Where do my Brain Macrophages come from?

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When I was a scientific ameoba the big question was what are the antigen presenting cells in the brain. Those around blood vessesl were the most likely first point of contact for an invading white blood cell. Back in the day many people called them pericytes. Back in the day people used to say they were derived from smooth muscle. This is a type of muscle distinct from heart and skeletal muscule in your muscles. I used to think this was a load of Jajce and called them perivascular (around blood vessels) microglia or perivascular macrophages as I thought they were more macrophage like. It seems that both ideas are partially correct. The macrophages derive from macrophages resident in the brain at birth but they gain characteristics because of contact with smoth muscles…You live and learn.

Cells | Free Full-Text | Pericytes, Mesenchymal Stem Cells and the Wound  Healing Process | HTML

Masuda T et al. Specification of CNS macrophage subsets occurs postnatally in defined niches. Nature. 2022. doi: 10.1038/s41586-022-04596-2. 

All tissue-resident macrophages of the central nervous system (CNS)—including microglia, as well as CNS-associated macrophages (CAMs) such as meningeal and perivascular macrophages are part of the CNS endogenous innate immune system that acts as the first line of defence during infections or trauma. It has been suggested that microglia and all subsets of CAMs are derived from prenatal cellular sources in the yolk sac that were defined as early erythromyeloid progenitors…Here it is shown, that only meningeal macrophages (on the edge of the brain and spinal cord) and microglia share a common prenatal (pre birth) progenitor. By contrast, perivascular (sat round the blood vessel) macrophages originate from perinatal meningeal macrophages only after birth in an integrin-dependent manner. The establishment of perivascular macrophages critically requires the presence of arterial vascular smooth muscle cells (Smooth muscle rings around the arterial vessel..but veins lack them). Together, our data reveal a precisely timed process in distinct anatomical niches for the establishment of macrophage subsets in the CNS.

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