Recent Advances in the Study on Early Development of the Central Nervous System : When Do Glial Cells Appear in CNS?

Abstract

There are two opposing concepts about the time of appearance of glial cells in the central nervous system (CNS). According to His, cells belonging to neuroglial cell lineage coexist with cells forming neurons from beginning of its development. On the other hand, from the results of autoradiographical study Fujita proposed that development of CNS takes place stepwise, and noted that glial cells are produced first after the production of neurons ends. Recently, Rakic and others claimed that migration of neuroblasts from the matrix layer to the cortical plate took place along the cell body of a kind of glial cells ("radial glia"), which were thought to exist in the time of neuroblast formation, and they insisted that glial cells exist in earlier stages than suggested by autoradiographical study. Fujita and his co-workers pointed out, however, that there has been no convincing evidences indicating glial nature of the radial elements in the stage of neuroblast-formation. From the findings of SEM-fractography they concluded that the bundle of processes of the matrix cells may play the role of guidance for the migration of neuroblasts. Using EM-autoradiography we have investigated transition of morphology glial cells retrospectively from adult age to embryonic stage, and could identify their progenitor cells, immature glioblasts, in stage III. These facts suggest strongly that no glial cells exist at the time of neuroblast-formation and before it.