Development of layer I of the human cerebral cortex after midgestation: Architectonic findings, immunocytochemical identification of neurons and glia, and in situ labeling of apoptotic cells

The development of layer I was studied in the human frontal cortex from 21 weeks of gestation (GW) to 2.5 postnatal months in series of adjacent secti ons processed for thionin staining, Bodian silver staining, and immunocytoc hemical labeling of neurons and glia. In addition, the terminal dUTP nick-e nd labeling (TUNEL) method was used to label in situ DNA fragmentation. A p rogressive decrease of cell density and the disappearance of the subpial gr anular layer (SGL) appeared as distinctive developmental features of human layer I, consistently with previous investigations. The neuronal antigen mi crotubule-associated protein(2) was found to label preferentially Cajal-Ret zius cells and dendritic processes extending from the cortical plate. At mi dgestation, the calcium binding protein calretinin stained in the marginal zone numerous neurons, including the Cajal-Retzius cells and their processe s. Calretinin-immunoreactive neurons decreased during the subsequent matura tion: such decline was abrupt in the SGL, whereas bipolar calretinin-immuno positive cells accumulated in the inner marginal zone to be presumably inco rporated into the cortical plate. Cajal-Retzius cells expressed calretinin throughout the examined developmental stages. The glial antigen vimentin wa s already expressed at midgestation, and vimentin immunopositivity decrease d progressively in cell bodies and fibers of layer I during development. Gl ial fibrillary acidic protein-positive elements gradually matured, and the positive cell bodies displayed the features of mature astrocytes at the end of gestation. Moreover, a decrease of free glial cells was observed in lay er I, suggesting their progressive incorporation into the cortical plate. T UNEL-positive cells were detected at midgestation in the marginal zone, and they were concentrated in the SGL until its disappearance; their number de creased dramatically throughout layer I after 30 gestational weeks, TUNEL-p ositive nuclei or regressive changes were not detected in Cajal-Retzius cel ls throughout the examined developmental stages. Thus, our data point out t hat naturally occurring cell death is an active mechanism contributing to t he disappearance of the SGL but not to the subsequent developmental reshapi ng of human layer I, in which, instead, migratory phenomena should play a m ajor role. In addition, our findings argue against a disappearance of Cajal -Retzius cells due to regressive processes. (C) 1999 Wiley-Liss, Inc.