The marginal layer in the neocortex of a 7 week-old human embryo. A light and electron microscopic study

Ultrastructural study of the molecular layer of the neocortex of a 7 week-old human embryo confirms recent observations on various laboratory animals that call for revision of some classical concepts of corticogenesis. 1. At 7 weeks, the subpial, marginal or molecular layer is the first layer to differentiate from the ventricular layer and represents almost half the thickness of the telencephalic vesicle. 2. The first cells that have already migrated from the ventricular zone, even before any cortical plate is visible, are to be found in this marginal layer. These large cells are well differentiated and most probably represent the so called Cajal Retzius cells. 3. The earliest synapses ever seen in human embryo are found in the marginal or plexiform layer; this indicates the presence of a precocious set-up for an elaborate neuronal circuitry at this level.