The evolutionary origins of glia

Abstract

The evolutionary origins of glia are lost in time, as soft tissues rarely leave behind fossil footprints, and any molecular footprints they might have been left we have yet to decipher. Nevertheless, because of the growing realization of the importance glia plays in the development and functioning of the nervous system, lessons we can draw about commonalities among different taxa (including vertebrates) brought about either from a common origin, or from common adaptational pressures, shed light on the roles glia play in all nervous systems. The Acoelomorpha, primitive interstitial flatworms with very simple cellular organization and currently at the base of the bilaterian phylogeny, possess glia-like cells. If they indeed represent the ancestors of all other Bilateria, then it is possible that all glias derive from a common ancestor. However, basal taxa lacking convincing glia are found in most major phyletic lines: urochordates, hemichordates, bryozoans, rotifers, and basal platyhelminths. With deep phylogenies currently in flux, it is equally possible that glia in several lines had different origins. If developmental patterns are any indication, glia evolved from ectodermal cells, possibly from a mobile lineage, and even possibly independently in different regions of the body. As to what functions might have brought about the evolution of glia, by-product removal, structural support, phagocytic needs, developmental programming, and circuit modulation may be the more likely. Explaining possible cases of glial loss is more difficult, as once evolved, glia appears to keep inventing new functions, giving it continued value even after the original generative need becomes obsolete. Among all the uncertainties regarding the origin of glia, one thing is certain: that our ideas about those origins will change with every rearrangement in deep phylogeny and with continued advances in invertebrate molecular and developmental areas.

Leptin regulates glutamate and glucose transporters in hypothalamic astrocytes

“…Astrocytes appeared evolutionarily subsequent to neurons, augmenting in abundance and complexity in parallel with increasingly complex brain functions (1)(2)(3), which supports the concept that they play a more sophisticated role than previously considered. Indeed, these glial cells are fundamental for normal brain development and function, as they modulate neuronal proliferation, survival and metabolism, synaptogenesis, and synaptic transmission and maintain local extracellular homeostasis, with new and more complex functions continuing to be described (2,3).…”

You Do Not Mess with the Glia

“…Persistence and inquisitiveness did bring the field a long way in understanding the actual roles of neurons and glial cells in animals. However, one simple, repeated observation could, or should, have raised much earlier suspicions of a fundamental role of glial cells in the nervous system: that every animal known to have neurons that are well differentiated from the ectoderm also has glial cells of some sort [3]. No matter what criteria are used to define glia, whatever their evolutionary or developmental origins, whether conserved across all animal clades, the fact remains that neurons do not come alone.…”

“…The developmental mechanisms that originate glial cells do appear to be shared and conserved across vertebrates, or even invertebrates, going back at least 500 million years [3][4][5][6][7]. Moreover, across a wide range of mammalian species, new evidence indicates that the mechanisms that regulate glial cell diversity and how they are added to brain tissue are indeed highly conserved in evolution.…”

The homeostatic astroglia emerges from evolutionary specialization of neural cells

“…In the human brain, the total number of glial cells is more or less similar to that of neurons, although with prominent regional differences [7,8]. Evolutionarily, emergence of neuroglia coincided with the appearance of the centralized nervous system; there is little sign of supportive cells in the diffuse nervous system [9]. The proto-astrocytes are well-defined in round worms, where they contribute to the development and maintenance of the nervous system (especially its the sensory arm; the glial cells form sensory organs of the worm, known as sensillas), although artificial ablation of this primeval glia does not exterminate the animal [10,11].…”