ABSTRACT
Currently, most of the available information about origins of macrophages comes from murine studies utilizing genetic fate-mapping techniques. F4/80+ cells rst appear in mouse yolk sac on the days 8-9 of embryo development, and then populate embryonic liver at day 10, reaching the peak at days 12-14 of development [1]. Yolk sac macrophages differentiate from restricted progenitors that give rise to macrophages and red blood cells, whereas fetal liver macrophages differentiate from hematopoietic stem cells (HSCs) [2]. Genetic fate-mapping experiments revealed that resident macrophages in many adult organs at least partially originate from yolk sac and fetal liver macrophages. For example, it is believed that microglia (resident macrophages of brain and spinal cord) almost exclusively originates from yolk sac macrophages [2]. Macrophages in other organs such as liver, heart, and skin partially originate from yolk sac. However, embryonic HSCs (in case of liver) or embryonic and adult HSCs (in case of skin and heart) also give rise to resident macrophage populations in these organs. In general, most of the organs in adult animals are composed of embryonically derived and adult-derived macrophage subpopulations (Figure 10.1). Moreover, recent studies conclude that populations of resident macrophages in most of the organs exist autonomously and are not completely replaced by circulating monocytes [2].
