Neuroglia: a reevaluation of their origin and development.

The development of the central nervous system (CNS) is often considered to consist of three main stages: cell proliferation, cell migration and a final phase involving cytological differentiation into mature cells. The development of neurons is frequently presented in this context but such a framework is as applicable to neuroglia as to neurons. The breakdown of glial development in this manner provides a framework for analyzing interactions of different stages upon one another and for comparing glial to neuronal development. The present chapter examines gliogenesis in terms of these three stages. Emphasis is placed on glial proliferation in normal development and in experimentally altered nervous tissue, primarily because most of our information about gliogenesis relates to cell proliferation. The final section of this chapter deals with the regulation of glial development by neurons and hormones.