Introduction

Abstract

The hypothesis that growth factors and hormones determine or permit the expression of the phenotypic properties that characterize different neuroendocrine cell types has gained universal acceptance. Much of the earlier work in the development of neuronal culture systems relying on different defined media and the purification of neuronotrophic factors has led to more recent advances in the separation and in vitro study of different glial populations. Our understanding of neural cell lineage and the factors that govern passage of cells to different end points of development is now more molecular as the external signals that control cellular development are isolated and analyzed in vivo and in vitro. Lastly, neurotransmitters have been shown to have trophic and tropic effects on neuronal structures that go beyond their previously recognized signalling role.