Defining glial cells during CNS development

Neuroglia are non-neuronal cells in the nervous system and are involved in virtually every aspect of neural function. Because of the ambiguity of glial function, the definition of glial cells relies chiefly on structural and biochemical characteristics. The use of molecular markers in identifying glial cells along their differentiation pathways is further complicated by recent findings that many of the molecules are also expressed by cells of the neuronal lineage. So, how specific are glial markers and how can a glial cell be defined during development?