© SCOTT LEIGHTON
Tissue-resident macrophages, which not only respond to local assaults but also function in normal development and physiology, originate in the yolk sac of the embryo and mature in one particular tissue in the developing fetus, where they acquire tissue-specific roles and change their gene expression profile. By contrast, circulating macrophages are produced throughout life by the bone marrow, then released into the vasculature to respond to infections and injury.
Circulating macrophages (called monocytes) primarily patrol the body for infection, but they can also specialize to perform tissue-resident roles, replacing embryonic macrophages that die. Some scientists believe this macrophage replacement may contribute to aging.
In the developing brain, macrophages called microglia release CD95L (orange triangles) and other signals that bind the CD95 receptor (blue shapes) on blood vessels and neurons, stimulating them to grow and branch, respectively. They also orchestrate a pruning process, so...
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