What embryologic failure leads to Treacher Collins syndrome?

• A. Failure of neural crest migration into the fourth arch
• B. Failure of neural crest migration into the first pharyngeal arch
• C. Abnormal maturation of endoderm
• D. Defective paraxial mesoderm segmentation

Answer: (B)

Treacher Collins arises when neural crest cells fail to migrate into the first pharyngeal arch during early development. These cells populate the first arch and form much of the midface, including the maxilla and zygomatic bones, as well as ear structures and part of the mandible. If their migration is defective, the facial skeleton underdevelops, producing the prominent features of Treacher Collins such as hypoplastic cheekbones, micrognathia, and ear anomalies.

Other options describe problems in different tissues or stages that don’t explain the facial bone hypoplasia seen in this condition: migration into a later arch would affect different facial structures, abnormal endoderm maturation would impact pharyngeal pouch derivatives rather than the facial bones, and issues with paraxial mesoderm development affect trunk structures and some muscles rather than the neural crest–derived facial skeleton.