Original article

Growth and remodeling of the human maxilla☆

The immature human palate functions in suckling and swallowing but transitions to participate in mastication and sound production as the individual grows and matures. Functional alterations manifest as changes in the morphology of the palate and may result from dental eruption. The aim of this study was to analyse the effect of dental eruption on morphological changes of the palate in immature individuals using geometric morphometrics.

Crania from 72 individuals were sourced from the Raymond A. Dart Collection of Human Skeletons, University of the Witwatersrand. Age groups corresponding to the eruption of the deciduous (birth – 5 years), mixed (6–12 years) and permanent dentition (13–20 years) respectively were used. Fourteen osteological landmarks were digitised across the oral surface of the palate and were converted into linear distances and 3D wireframes. Data analysis focused on alterations in shape and size relative to the state of the dentition.

A significant increase in the geometric mean, length and width was observed between the different states of the dentition. The palate shape was long and narrow in the permanent dentition group, resulting in a more acute angle anteriorly and a protruded orale. A protrusion of the staphylion and deeper palatal dome and change in the horizontal alignment of the alveolar region were also observed in the permanent dentition group.

The subtle changes identified in the growing palate during different stages of the dentition may assist forensic scientists during detailed case analysis.

Facial orientation (projection and degree of prognathism) and form in hominins is highly variable, likely related to evolutionary modifications of the microscopic process of bone modeling (the simultaneous cellular activities of bone formation and resorption) during ontogeny. However, in anteriorly projected faces such as those of early hominins, little is known about the link between bone modeling and facial developmental patterns. Similarly, these aspects have been infrequently investigated in extant great apes. In this study, quantitative methods were applied to a cross-sectional ontogenetic sample of 33 chimpanzees (Pan troglodytes verus) and 59 modern humans (Homo sapiens) to compare the development of maxillary prognathism to orthognathism at both microscopic and macroscopic (or morphological) scales using surface histology and geometric morphometric techniques. Chimpanzees express on average lower amounts of bone resorption than humans on the maxillary periosteum throughout ontogeny; however, the premaxilla is consistently resorbed from early stages on. The presence of bone resorption in the chimpanzee premaxilla, such as that seen in some early hominins, suggests a more ape-like pattern of maxillary bone modeling in these specimens. However, this shows that similarities in bone modeling patterns can lead to variations in shape, suggesting that other aspects of facial growth (such as modifications of rates and timings of development, as well as sutural growth) also played a crucial role in facial evolution.

The reduction of masticatory strains is considered one of the main factors that led to a pronounced morphological variation of the facial skeleton among modern humans. Although the archaeological record has provided evidence of bone remodeling activity being linked to craniofacial variation, its link with subsistence strategies has been proposed but not yet tested. Here, we evaluate the relationship between the strains arising from masticatory loads in the facial bones and the observed surface bone remodeling activity in adults and subadults from archaeological sites from South America that exerted different masticatory loads during life. We simulated the impact of mechanical loading during I1 and M1 bite using finite element analysis in six skulls from two archaeological samples, one of hunter-gatherers from Patagonia and the other of horticulturists from Northwest Argentina. The extension and distribution of bone formation and resorption were registered by a periosteal bone surface analysis on facial bones. We found a similar spatial distribution of high and low strains between samples and across ages, but different magnitudes. In general, compression strains corresponded with resorption activity, while tension strains corresponded with formation activity. Our results show a relationship between mechanical bone response to masticatory loading and bone remodeling activity, which can ultimately shape cranial morphology. We propose that although there are differences in skull morphology among populations that are established early in ontogeny, mechanical loading produced during mastication can enhance such differences. These results then support the idea that craniofacial morphology can contribute to reconstructing the history of past populations.