Cells in focusGlial cells more than support cells?
Introduction
The nervous system is composed of two major cell types: neurons and glia. Strikingly, glial cells constitute 90% of cells in the human brain. The name of glia originates from Greek for glue, but now we know glia do not just hold nerve cells together and thus should not just be considered as supportive cells to neurons in the nervous system. Instead, recent compelling studies indicate that glial cells play more active and essential roles in brain development and brain function than previously acknowledged (Allen & Barres, 2005; Doetsch, 2003, Haydon, 2001; Seifert, Schilling, & Steinhauser, 2006).
There are three types of glial cells in the central nervous system (CNS): astrocytes, oligodendrocytes and microglia. Astrocytes are the most abundant glial cells with irregular star-shaped cell bodies and broad end-feet on their processes. They interact extensively with neurons and provide them with important structural and metabolic support. Oligodendrocytes have relatively small amounts of cytoplasm around the nucleus but have several processes which wrap themselves around axons to form myelin sheaths. Microglia are the smallest of the glial cells which act as phagocytes, the immune cells of the CNS, to clean up debris.
Section snippets
Cell origin and plasticity
The mammalian cerebral cortex originates from a single layer of proliferating neuroepithelial cells. These neural progenitor cells (NPC, a subpopulation of which is also referred to as neural stem cells, NSC) line the ventricles, forming the ventricular zone (VZ). NPCs in the VZ proliferate and sequentially give rise to three major cell types of the brain: neurons, astrocytes, and oligodendrocytes (Sauvageot & Stiles, 2002; Sun, Martinowich, & Ge, 2003). Unlike astrocytes and oligodendrocytes,
Functions
One of the well-known functions of glial cells is to help form an impermeable lining in the brain's capillaries and venules, the blood-brain barrier, that prevents toxic substances in the blood from entering the brain. In addition, glia control the levels of neurotransmitters such as glutamate and dopamine. For example, astrocytes facilitate rapid removal of synaptic glutamate following its release from the presynaptic terminal, thus prevent glutamate-mediated neurotoxicity which can induce
Associated pathologies
Astrocytes underlie the pathological states of reactive gliosis and glial scar formation accompanying nerve injury (Sofroniew, 2005). Gliosis and neuronal loss in certain brain regions are seen in a variety of neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease and Amyotrophic lateral sclerosis (ALS) (Boillee, Vande Velde, & Cleveland, 2006). Microglia are believed to be responsible for producing neuroinflammation in these conditions (Hoozemans, Veerhuis, Rozemuller, &
Acknowledgements
We thank Dr. Volkan Coskun and Ryan Schmidt (University of California, Los Angeles) for critical reading of the manuscript and members of the Sun laboratory for helpful discussions. We apologize for our inability to include and discuss all the studies in this area within the limitations of this article. FH was supported by UCLA Dissertation Year Fellowship. This work is supported by NIH RO1 grant MH066196 and NIH project grant HD006576.
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