[Expression and localization of cell growth factors in the salivary gland: a review]

The salivary gland secretes not only digestive enzymes but also various cell growth factors. Especially in rodents, granular ducts which develop between striated and intercalated ducts, are known to secrete epidermal growth factor (EGF) and nerve growth factor (NGF). Out of newly discovered growth factors, we have examined hepatocyte growth factor (HGF), transforming growth factor-beta (TGF-beta), insulin-like growth factor I (IGF-I), and basic fibroblast growth factor (FGF-2) and have demonstrated their expression and localization in the rat submandibular gland. HGF and TGF-beta show a very similar distribution pattern to EGF, i.e., exclusive localization in the secretory granules of granular duct cells. These factors are suggested to function as i) exocrine salivary factors, or ii) endocrine factors after reabsorption. In contrast, FGF-2 shows a different pattern of cellular and subcellular localization from EGF and others. FGF-2 is localized in cytoplasm of striated and excretory duct cells and pillar cells in granular ducts, but not in secretory granules of granular duct cells. Since the receptor for FGF also present in the same cells as its ligand, FGF is suggested to function in the salivary gland as an autocrine/paracrine factor. The possible physiological roles of salivary growth factors in the digestive organs are discussed based on our own data and an extensive literature.