Fig. 1

Cellular origins and functions of TAMs. As the major primary resource of macrophages, monocytes are generated from hematopoietic stem cells (HSCs) that differentiate into granulocyte-macrophage progenitors (GMPs) and then into monocyte-dendritic cell progenitors (MDPs). Besides, tissue-resident macrophage stem from yolk sac progenitors are another key resources of macrophages, which proliferate or differentiate in situ, such as alveolar macrophages, brain macrophages, and Kupffer cells. The mature monocytes released in the blood and tissue-resident macrophages are recruited and activated by various signals in the TME and then exhibit dramatic impacts on the tumor initiation and promotion, metastasis, immune regulation and angiogenesis