Expressions and functions of neurotrophic factors in tooth development

Neurotrophic factors are soluble growth factors predominantly expressed in vertebrate nervous systems and have been well-characterized for their critical roles in neural tissues. Recent studies have revealed that neurotrophin factors and their receptors are also expressed in multiple non-neural tissues, and play a role in a wide range of biological functions, such as regulation of cellular proliferation, survival, migration, and differentiation. The neurotrophic factor family is defined by its structural and functional similarities to 4 ligands ; nerve growth factor (NGF),brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4, also known as NT-5).They activate 2 different receptors, trk tyrosine kinase and p75, the latter of which is a member of the tumor necrosis factor receptor superfamily. During tooth development, observations of dynamic changes of specific expression patterns of neurotrophic factors and their receptors imply their important functions in odontogenic processes. In addition, our recent study demonstrated that NT-4 regulates proliferation and differentiation of dental epithelium, and promotes the production of enamel matrixes. In this review, we describe the expression patterns and functions of neu-rotrophic factors in the tooth germ, and discuss the relationships with tooth development.