Postnatal Gliogenesis in the Mammalian Brain

This chapter summarizes the recent advances on postnatal gliogenesis obtained by electron microscope and radioautography and indicates the possibility that apparently mature macroglial cell types transform into each other or into microglial cells needs to be explored.

Abstract

This chapter summarizes the recent advances on postnatal gliogenesis obtained by electron microscope and radioautography. The electron microscope has provided precise criteria for the identification of glial cells, and has given several indications of their possible function. Radioautography has permitted investigators to label immature cells through incorporation of labeled nucleotides, and to follow their ultimate fate. Evidence of protracted gliogenesis throughout life, in several mammalian species, has been firmly established, and a common origin for astrocytes and oligodendrocytes from a neuroectodermal precursor has been demonstrated. It has also been shown that the production of either type from the matrix layer is not random, and that astrocytes are generated earlier than oligodendrocytes, although both are still produced, at different rates, in adult rats. The origin of microglial cells is not so evident; nevertheless, several points have been clarified. Brain macrophages may originate from several sources and, when extensive necrosis occurs, blood leukocytes are always involved. In other cases indigenous elements are responsible for phagocytosis, and they most likely represent microglial cells. These might in turn originate from mesenchymal elements of the brain, or from neuroectodermal cells. The neuroectodermal origin of microglial cells needs to be unequivocally demonstrated. The possibility that apparently mature macroglial cell types transform into each other or into microglial cells needs to be explored.