Discussion

Abstract

The aim of this study was to investigate when and where nonradial glial cells develop in the embryonic neocortex. Therefore, our work had to be concerned, first, with cytological criteria by which immature glial cells can be recognized and, second, with the temporospatial sequence of zones developing in the “pallial anlage” (Rickmann et al. 1977). Our main findings are the following:

• 1. The earliest nonradial glial cells are found in the marginal zone, i.e., nonradial glial cells belong to the earliest-born cells of the neocortex. It should be emphasized that the marginal zone, being the substratum in which the cortical plate develops, has both a neuronal and a glial complement of cells. This is in agreement with the view that the marginal zone represents a “pallial anlage” (Rickmann et al. 1977).

• 2. The cytological criterion by which nonradial glial precursors can first be distinguished from preneurons is their polarity with respect to the surfaces of the neuroepithelium. Even the earliest glial precursors have lost their ventricular contacts while remaining in contact with the pial surface. In this way they differ from ventricular and radial glial cells (which maintain both kinds of contacts) and from preneurons (which have lost both).

• 3. AS the glial cell population of the marginal zone matures, its superficial portion gives rise to glial precursors, in lamina I, which are quite distinct from neighboring neurons. We have called these cells quiescent presumptive astrocytes because they are characterized by a period of arrested cell proliferation, pial contacts, bundles of intermediate filaments, and expression of both GFAP and C 1-antigen.

• 4. The deep marginal zone gives rise to the subcortical zone of the neocortex. It contains a population of proliferating cells which have been termed polymorphic cells. Their cytoplasm appears immature, lacking bundles of intermediate filaments and a specialized form of RER. However, some of the cells provide lamellar sheaths around preneurons, while others have vascular contacts or take part in phagocytosis. Thus, at least some of the polymorphic cells can be interpreted as glial. Polymorphic cells are the main constituent of the prenatal subventricular zone. They appear to be a major source of glial cells in the intermediate zone, and probably also in the cortical plate.

• 5. Immature glial cells are involved in a number of special intercellular relationships, the most apparent being that between nonradial and radial glial cells which are interconnected by specialized intercellular contacts and seem to form a glial template throughout the developing neocortex.

• 6. Prenatal glial cell populations mature heterochronously, expressing different features at different locations of the neocortex at different times. In general, cytoplasmic features of young glial cells seem to appear later than their cell surface properties do.