An essential element in the developmental and functional integrity of all organisms is intracellular communication. This is achieved by the secretion of soluble messenger molecules, or signal substances, which interact with a corresponding receptor molecule on the target cell surface. Hormones, such as growth hormone（GH）, are defined as the messengers synthesized by endocrine glands. Growth factors such as insulin-like growth factor-I（IGF-I）are hormone-related substances produced by many tissues and play an important role in controlling growth and development. Although the physiological roles of growth factors have yet to be completely elucidated, they play important roles in the regulation of cellular proliferation and/or differentiation. Recently, there have been substantial developments in research related to peptide hormones, growth factors, and their receptors. With the discovery and characterization of numerous growth factors, it is evident that growth factors have multiple features in common with classic hormones
