Components of the nervous system Components of the nervous system
Estimates of the number of glial cells in the central nervous system vary so widely that it is safe to conclude only that we do not know how many there are. In crickets, for example, they are suggested to outnumber neurons by as much as 8:1 in the central nervous system and to account for as much as half of its volume whereas in the brain of bees they are suggested to represent only 15% (∼19 000 glial cells in a worker bee) of the total number of cells within the Neuropil. In Drosophila, the gene repo is expressed in most glial cells but not in neurons of the developing nervous system. In an abdominal ganglion, it is present in only 60 glial cells. Lack of knowledge of their numbers is matched only by the paucity of information about the structure of the different types of glial cell and of the role that they might be playing in the functioning of the central nervous system. The shining exception to this is the considerable amount of information on the glial cells of the perineurium that surrounds the central nervous system and forms a barrier with the haemolymph.
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