Hematopoietic stem cells, also called hemocytoblasts, give rise to all cell types found in the blood. These pluripotent cells pass through many stages of progenitors, which are still stem cells but are more restricted in their options. Each new progenitor stage involves a step of differentiation closer to final maturation.

Hemocytoblasts divides into two separate lineages: lymphoid stem cells and myeloid stem cells. Lymphoid stem cells further differentiates into the B lymphocytes and T lymphocytes of the immune system. Myeloid stem cells differentiate into other blood cell lines such as erythrocytes, granulocytes, monocytes, and platelets. (1)

The progeny of lymphoid and myeloid cell lines are committed, capable of forming only one type of end cell. However they must still pass through the several stages of differentiation before becoming mature blood cells. Cytokine requirements and transcriptional regulators control hematopoietic entry into the cell cycle, maintaining the delicate balance between self-renewal and commitment to differentiation.