Growth factors

Growth factor is a protein that acts as a signaling molecule between cells (like cytokines and hormones) that attaches to specific receptors on the surface of a target cell and promotes differentiation and maturation of these cells.

Growth factors and cytokines
The term growth factor is sometimes used interchangeably among scientitsts with the term cytokine. Historically, cytokines were associated with hematopoietic (blood forming) cells and immune system cells (e.g., lymphocytes and tissue cells from spleen, thymus, and lymph nodes). For the circulatory system and bone marrow in which cells can occur in a liquid suspension and not bound up in solid tissue, it makes sense for them to communicate by soluble, circulating protein molecules. However, as different lines of research converged, it became clear that some of the same signaling proteins the hematopoietic and immune systems used were also being used by all sorts of other cells and tissues, during development and in the mature organism.

Growth factor signifies a positive effect on cell growth and cellular differentiation, but cytokine is a neutral term in regards to what it is being signaled. In this sense, some cytokines can be growth factors such as G-CSF and GM-CSF as listed below. However some cytokines are actually used as "death" signals, such as the FAS ligand, which causes target lymphocytes to commit a form of suicide known as programmed cell death or apoptosis.

Types
Individual growth factor proteins tend to occur as members of larger families of structurally and evolutionarily related proteins. There are dozens and dozens of growth factor families such as TGF-beta (transforming growth factor), BMP (bone morphogenic protein), neurotrophins (NGF, BDNF, and NT3), fibroblast growth factor (FGF), and so on.

Several well known growth factors are:
 * granulocyte-colony stimulating factor (G-CSF)
 * granulocyte-macrophage colony stimulating factor (GM-CSF)
 * nerve growth factor (NGF)
 * neurotrophins
 * platelet-derived growth factor (PDGF)
 * erythropoietin (EPO)
 * thrombopoietin (TPO)
 * myostatin (GDF-8)
 * Growth Differentiation factor-9 (GDF9)
 * basic fibroblast growth factor (bFGF or FGF2)

Uses in medicine
For the last two decades, growth factors have been increasingly used in treatment of hematologic and oncologic diseases like:
 * neutropenia
 * myelodysplastic syndrome (MDS)
 * leukemias
 * aplastic anaemia
 * bone marrow transplantation