Laminar Organization of the Marginal Zone in the Human Fetal Cortex

Abstract

Background: Recent studies have shown remarkable complexity of neuronal differentiation and cellular composition of the marginal zone in the fetal cortex. New immunocytochemical parameters are needed for analysis of dynamic neurogenetic events at the interface between the afferent fibers of the marginal zone and the superficial cortical plate. Aims: The aim of this study was to analyze transient cytoarchitectonic and immunocytochemical patterns of organization of the marginal zone of the human fetal cortex. Results: Fine cytoarchitectural parameters revealed a very complex six-layered organization of the marginal zone of the human fetal cortex between 18 and 28 weeks of gestation (w.g.). Starting from the pia to the cortical plate, the following layers can be distinguished: (1) cell-poor marginal stripe (Randstreifen); (2) subpial granular layer; (3) marginal zone proper; (4) stratum lucidum (SL); (5) deep granular layer (DGL), and (6) stratum radiatum (SR). The subpial granular layer develops around 13 w.g. and is not visible at 34 w.g. The SL, characterized by immunostaining with SNAP-25, MAP1b and SMI 312 antibodies and acetylcholinesterase histochemistry, appears on Nissl sections at around 20 w.g. and disappears after 28 w.g. The DGL contains NeuN and some MAP2-positive cells, presumably neurons, appears at 15 w.g. and disappears by 34 w.g. The SR shows profound transformation in parallel to the changes in the most superficial part of the cortical plate. Conclusions: The marginal zone displays a very complex organization in the human fetal cortex due to the complexity of cortical inputs from both increased population of Cajal-Retzius neurons and abundant afferents to the apical dendrites of cortical plate neurons. Deep transitional sublayers, namely SL, DGL and SR, serve as a compartment for ingrowth of afferents and migratory routes to the superficial part of the cortical plate. A similar developmental role is played by the subplate zone at the deep border of the cortical plate.