Any of various chemicals, particularly polypeptides, that have a variety of important roles in the stimulation of new cell growth and cell maintenance. They bind to the cell surface on receptors. Specific growth factors can cause new cell proliferation (epidermal growth factor, insulin-like growth factor, haemopoietic growth factor—see haemopoietic tissue) and the migration of cells (see fibroblast growth factor). Platelet-derived growth factor (PDGF) is released by platelets in response to tissue damage. It stimulates the proliferation of fibroblasts, which contribute to wound healing. Some growth factors act in the embryonic stage of development; for example, nerve growth factor (see neurotrophin) stimulates the growth of axons and dendrites from developing sensory and sympathetic neurons. Some growth factors or their receptors are involved in the abnormal regulation of growth seen in cancer when produced in excessive amounts or permanently activated. See also bone morphogenetic factor; transforming growth factor beta.