Gliogenesis

Glia, like neurons, develop from relatively few cells that form a primitive epithelium. During maturation, glial precursors proliferate, migrate, develop processes, and interact with nearby and more distant cells. In most regions, glial and neuronal precursors could not be distinguished during early stages by morphological, physiological, or cytochemical criteria. Distinguishing features appeared during subsequent cytodifferentiation. Primitive glia and neurons became identifiable by differences in cellular size, shape, and content of nucleus and cytoplasm. Differences in cytoplasmic constituents, surface membrane components, and physiological properties also became evident. Junctional complexes differing in structure and function appeared and were thought to have an important role in intercellular exchange and the development of permeability barriers. Relatively late in development, oligodendroglia and Schwann cells could be identified. They became associated with neuronal perikarya and axons and some of them formed all of the myelin sheaths found in central and peripheral nervous tissue.