In humans, hematopoiesis, the formation and development of red and white blood cells from stem cells, begins in the yolk sack in the first week of embryonic development. Here yolk sac stem cells differentiate into primitive erythroid cells containing embryonic hemoglobin. In the the third month of gestation, the stem cells migrate from the yolk sac to the fetal liver and then to the spleen; these two organs have the major role in hematopoeisis from the third to the seventh months of the gestation. As gestation continues, the bone marrow becomes the major hematopoietic organ; by birth hematopoeisis has ceased within the liver and spleen. Every mature blood cell is derived from a common stem cell.

In contrast to a uni-potent cell, which differentiates into a single type, a hematopoietic stem cell is pluripotent, able to differentiate along a number of pathways and there by generate erythrocytes, granulocytes, monocytes, mast cells, lymphocytes,and megakaryocytes.

A pluripotent stem cell differentiate among among one of two pathways, giving rise to either a a lymphoid stem cell or a myeloid stem cell.

Lymphoid and myeloid stem cells differentiate into progenitor cells, which have lost the capacity of self renewal and are committed to a given cell lineage. The lymphoid stem cell generates T and B lymphocytes. The myeloid stem cell generates progenitor cells for red blood cells (erythrocytes), the various white blood cells ( neutrophils, eosinophils, basophils, monocytes, mast cells), and platelets. Progenitor commitment depends on the acquisition of responsiveness to particular growth factors.

The heamatopoietic cells grow and mature on a meshwork of stromal cells which are nonhematopoietic cells that support the growth and differentiation of the hematopoietic cells. Stromal cells include fat cells, endothelial cells, fibroblasts and macrophages. Stromal cells influence hematopoetic stem-cell differentiation by providing a hematopoetic inducing microenvironment consisting of a cellular matrix and either membrane bound or diffusible growth factors. As hematopoetic stem cells differentiate in this microenvirenment, their membranes acquire deformability, allowing the mature cells to pass through the sinusoidal wall into the sinuses of the bone marrow, where the enter the circulation.