Early areal differentiation of the human cerebral cortex: entorhinal area

The early cytoarchitectonic specialization and area-specific differentiation of the prospective entorhinal cortex were studied in the postmortem human fetal brains (9.5-13.5 postovulatory weeks). At 10 weeks, using the Golgi method, we saw the appearance of area-specific large neurons (promoter neurons) with widely bifurcating apical dendrites situated at the outer margin of the cortical plate of the prospective entorhinal cortex. The analysis of the serial Nissl-stained sections revealed the concomitant appearance of a one-cell-thick layer (monolayer) at the interface between the cortical plate and the marginal zone and multilaminated spread of the deep part of the cortical plate. This is the earliest sign of area-specific cytoarchitectonic differentiation of all pallial regions characterized by the presence of the typical cortical plate. The first subareal differentiation within the entorhinal cortex begins at 13 postovulatory weeks with uneven development of fiber-rich lamina dissecans, which separates two cellular laminae principals (externa and interna), and with the appearance of characteristic cell islands of the prospective layer II. At rostral levels, cell islands begin to develop in the rostromedial entorhinal area at the subpial depths where large promoter neurons reside. At intermediate levels, both lamina dissecans and lamina principalis interna are well delineated. At caudal levels, lamina principalis interna is continuous with the upper subplate zone of the adjacent neocortex. Both area-specific neurons (promoters) and fiber-rich (afferent) strata develop synchronously during the earliest areal differentiation of the cerebral cortex. The precocious lamination of the cortical plate is the crucial event in the histogenesis of the entorhinal cortex.