Introduction

Virchow (1846) was the first to find cells other than neurons in the brain. He thought that it was the connective tissue of the brain, which he called ''nervenkitt'' (nerve glue) (i.e., neuroglia). The name survived, although the original concept radically changed. Two main types of glial cells were originally described in 1893 by Andriezen: protoplasmic in gray matter and fibrous in white matter. They were both later proven to be astrocytes and their first clear description was provided by Ramon y Cajal in 1913; he showed that they were independent from neurons and capillaries. In 1928, using staining techniques based on silver carbonate impregnation, as introduced by Golgi, Rio Hortega found two other cell types, the oligodendrocyte (first called interfascicular glia) and another cell type that he distinguished from the two macroglial cells (i.e., macroglia) and that he called microglia.

Astrocytes with oligodendrocytes and microglia constitute the glial cells (Fig. 1) in the central nervous system (CNS). The importance of glial cells is suggested by their increasing number in relation to the evolution of species, from 25% in the Drosophila brain to 65% in murine species and 90% in man. Astrocytes

Figure 1 Schematic representation of the different types of glial cells in the CNS and their interactions among themselves and with neurons. Astrocytes are stellate cells with numerous processes contacting several cell types in the CNS: soma, dendrites, and axons of neurons and soma and processes of oligodendrocytes and other astrocytes. Astrocytic feet also ensheath endothelial cells around blood capillaries, forming the blood-brain barrier, and terminate to the pial surface of the brain, forming the glia limitans. Oligodendrocytes are the myelinating cells of the CNS. Many interactions between glial cells, particularly between astrocytes in the mature CNS, are regulated by gap junctions, forming a glial network (adapted from C. Giaume and L. Venance, 1995, Perspect. Dev. Neurobiol. 2, 335-345).

Figure 1 Schematic representation of the different types of glial cells in the CNS and their interactions among themselves and with neurons. Astrocytes are stellate cells with numerous processes contacting several cell types in the CNS: soma, dendrites, and axons of neurons and soma and processes of oligodendrocytes and other astrocytes. Astrocytic feet also ensheath endothelial cells around blood capillaries, forming the blood-brain barrier, and terminate to the pial surface of the brain, forming the glia limitans. Oligodendrocytes are the myelinating cells of the CNS. Many interactions between glial cells, particularly between astrocytes in the mature CNS, are regulated by gap junctions, forming a glial network (adapted from C. Giaume and L. Venance, 1995, Perspect. Dev. Neurobiol. 2, 335-345).

present a wide range of functions: They participate to brain construction and energy metabolism, and they also participate in the formation and maintenance of the blood-brain barrier. Their organization and functions are modulated by circadian rhythms, parturition, lactation, and osmotic stimulation. They respond to many changes in the cellular and extracellular environment. Astrocytes are also directly or indirectly involved in pathological states.

The diverse roles of these cells are still being unraveled and may be more complex than previously thought. Indeed, recently it was shown that astrocytes possess a calcium-dependent form of excitability, and that they are involved in the functional modulation of synaptic activity. The existence of a coordinated and continuous bidirectional neuron-astrocyte signaling provides a new view of the physiology of these glial cells.

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