Figure 1. Macrophages in heart.

This figure shows the origin of infiltrating monocytes in the bone marrow after tissue injury. Ly6C^hi macrophages can mature and differentiate into Ly6C^lo macrophages after infiltrating tissues. In the heart at homeostasis, tissue-resident macrophages are the predominant macrophage type, which originate from the yolk sac and are low in expression of MHC II (MHC II^lo); during periods of stress in the heart, the number of cardiac macrophages increases significantly through local proliferation and monocyte recruitment. CCR2⁺ monocyte-derived macrophages and heart-resident macrophages produce and secrete large amounts of pro-inflammatory cytokines that promote cardiomyocyte regeneration and contribute to cardiac tissue remodeling.