Evolution of primordial glial cells and glial genes in invertebrates

Glial cells were first observed in the nervous system of vertebrates as neuronal supportive cells. In recent years they have been credited for not only supporting and sustaining neuronal cells but for playing roles in testis development, neuron migration and the formation of blood brain barriers, to name a few. The glial cells increasing diversity in higher complex nervous systems, has also contributed to the belief that the emergence of glial cells gave rise to the evolution of the central nervous system. Here we examine the possibility of this theory through the study and distinguishing of glial cells in the phylum Cnidaria and the phylum Platyhelminthes, which are considered to possess one of the simplest central nervous systems in metazoans. Through the analysis of Golgi staining, In situ hybridization, transmission electron microscopy (TEM) and immunofluorescence staining we were able to distinguish glial-like cells in the metazoans Nematostella vectensis, Dugesia tigrina, Dugesia dorotocephala and Carybdea species. These glial-like cells appear to have an equivalent relationship with neuronal cells as the glial cells do in the nervous system of vertebrates.