Overview: The Mechanisms Controlling Root Morphogenesis in Mammalian Molars: —Morphological Relevance of the Tooth Root and Crown, Developmental Mechanisms, and Phylogenetic Aspect of the Tooth Roots—

Diet is one of the most important factors driving the process of natural selection in evolution. The dentition is adapted for the diet, contributing to the diversification and adaptation of extant vertebrates. Importantly, the structure of the molar shows marked variation in extant mammals to facilitate the processing of many kinds of food and support the occlusal force effectively. A tooth root is an important element, connecting a tooth with the jaw to support the occlusal force. However, morphological studies on teeth mainly focus on the crown, whereas there has been an insufficient accumulation of information on the morphogenic control mechanism of tooth root structures. There have been some reports on the root form in humans due to clinical demand, and recent studies on Hertwig's epithelial root sheath (HERS), which plays an important role in root development, have progressed rapidly. However, studies on the regulatory mechanism of the tooth root structure in relation to the jaw and crown are still awaited. In this paper, we highlight the following three issues : the morphological relevance of the tooth root and crown, and developmental mechanisms and phylogenetic aspects of the tooth roots. HERS produces several signaling molecules including bone morphogenic protein (BMP), Sonic Hedgehog (Shh), Wnt, ectodysplasin (Eda), and fibrobast growth factor (FGF). These factors regulate the closure of the pulp chamber floor to determine the numbers and width of the tooth roots, and then regulate the fusion, curve, and length. The tooth root differentiates and develops in conjunction with its crown. However, few studies have examined the tooth root from an evolutionary perspective. We need to advance research on the tooth root including the whole tooth in developmental biology, evolutionary biology, and comparative odontology.