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

Abstract

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, a s 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 a s 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 (Rp) and rate of differentiation (R,) during the attainment of final crown height measured a t 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.Because of their great durability and the wealth of developmental and morphologic information they contain at the gross and microscopic levels (Sarnat and 0 1991 Wiley-Liss, Inc.

Associations between Carabelli trait and cusp areas in human permanent maxillary first molars

“…Putting together the various aspects of molar morphogenesis described above, and drawing on the morphogenetic triangle concept of Keene (1991), we propose the following explanation for observed relationships between overall molar crown size, cusp areas, and Carabelli trait (Fig. 3).…”

Genotype, phenotype, and developmental biology of molar tooth characters

“…Thus the patterning cascade parameters are inheritable, but to what extent remains to be determined. It is worthwhile to point out that the patterning cascade mode of cusp development is quite compatible with the "morphogenetic triangle" that Keene (1991) used to approach heterochrony in tooth development. For example, the iterations that the patterning cascade is allowed to go through appear to be controlled independently by crown height (Jernvall, 2000).…”

“…Since the previous Yearbook articles addressing tooth development (Weiss, 1990;Keene, 1991), there has been a renaissance of developmental biology, mostly propelled by new discoveries from developmental genetics. Indeed, enough new investigators have begun to examine development in the context of evolution that a new field of evolutionary developmental biology (evo-devo) is in the making.…”

Three‐dimensional (3D) geometric morphometric analysis of human premolars to assess sexual dimorphism and biological ancestry in Australian populations

“…In the European group, a larger interfoveal distance is present (with more slender marginal ridges), corresponding to a “square” outline. During tooth morphogenesis, variations in cusp alignment, interfoveal distance and occlusal outline are likely to be due to factors such as the spatial relationship between developing enamel knots (Jernvall & Thesleff, ; Thesleff, Keranen & Jernvall, ) and the timing and duration of critical phases of dental development (Brook, ; Brook, Jernvall, Smith, Hughes & Townsend, ; Keene, ). For example, a longer growth duration and increased tooth germ size favor widened cusp tip spacing and more prominent ridge development.…”