Variation and Covariation of Size and Shape of the Sphenoidal Sinus in H. sapiens

Abstract

No consensus about the function or evolutionary forces that influence variation of paranasal sinuses among humans has been established. By establishing the covariance of sinus shape with surrounding morphology, researchers may better develop hypotheses about the potential for independent evolution in the sinuses. The sphenoidal sinuses are rarely studied, despite being centrally located within the basicranium, at the intersection of the three cranial fossae and adjacent to the synchondroses, the location of cranial base angulation. The angulation of the cranial base and its relative orientation with the face is a distinctive, derived trait in Homo sapiens and is important to our understanding of the evolution of our species, yet there are aspects of that anatomy (the sphenoidal sinuses) that have not yet been investigated for their variation and influence in this highly integrated region. Previous research has shown that cranial base angulation is correlated with sphenoidal sinus size variation. Yet, the covariance of size, shape, and orientation of the cranial fossae have not been explored, though these likely have a developmental and evolutionary relationship with the sphenoidal sinus. Here we analyze the covariance of size and shape in the sphenoid sinus with metrics of the cranial base captured using geometric morphometric techniques. Computed tomography (CT) scans of 45 adult human crania were digitally segmented for analysis. Cranial fossa shape was captured with landmarks, while sinus volume was measured from manual segmentation of CT images. Regression analyses were performed between the landmark data and the sinus volume data to assess their covariation. Using a separate sample, we will analyze the three-dimensional shape of the sphenoidal sinus (i.e., the volumetric apportionment) using spherical harmonic coefficients, and compare its variance to variation in the shape of the cranial fossae and anterior cranial base using Two Block Partial Least Squares (TB-PLS) analyses. Previous findings support the existence of a relationship between basicranial angulation and sinus volumetric variation, but not the lateral expansion or shape of the posterior brain. These findings suggest that the sphenoidal sinus may be influenced by the shape and relative position of anterior fossa. These relationships likely represent the influence of the soft tissue anatomy (including the brain, pituitary gland, and the internal epithelial tissues) on the dimensions of the sphenoid, and thus the trajectory of its pneumatization. We expect the morphology of the sinus to demonstrate similar relationships. These findings on the pattern of covariation inform the directions of future research on the evolution and development of the sphenoidal sinuses and basicranium.