Masticatory properties in pre-modern Holocene populations from Northern China.

Recent studies indicate that evolution of the craniofacial skeleton is influenced by dietary behavior, which in turn alters masticatory efficacy and temporomandibular joint (TMJ) stability. In this study the mechanical properties of the masticatory system and the integrity of the TMJ in human populations from Northern China, dated to between 3800 BCE and 100 CE, were assessed. The results demonstrate that the mechanical efficiency is comparable to other modern human populations, though variations are present across different populations. While the ratio of overall weighted muscle efficiency for incisor loading vs. molar loading in pastoral and some recent agricultural groups is similar to early Homo sapiens, the ratio in more ancient agricultural groups is similar to the ratio in populations with heavy anterior paramasticatory activities, such as Neandertals, Inuits, and Native Americans. The TMJ vulnerability negatively correlates with the maxillary dental arch size, and positively with the condylar size. These findings suggest that there are multi-directional strategies in adaptation to heavy anterior teeth loading, such as increasing anterior teeth loading efficiency, increasing facial height, increasing facial breath and facial orthognathy, or decreasing anterior facial length. Furthermore, populations or individuals with a smaller dental arch and high biting efficiency could more easily injure the TMJ during unilateral loadings, which may explain the higher prevalence of TMJ disorders in modern humans, especially in women. These findings further reflect the impact of diachronic changes of the masticatory apparatus and lifestyle and their impact on oral health during recent human history.

Divided zygoma in Holocene human populations from Northern China.

OBJECTIVES: Divided zygoma (DZ) occurs in contemporaneous human populations, with the highest incidences in people from East Asia and Southern Africa. The present study examines the prevalence and variation of this condition in the Holocene populations of Northern China for the first time. METHODS: In this study, 1145 skulls from various human populations living in Northern China from the Neolithic Age to recent dynasties (5000-300 years BP) were examined. Specifically, cranial measurements and a CT scan were conducted to quantify craniofacial morphology. RESULTS: Fifteen skulls were identified with DZ, revealing an overall prevalence of 1.3% in the collection, while it was determined to be higher in North Asian and Northeast Asian regional groups. In skulls with unilateral DZ, the superior division of the zygoma was generally slender, while the inferior division of the zygoma was more robust. In skulls with bilateral DZ, the maxillae were generally more laterally extended. Moreover, unilateral DZ skulls displayed differences in cortical bone thickness between two sides of the facial skeleton. DISCUSSION: In context, the distribution pattern within these data points toward a greater prevalence of the DZ phenotype in North and Northeast Asian regional groups, suggesting a hypothesis that the DZ trait is more frequent in populations characterized by flat and broad faces. Accordingly, further studies into the DZ condition will deepen our understanding of developments in plasticity, variation, and recent evolution of the human cranium.

Orexin regulates bone remodeling via a dominant positive central action and a subordinate negative peripheral action.

Orexin neuropeptides promote arousal, appetite, reward, and energy expenditure. However, whether orexin affects bone mass accrual is unknown. Here, we show that orexin functions centrally through orexin receptor 2 (OX2R) in the brain to enhance bone formation. OX2R null mice exhibit low bone mass owing to elevated circulating leptin, whereas central administration of an OX2R-selective agonist augments bone mass. Conversely, orexin also functions peripherally through orexin receptor 1 (OX1R) in the bone to suppress bone formation. OX1R null mice exhibit high bone mass owing to a differentiation shift from marrow adipocyte to osteoblast that results from higher osseous ghrelin expression. The central action is dominant because bone mass is reduced in orexin null and OX1R2R double null mice but enhanced in orexin-overexpressing transgenic mice. These findings reveal orexin as a critical rheostat of skeletal homeostasis that exerts a yin-yang dual regulation and highlight orexin as a therapeutic target for osteoporosis.