108 - Mechanical Influences on Synchondroses Mediate Cranial Variation

Poster Board Number: 108
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Abstract Body : Synchondroses function as growth centers of the skull. Proliferation of chondrocytes and interstitial expansion of the cartilage matrix drive differential growth of skeletal structures. As a result, the orientation of synchondroses predicts subsequent growth trajectories, affecting overall cranial shape. Here, we consider a comparative sample of primates to test the association of synchondroseal orientation at the newborn stage with cranial traits in adults that distinguish major clades of primates. Three growth centers are the focus of this study: the presphenoseptal synchondrosis, the intrasphenoidal synchondrosis, and the bilateral ali-basisphenoid synchondroses. Results are interpreted in the context of mechanical influences of functional matrices, including the brain and oronasal structures.

We created virtual reconstructions of the skull using microCT images of newborn and adult primates from over 30 representative species of strepsirrhines (galagos, lemurs, lorises) and haplorhines (tarsiers, platyrrhines, and non-hominoid catarrhines). Adult morphology was estimated using morphometric analysis and data reduction techniques based on landmark coordinate data from the cranium. Orientation of synchondroses in the newborns was assessed qualitatively and quantitatively. For example, the midsagittal orientation of the presphenoseptal synchondrosis was quantified using sliding semilandmarks on the rostral edge of the presphenoid in the midsagittal plane. Associations between newborn synchondroseal metrics and adult shape were tested using nonparametric regression. For a subset of the sample, histological imaging was used to describe the orientation of proliferative columns of chondrocytes relative to surrounding anatomical structures.

As expected, our results demonstrate that orientation of synchondroses in newborn primates is associated with phylogenetically informative cranial traits in adults. Shape of the presphenoseptal synchondrosis predicts the degree of facial reduction. A curved synchondrosis and radiating proliferative columns in the newborn is associated with facial reduction in the adult. The intrasphenoidal synchondrosis is characterized by “bulging” cartilage at the endocranial surface and an arched orientation in primates that display a more acute cranial base angle as adults. This likely reflects increased proliferation of chondrocytes at the endocranial compared to the ectocranial margin of the intrasphenoidal synchondrosis. In contrast, the spheno-occipital synchondrosis is oriented similarly across primates. Finally, the orientation of ali-basisphenoid synchondroses in newborns is associated with the structure of the sphenoid in adults. As a result, the adult alisphenoid extends farther rostrally and ventrally relative to the basisphenoid in haplorhines compared to strepsirrhines. The orientation of the ali-basisphenoid synchondroses is likely a result of relative expansion of the temporal lobes of the brain. Three-dimensional reconstructions of soft tissue functional matrices (brain and oronasal structures) provide evidence that mechanical influences mediate variation in orientation of synchondroses, and therefore phylogenetically informative traits in the adult skull.