More than a century and a half ago, Virchow (1846) introduced the word "neuroglia" to describe the tissue that fills the space between the nerve elements in the brain. Although simplistic when compared to our current concepts of neuroglia, the initial definition was correct. Almost four decades passed before the pioneering staining techniques developed by Golgi (1885) paved the way for others in the early 1900s to describe the morphology of the basic cell types that make up the neuroglia: astrocytes, oligodendrocytes, microglia, and ependymal cells.

With regard to establishing functions for neuroglia, His (1889) suggested that embryonic glial cells were responsible for guiding the migration of developing neurons to their final destination within the brain. In 1907, Lugaro proposed that adult astrocytes police the

Encyclopedia of the Human Brain Volume 2

Copyright 2002, Elsevier Science (USA).

All rights reserved.

interstitial milieu and maintain it in a state compatible for neuronal function. He also postulated that astrocytes were the cells responsible for the chemical degradation and uptake of substances released by neurons, which allowed communication and excitation, thus establishing the basis for synaptic transmission of nerve impulses. The concept that glial cells and their processes insulate nerve fibers to enhance neural impulse transmission was proposed in 1909 by Ramon y Cajal. He further postulated that glial cells fill the spatial void created by pathologic neuronal death, setting the precedent for gliosis.

In the 1920s, del Rio Hortega, by use of an innovative and selective silver impregnation technique, was the first to give a detailed morphological description of oligodendrocytes. He also identified oligodendrocytes as the cells that produce the myelin sheath that enwraps the axons of central nervous system neurons. In 1932, by use of the same silver impregnation technique, Rio Hortega also gave the first morphological description of microglia. Unlike astrocytes and oligodendrocytes, microglia have markers normally associated with hematopoietic mono-cytes. The origin, lineage, and mode of differentiation of microglia are incompletely understood.

Beginning in the 1950s, electron microscopy yielded information about the ultrastructural characterization of the various organelles in glial cells. From these studies came the finding that fibrous and protoplasmic astrocytes contain gliofilaments, which were subsequently determined to be intermediate filaments containing glial fibrillary acidic protein (GFAP) and vimentin. Although not all astrocytes in the normal brain immunochemically stain positive for GFAP, especially those in the gray matter, GFAP immunor-eactivity is a major characteristic used to identify astrocytes.

Later in the 20th century, the focus of neuroglial study shifted toward the understanding of various interactive processes that occur between different glial cell types and neurons. These interactions are required to establish, maintain, and regulate normal brain functions and to determine how they may contribute to pathology. In synaptic and nonsynaptic transmission of nerve impulses, neurons and glia were shown to communicate reciprocally. In nonsynaptic regions of the brain, neuron to glia and glia to glia signals were shown to be mediated by neurotransmitters. Prominent among other molecules that are involved in interactive signaling between glia and other cell types of the brain are growth factors, neurotrophic factors, hematopoietic factors, cytokines, and chemokines. In the following sections, the morphology and functions of the subtypes of cells, which make up the "neuroglia," are described along with cellular activities associated with pathology or specific diseases.

Conquering Fear In The 21th Century

Conquering Fear In The 21th Century

The Ultimate Guide To Overcoming Fear And Getting Breakthroughs. Fear is without doubt among the strongest and most influential emotional responses we have, and it may act as both a protective and destructive force depending upon the situation.

Get My Free Ebook

Post a comment