Figure 2.

Macrophage ontogeny.

Within tissue, two distinct macrophage types, tissue-resident macrophages and monocyte-derived macrophages, can be discriminated. While tissue-resident macrophages populate tissues before birth (prenatal origin) and proliferate locally, monocyte-derived macrophages are replenished from circulating blood monocytes (postnatal origin). Circulating blood monocytes originate from bone marrow-derived hematopoietic stem cells and give rise to classical monocytes, which then can further differentiate to intermediate and non-classical monocytes on request.