Origin
Microglial cells differentiate in the bone marrow from hematopoietic stem cells, the progenitors of all blood cells. During hematopoiesis, some of these stem cells differentiate into monocytes and travel from the bone marrow to the brain, where they settle and further differentiate into microglia.
Monocytes can also differentiate into myeloid dendritic cells and macrophages in the peripheral systems. Like macrophages in the rest of the body, microglia use phagocytic and cytotoxic mechanisms to destroy foreign materials. Microglia and macrophages both contribute to the immune response by acting as antigen presenting cells, as well as promoting inflammation and homeostatic mechanisms within the body by secreting cytokines and other signaling molecules.
In their downregulated form, microglia lack the MHC class I/MHC class II proteins, IFN-γ cytokines, CD45 antigens, and many other surface receptors required to act in the antigen-presenting, phagocytic, and cytotoxic roles that hallmark normal macrophages. Microglia also differ from macrophages in that they are much more tightly regulated spatially and temporally in order to maintain a precise immune response.
Another difference between microglia and other cells that differentiate from myeloid progenitor cells is the turnover rate. Macrophages and dendritic cells are constantly being used up and replaced by myeloid progenitor cells which differentiate into the needed type. Due to the blood brain barrier, it would be fairly difficult for the body to constantly replace microglia. Therefore, instead of constantly being replaced with myeloid progenitor cells, the microglia maintain their status quo while in their quiescent state, and then, when they are activated, they rapidly proliferate in order to keep their numbers up. Bone chimera studies have shown, however, that in cases of extreme infection the blood–brain barrier will weaken, and microglia will be replaced with haematogenous, cart-marrow derived cells, namely myeloid progenitor cells and macrophages. Once the infection has decreased the disconnect between peripheral and central systems is reestablished and only microglia are present for the recovery and regrowth period.
Read more about this topic: Microglia
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