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A growth factor is a naturally occurring substance capable of stimulating cellular growth, proliferation, healing, and cellular differentiation. Usually it is a protein or a steroid hormone. Growth factors are important for regulating a variety of cellular processes.
They often promote cell differentiation and maturation, which varies between growth factors. For example, epidermal growth factor (EGF) enhances osteogenic differentiation,  while fibroblast growth factors and vascular endothelial growth factors stimulate blood vessel differentiation (angiogenesis).
Growth factor is sometimes used interchangeably among scientists with the term cytokine. Historically, cytokines were associated with hematopoietic (blood and lymph 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 which the hematopoietic and immune systems use were also being used by all sorts of other cells and tissues, during development and in the mature organism.
While growth factor implies a positive effect on cell division, cytokine is a neutral term with respect to whether a molecule affects proliferation. While some cytokines can be growth factors, such as G-CSF and GM-CSF, others have an inhibitory effect on cell growth or proliferation. Some cytokines, such as Fas ligand, are used as "death" signals; they cause target cells to undergo programmed cell death or apoptosis.
Individual growth factor proteins tend to occur as members of larger families of structurally and evolutionarily related proteins. There are many families, some of which are listed below:
- Adrenomedullin (AM)
- Angiopoietin (Ang)
- Autocrine motility factor
- Bone morphogenetic proteins (BMPs)
- Ciliary neurotrophic factor family
- Colony-stimulating factors
- Epidermal growth factor (EGF)
- Erythropoietin (EPO)
- Fibroblast growth factor (FGF)
- Fibroblast growth factor 1(FGF1)
- Fibroblast growth factor 2(FGF2)
- Fibroblast growth factor 3(FGF3)
- Fibroblast growth factor 4(FGF4)
- Fibroblast growth factor 5(FGF5)
- Fibroblast growth factor 6(FGF6)
- Fibroblast growth factor 7(FGF7)
- Fibroblast growth factor 8(FGF8)
- Fibroblast growth factor 9(FGF9)
- Fibroblast growth factor 10(FGF10)
- Fibroblast growth factor 11(FGF11)
- Fibroblast growth factor 12(FGF12)
- Fibroblast growth factor 13(FGF13)
- Fibroblast growth factor 14(FGF14)
- Fibroblast growth factor 15(FGF15)
- Fibroblast growth factor 16(FGF16)
- Fibroblast growth factor 17(FGF17)
- Fibroblast growth factor 18(FGF18)
- Fibroblast growth factor 19(FGF19)
- Fibroblast growth factor 20(FGF20)
- Fibroblast growth factor 21(FGF21)
- Fibroblast growth factor 22(FGF22)
- Fibroblast growth factor 23(FGF23)
- Foetal Bovine Somatotrophin (FBS)
- GDNF family of ligands
- Growth differentiation factor-9 (GDF9)
- Hepatocyte growth factor (HGF)
- Hepatoma-derived growth factor (HDGF)
- Insulin-like growth factors
- IL-1- Cofactor for IL-3 and IL-6. Activates T cells.
- IL-2 – T-cell growth factor. Stimulates IL-1 synthesis. Activates B-cells and NK cells.
- IL-3 – Stimulates production of all non-lymphoid cells.
- IL-4 – Growth factor for activated B cells, resting T cells, and mast cells.
- IL-5 – Induces differentiation of activated B cells and eosinophils.
- IL-6 – Stimulates Ig synthesis. Growth factor for plasma cells.
- IL-7 – Growth factor for pre-B cells.
- Keratinocyte growth factor (KGF)
- Migration-stimulating factor (MSF)
- Macrophage-stimulating protein (MSP), also known as hepatocyte growth factor-like protein (HGFLP)
- Myostatin (GDF-8)
- Placental growth factor (PGF)
- Platelet-derived growth factor (PDGF)
- Renalase (RNLS) – Anti-apoptotic survival factor
- T-cell growth factor (TCGF)
- Thrombopoietin (TPO)
- Transforming growth factors
- Tumor necrosis factor-alpha (TNF-α)
- Vascular endothelial growth factor (VEGF)
- Wnt Signaling Pathway
Uses in medicine
- myelodysplastic syndrome (MDS)
- aplastic anaemia
- bone marrow transplantation
- angiogenesis for cardiovascular diseases
- Bone growth factor
- Growth factor receptor
- Human Genome Organisation
- Neurotrophic factor
- Receptor (biochemistry)
- Signal transduction
- Wound healing#Overview of involved growth factors
- "growth factor" at Dorland's Medical Dictionary
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- Growth+Factors at the US National Library of Medicine Medical Subject Headings (MeSH)
- FGF5 in Hair Tonic Products
- FGF1 in Cosmetic Products