The human transient subpial granular layer: an optical, immunohistochemical, and ultrastructural analysis.

The cytological features, origin, migration, and fate of the subpial granular layer cells of the human embryonic cerebral cortex are studied with light and electron microscopy, Golgi impregnations, and immunocytochemical staining with the microtubule associated protein 2 and glial fibrillary acidic protein antibodies. Subpial granular layer (SGL) cells form a distinct neuronal population in the molecular layer, characterized by a small dark nucleus with abundant chromatin clumps and prominent nucleoli, and a lightly stained cytoplasm containing few organelles. Somata and processes of SGL cells are intensively stained with microtubule-associated protein 2 antibody but do not express glial fibrillary acidic protein antibody. These cells apparently originate from the olfactory germinative zone. They follow two major strands from the olfactory subventricular zone to the subpial region. Subsequently, they migrate tangentially at the subpial level to all cortical regions, as is observed on Golgi and ultrastructural preparations. They constitute a transient population that penetrates the deep molecular layer and subsequently disappear from it. Several cytological features of these cells suggest an inward migration with growth of a radial process toward the cortical plate and subsequent nuclear translocation. The fate and the role of this new phylogenetic neuronal population has yet to be determined although the abundance of degenerating SGL cells in the deep molecular layer suggests at least partial degeneration.