Cranial base angulation and growth of the human fetal pharynx

The upper airway controls the mechanisms of breathing, swallowing, and vocalization. Information on its normal ontogenetic associations is therefore crucial for understanding human speech evolution as well as the developmental etiology of speech abnormalities. Of particular interest to evolutionary morphologists is the proposed structural association between the upper airway and the cranial base. The present study aims to elucidate whether cranial base angulation is linked to growth of the upper airway during human fetal life. Forty postmortem human fetuses ranging from 10 to 29 weeks of gestation were imaged with high-resolution magnetic resonance imaging. Measurements of basicranial angle, hyoid depth, hormion height, and pharyngeal area were taken from sagittal images. Results show a significant correlation between the internal and external basicranial angles but the correlation between relative hyoid depth and internal cranial base angle was not significant. Findings show a significant correlation between increases in the size of the oropharynx relative to the nasopharynx and increases in the internal cranial base angle (retroflexion). These findings give limited support (rrank=0.46; P<0.01) for the hypothesis that restructuring of the upper airway underlies basicranial retroflexion during prenatal life.