EARLY AREAL DIFFERENTIATION OF THE HUMAN CEREBRAL-CORTEX - ENTORHINALAREA

The early cytoarchitectonic specialization and area-specific different iation of the prospective entorhinal cortex were studied in the postmo rtem human fetal brains (9.5-13.5 postovulatory weeks). At 10 weeks, u sing the Golgi method, we saw the appearance of area-specific large ne urons (promoter neurons) with widely bifurcating apical dendrites situ ated at the outer margin of the cortical plate of the prospective ento rhinal cortex. The analysis of the serial Nissl-stained sections revea led the concomitant appearance of a one-cell-thick layer (monolayer) a t the interface between the cortical plate and the marginal zone and m ultilaminated spread of the deep part of the cortical plate. This is t he earliest sign of area-specific cytoarchitectonic differentiation of all pallial regions characterized by the presence of the typical cort ical plate. The first subareal differentiation within the entorhinal c ortex begins at 13 postovulatory weeks with uneven development of fibe r-rich lamina dissecans, which separates two cellular laminae principa les (externa and interna), and with the appearance of characteristic c ell islands of the prospective layer II. At rostral levels, cell islan ds begin to develop in the rostromedial entorhinal area at the subpial depths where large promoter neurons reside. At intermediate levels, b oth lamina dissecans and lamina principalis interna are well delineate d. At caudal levels, lamina principalis interna is continuous with the upper subplate zone of the adjacent neocortex. Both area-specific neu rons (promoters) and fiber-rich (afferent) strata develop synchronousl y during the earliest areal differentiation of the cerebral cortex. Th e precocious lamination of the cortical plate is the crucial event in the histogenesis of the entorhinal cortex.