On heterochrony in heterodonty: A review of some problems in tooth morphogenesis and evolution

Although initial discussions and speculations about the role of acceleration and retardation in the relative timing of ontogenetic events associated with dental evolution, dental development, and heterodonty can be found in the literature as early as a century ago, morphogenetic processes and mechanisms remain elusive and formal heterochronic and mechanistic models employing mathematical principles, especially those that are testable, have only recently begun to appear. Even the seemingly simplest of developmental problems, such as primate canine tooth size variation in ontogeny and phylogeny, continue to be the subject of much controversy and debate. At least part of the problem, as well as its solution, may be found in the uniqueness of the process of amelogenesis during tooth formation and the dichotomy between inner enamel epithelium growth parameters and those dealing with ameloblast secretory functions, including duration and rate. Certain aspects of the life cycle of the inner enamel epithelium lend themselves to mathematical analysis and can be visualized graphically as a morphogenetic triangle , which illustrates the relationship between proliferation and differentiation during tooth crown formation. Components of the triangle model include the times of onset of proliferation (P), onset of differentiation (D), and crown completion (C), which provide the basis for calculations of rate of proliferation (R P ) and rate of differentiation (R d ) during the attainment of final crown height measured at the dentinoenamel junction. Thus, tooth crown size is partially controlled by a “framework” established by the proliferating cells of the inner enamel epithelium, and partially by the variable thickness of enamel matrix produced by functional ameloblasts and laid down on the “framework” following their terminal differentiation. From a developmental point of view, it would seem appropriate to consider tooth size (inner enamel epithelium, morphogenetic triangle) and enamel thickness (secretory ameloblasts) as independent parameters of growth that contribute differentially to external crown dimensions. Morphogenetic triangle parameters that are actually related to real ontogenetic events during tooth formation facilitate heterochronic analysis and modelling and provide a theoretical basis for a broader understanding of basic mechanisms involved in certain troublesome size and shape problems in dental morphogenesis and evolution.