The natural history of neuroglia: an agenda for comparative studies

Neuroglia, which are the non-neuronal cells of the nervous system, except for meninges and vasculature with its endothelium, blood vessel walls and relatively rare smooth muscle cells, have received much attention in mammalian investigations and occupy a voluminous literature. The term has come to be understood as embracing at least three diverse types of cells: astrocytes, oligodendroglia and microglia, each of which might be heterogeneous in some degree. Schwann cells ensheathing the medullated axons in the PNS resemble oligodendrocytes in the CNS and might be considered a fourth cell type, distinct from the connective tissue elements of the sheaths of bundles of axons or nerves. Although no one would claim we know enough about neuroglia in the best studied animals, the fact I want to point out here is that we know far less about these classes of cells in non-mammalian vertebrates and invertebrates. My thesis is that we may be overlooking major clues to the understanding of their roles and functions that could come from relatively modest effort to uncover their natural history in animals with less complex brains among the lower vertebrates and invertebrates. The techniques and findings that have been worked out in laboratory mammals can be expected to facilitate greatly the discerning of commonalities and differences in less familiar animals.