Osteopontin (OPN) is an acidic phosphorylated glycoprotein involved in a wide range of biological activities, such as cell proliferation and differentiation, as well as immunomodulatory functions. OPN contains integrin and CD44 binding sites, and it exerts its multiple functions by binding to its receptors on the cell membrane to trigger various cellular signaling pathways. It is generated by a variety of cell types, including epithelial cells and immune cells. OPN appears in most body fluids, such as milk and blood, and is present at a high concentration in human milk but not in bovine milk. Milk OPN is relatively resistant to digestion, and orally ingested OPN can enter the circulatory system. Milk OPN may, therefore, play essential roles in the development in early life. The impact of milk OPN on development has been investigated using cell models, animal models, and randomized clinical trials. Recent OPN studies strongly suggest that milk OPN plays important roles in intestinal proliferation and maturation, brain myelination, and neurodevelopment, as well as immune development.
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