Figure 1.

Vascular macrophages and monocytes in the healthy mouse aorta. Influx of EMP-derived macrophages into the tissue during embryogenesis starts around embryonic day 9.5. Macrophages settle within the aortic adventitia and sustain solely through local proliferation. Around embryonic day 18.5, monocytes from the bone marrow seed the aorta and differentiate within the adventitia (forming a population of BM-derived tissue resident macrophages) as well as within the intima (forming a separate population of intima-resident macrophages). This recently defined population of intimal macrophages is heavily seeded perinatally but maintains solely through local proliferation. Thus, the adventitia is colonised with macrophages of dual origin which self-sustain numbers through proliferation, and in the case of adventitial BM-derived cells, replenishment from circulating monocytes. The number of cells of different ontogeny varies throughout life, with numbers changing in age adopted from (37). The fate of monocytes migrating into the steady-state aorta follows several possible fates: differentiation into BM-derived macrophages, further migration towards lymphatics, apoptosis, or migration back into circulation.