgi staining study].
The telencephalic vesicle (or hemisphere) of human embryos and fetuses has been studied by electron microscopy and Golgi impregnations. Our observations confirm recent ones on various laboratory animals and revise some concepts of corticogenesis. At 4/6 weeks, the neural tube was a single pseudostratified epithelium in which mitoses occurred. At 7 weeks the telencephalic vesicle was made up of two layers, the ventricular layer or matrix and the outer plexiform or marginal layer. The first cells, appearing in the plexiform layer before any cortical plate could be identified, had the morphological attributes of Cajal-Retzius cells. In this primitive layer we found the earliest synapses yet described in human embryos. From 8 weeks on, the cortical plate was formed by successive waves of cells, migrating from inside-out. The next synapses found were in the subcortical layer. On Golgi impregnations the Cajal-Retzius cells were already visible at 15 weeks, and from 20 weeks on they presented the usual morphological characteristics.