General Introduction

Abstract

The diversity of cells constituting the central nervous system did not deceive last century neurohistologists in recognizing that this organ contained essentially two cell types: the nerve cells, or as termed according to the emerging concept of neural contiguity, the neurons, and the neuroglial cells. Neurons were clearly shown to be the means of excitability, impulse generation, impulse transmission, and connectivity in the neural tissue. The neuroglia, as indicated by its name (γλια=cement or glue) given by Virchow (1860), was thought to be the cementing material ensuring the coherence of the nervous tissue, filling in the spaces of the neuropil, and isolating neuronal cell bodies. While this supposedly passive role did not attract multidisciplinary research on the neuroglia, successful efforts were made to extend our knowledge of the physiology, morphology, and biochemistry of neurons. As a result of this, the investigation of the neuroglia carried out in the first half of this century was mainly confined to morphology, often as a by-product of comprehensive analyses of neuronal systems. At any rate, the histological classification of the neuroglia was accomplished, laying a framework which has been used to the present day. Accordingly, the glia was divided into two major groups: the macro- and microglia. The former comprises two further subclasses, the astroglia and oligodendroglia. The microglia are still a matter of debate as far as its cell types and origin are concerned (Stensaas and Stensaas 1968; Vaughn and Peters 1968; Mori and Leblond 1969 a; Peters et al. 1970; Privat and Leblond 1972). Since the late 1960s electron microscopic research has repeatedly suggested (King 1968) that the astroglia and oligodendroglia are two distinct groups of ectodermal origin, while the microglia comprises glial cells derived from the mesoderm.