The salivary gland is developed under the epithelial-mesenchymal interaction and formed by repeating branching morphogenesis, which is a common process in the development of the glandular tissues. Although the differentiation and maturation of salivary gland is continued into early postnatal life, the fundamental ability of saliva secretion is already expressed after birth.
These development, differentiation, and maturation of salivary gland are regulated by many growth/ differentiation factors, which are initially synthesized as inactive precursors and activated by the limited proteolysis. On the other hand, saliva secretion is one of the important physiological functions of the salivary gland and occurs dependently on the increase of the osmolality in the lumen through two pathways, paracellular and transcellular pathways. It is revealed that a water channel aquaporin 5 (AQP5) is involved in saliva secretion through the latter pathway. Saliva contains various components, which express various physiological functions of saliva. Because the oral cavity is confronted with hazards of various allergens from the outside continuously, the salivary gland produces and secretes various factors as the defense system. Saliva is, therefore, one of an important body fluid to maintain the oral health, and the decrease of saliva secretion causes xerostomia/dry mouth, which affects not only oral disease and dysfunction but also systemic disease. We previously reported the involvement of a subtilisin-like proprotein convertase PACE4 in the development and differentiation of salivary gland, lipopolysaccharide-mediated induction of inflammatory cytokines in the salivary gland, degradation of salivary AQP5 by parasympathetic denervation, and so on. This review will describe the structure and function of the salivary gland, from its development to functional expression and regulation.