The chondrocranial key: Development of the sphenoid bone in primates

Abstract

It has been hypothesized that the human sphenoid bone is uniquely truncated, which in turn contributes to a reduction of forward midfacial growth. If so, the perinatal fusion of the intrasphenoidal synchondrosis (ISS) in humans may contribute to midfacial reduction. However, there is a lack of detailed knowledge on sphenoid development of non‐human primates. In this study, orientation and direction of growth of basicranial interface with the midface of late prenatal and early postnatal sphenoid development was examined in ontogenetic samples of primates including three species of monkeys (n = 25) and four species of lemurs and bushbabies (strepsirrhines; n= 28). Micro‐computed tomographic (CT) and histological methods were used to track cross‐sectional age changes in the sphenoid bone. In monkeys (Saguinus spp.), histological findings indicated the number of proliferating chondrocytes is reduced across age leading to a reduction in absolute anteroposterior length of the proliferating zone. Preliminary data from micro‐CT reconstructions suggest that absolute length of the ISS decreases more rapidly in monkeys than in strepsirrhines. Measurements of presphenoid (PS) and basisphenoid (BS) length indicate that in strepsirrhines, these bones grow similarly to one another as cranial length increases. In contrast, in monkeys, the PS increases at a faster pace (i.e. higher linear regression line slopes) than the BS. However, unlike humans, the monkeys studied have prolonged postnatal patency of the ISS. Thus, the reduced midfacial projection in humans and these monkeys cannot be explained by the timing of ISS fusion alone. Dichotomous growth patterns of the cranial base and midface among primates suggest different patterns of regional integration of the cranium. In particular, our results suggest that in monkeys, integration of the midface is stronger with the anterior compared to the posterior portion of the sphenoid bone. In strepsirrhines by comparison, growth of the midface progressively deviates from that of the sphenoid bone over time. Support or Funding Information NSF; grant numbers BCS‐1830894, BCS‐1830919, BCS‐0959438