The development of the blood forming elements (blood cell production) proceeds gradually, so hematopoietic cells are generally classified into six classes.

Class I: pluripotent hematopoietic stem cells (HSCs)

• give rise to any formed element
• are able to maintain their number
• seldom divide, stay mainly at the G₀ stage of mitotic cycle

Class II: partially determined multipotent ancestor (progenitor) cells; also called hemistem cells or colony-forming units (CFU)

• have a limited potential to maintain their number
• there are progenitor cells of myelopoiesis (CFU-GEMM) to give rise to granulocytes, red blood cells, monocytes, and megakaryocytes
• there are progenitor cells of lymphopoiesis (CFU-L) to give rise to various lymphocytes

Class III: committed ancestor (progenitor) cells or colony-forming units (CFU)

• divide often with no potential to maintain their number
• are unipotent as they are determined to develop a certain type of formed elements:
  • BFE-E (burst-forming unit) and CFU-E for red blood cells
  • CFU-Meg for platelets
  • CFU-Eos for eosinophils
  • CFU-B for basophils
  • T and B cell progenitors (also for NK cells)
• however, CFU-GM is not unipotent but oligopotent; it gives rise to neutrophils and monocytes

Stem and progenitor cells are not identifiable by their morphology. You need to test certain marker expression on the cell surface to identify them

Class IV: unipotent progenitor cells (blasts)

• correspond to certain blood cell lineages
• divide frequently resulting in differentiating cells

Class V: maturing (differentiating) precursor cells

• undergo morphological and functional differentiation, thus gradually losing their proliferative potential (except lymphocytes and monocytes)
• transform into certain blood cell types

Class VI: mature (differentiated) cells

• are cells at the terminal stage of development from HSCs (except lymphocytes and monocytes)
• are not able to divide (except lymphocytes and monocytes)

T and B cells and monocytes continue their development outside the red bone marrow:

• monocytes migrate to tissues where they turn into macrophages
• T cells proceed to the thymus becoming T killers and T helpers there; then, they migrate to peripheral lymphoid organs and take part in cellular and humoral immune responses to antigens (with T memory cell formation)
• B cells become plasma cells or B memory cells in the lymphoid tissue to ensure humoral immunity

Hematopoietic cells of class V and VI, as well as some cells from class IV, may be distinguished according to certain typical morphological signs using certain hematological stains