All blood cells are derived from pluripotent haematopoietic stem cells. These comprise only 0.01% of the total marrow cells, but they can self-renew (i.e. make more stem cells) or differentiate to produce a hierarchy of lineage-committed progenitor cells. The resulting primitive progenitor cells cannot be identified morphologically, so they are named according to the types of cell (or colony) they form during cell culture experiments. CFU-GM (colony-forming unit - granulocyte, monocyte) is a progenitor cell that produces granulocytic and monocytic lines, CFU-E produce erythroid cells, and CFU-Meg produce megakaryocytes and ultimately platelets.
Growth factors, produced in bone marrow stromal cells and elsewhere, control the survival, proliferation, differentiation and function of stem cells and their progeny. Some, such as, interleukin-3 (IL-3), stem cell factor (SCF) and granulocyte, macrophage-colony-stimulating factor (GM-CSF), act on a wide number of cell types at various stages of differentiation. Others, such as erythropoietin, granulocyte-colony-stimulating factor (G-CSF) and thrombopoietin (Tpo), are lineage-specific. Many of these growth factors are now synthesized by recombinant DNA technology and used as treatments: for example, erythropoietin to correct renal anaemia and G-CSF to hasten neurtrophil recovery after chemotherapy.
The bone marrow also contains stem cells that can differentiate into non-haematological cells. Mesenchymal stem cells differentiate into skeletal muscle, cartilage, cardiac muscle, and fat cells while others differentiate into nerves, liver and blood vessel endothelium. This is termed stem cell plasticity and may have exciting clinical applications in the future.
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