Microglia

may have different origins, i.e. those arising from the primitive macrophages from the yolk sac and those newly differentiated from monocytes or their progenitors. Over 95% of all microglia are generated after birth and after the formation of the blood–brain barrier (BBB) and there has been an ongoing debate regarding the maintenance of the microglial population in the adult CNS. One hypothesis is that adult microglia are maintained via self-replication or by the division What are microglial cells? Microglia — from micro (small) and glia (glue) — are the resident immune cells of the brain and constantly patrol the cerebral microenvironment to respond to pathogens and damage. These cells are present throughout the central nervous system (CNS), including the spinal cord, although some regions are more populated than others, with the white matter generally containing fewer microglia than the grey matter. Microglia are highly ramified cells and their processes are very active and dynamic even under non-pathological conditions. Microglia that are found close to blood vessels seem to lose their ramifications during chronic immune challenges, becoming more amoeboid in such conditions. It is not clear whether this process depends on phenotypic changes of the resident cells or whether it is a step involved during the differentiation of blood-derived cells. Amoeboid cells are also found throughout all stages of development of the CNS. Microglial cells are found in similar numbers to neurons, representing around 10–20% of all glial cells and ranging from 100 to 200 billion cells depending on the condition (i.e., healthy, infected, diseased). In contrast to neurons, microglia can proliferate, particularly during infection and injury and in the presence of endogenously produced toxic proteins. What is the origin of microglia? Although the exact origin of microglia still remains to be fully established, both perivascular and parenchymal microglial cells and macrophages derive from myeloid progenitors. It is currently believed that parenchymal microglia originate from neuroectodermal matrix cells and that pial macrophages or mesenchymal progenitors originate from the yolk sac, establishing themselves in the brain during the embryonic stage (Figure 1). However, recent data suggest the existence of other subpopulations of microglial cells, each of which of progenitor cells already present in the brain. Another hypothesis suggests that circulating precursors are able to infiltrate the CNS and differentiate into microglial cells. Recent studies have demonstrated the capacity of bone marrow stem cells (BMSCs) to populate the CNS and differentiate into functional …