Evaluating the patterning cascade model of tooth morphogenesis in the human lower mixed and permanent dentition

Abstract

Objective: The patterning cascade model of crown morphogenesis has been studied extensively in a variety of organisms to elucidate the evolutionary history surrounding postcanine tooth form. The current research examines the degree to which model expectations are reflected in the crown configuration of lower deciduous and permanent molars in a modern human sample. This study has two main goals: 1) to determine if metameric and antimeric pairs significantly differ in size, accessory trait expression, and relative intercusp spacing, and 2) to establish if the relative distance among early-forming cusps accounts for observed variation in accessory cusp expression. Methods: Tooth size, intercusp distance, and morphological trait expression data were collected from 3D scans of mandibular dental casts representing 124 individual participants of the Harvard Solomon Islands Project. Paired tests were utilized to compare tooth size, accessory trait expression, and relative intercusp distance between diphyodont metameres and permanent antimeres. Proportional odds logistic regression was implemented to investigate how the likelihood of accessory cusp formation varies as a function of the distance between early-developing cusps. Results/Significance: For paired molars, results indicated significant discrepancies in tooth size and cusp 5 expression, but not cusp 6 and cusp 7 expression. Several relative intercusp distances emerged as important predictors of accessory cusp expression. These findings support previous quantitative genetic results and suggest the development of neighboring crown structures represents a zero-sum partitioning of cellular territory and resources. As such, this study contributes to a better understanding of the evolution of deciduous and permanent molar crown configuration in humans.