Summary Introduction Patients with dentofacial disharmonies (DFDs) seek orthodontic care and orthognathic surgery to address issues with mastication, esthetics, and speech. Speech distortions are seen 18 times more frequently in Class III DFD patients than the general population, with unclear causality. We hypothesize there are significant differences in spectral properties of stop (/t/ or /k/), fricative (/s/ or /ʃ/), and affricate (/tʃ/) consonants and that severity of Class III disharmony correlates with the degree of speech abnormality. Methods To understand how jaw disharmonies influence speech, orthodontic records and audio recordings were collected from Class III surgical candidates and reference subjects (n = 102 Class III, 62 controls). A speech pathologist evaluated subjects and recordings were quantitatively analysed by Spectral Moment Analysis for frequency distortions. Results A majority of Class III subjects exhibit speech distortions. A significant increase in the centroid frequency (M1) and spectral spread (M2) was seen in several consonants of Class III subjects compared to controls. Using regression analysis, correlations between Class III skeletal severity (assessed by cephalometric measures) and spectral distortion were found for /t/ and /k/ phones. Conclusions Class III DFD patients have a higher prevalence of articulation errors and significant spectral distortions in consonants relative to controls. This is the first demonstration that severity of malocclusion is quantitatively correlated with the degree of speech distortion for consonants, suggesting causation. These findings offer insight into the complex relationship between craniofacial structures and speech distortions.

SUMMARY The mouth arises from the extreme anterior domain (EAD), a region where the ectoderm and endoderm are directly juxtaposed. Here we identify a ‘pre-mouth array’ in Xenopus that forms soon after the cranial neural crest has migrated to lie on either side of the EAD. Initially, EAD ectoderm comprises a wide and short epithelial mass that becomes narrow and tall with cells and nuclei changing shape, a characteristic of convergent extension. The resulting two rows of cells - the pre-mouth array - later split down the midline to surround the mouth opening. Neural crest is essential for convergent extension and likely signals to the EAD through the Wnt/PCP pathway. Fzl7 receptor is locally required in EAD ectoderm, while Wnt11 ligand is required more globally. Indeed, heterologous cells expressing Wnt11 can elicit EAD convergent extension. The study reveals a precise cellular mechanism that positions and contributes to the future mouth.