Differentiation and development of myeloid-derived suppressor cells (MDSCs) in the tumor microenvironment (TME). Under physiological conditions, neutrophils, dendritic cells (DCs), and monocytes originate from hematopoietic progenitor cells (HPCs) in the bone marrow. HPCs differentiate into granulocyte-macrophage progenitors (GMPs) after common myeloid progenitors (CMPs), and then GMPs differentiate into monocyte/macrophage and dendritic cell precursors (MDPs) and myeloblasts (MBs). Among them, MDPs are the precursors of DCs and monocytes, and MBs are the precursors of neutrophils. However, under pathological conditions, such as cancer, myeloid cells are induced to differentiate into suppressor cells, including monocytic myeloid-derived suppressor cells (M-MDSCs), tumor-associated macrophages (TAMs), polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs), and tumor-associated neutrophils (TANs). TME, tumor microenvironment; HPCs, hemopoietic progenitor cells; CMPs, common myeloid progenitors; GMPs, granulocyte-macrophage progenitors; MBs, myeloblasts; MDPs, monocyte/macrophage and dendritic cell precursors; M-MDSCs, monocytic myeloid-derived suppressor cells; PMN-MDSCs, polymorphonuclear myeloid-derived suppressor cells; TAMs, tumor-associated macrophages; TANs, tumor-associated neutrophils; DCs, dendritic cells.