Embryology · Head and neck: pharyngeal arches

First-arch syndromes: Treacher Collins and Pierre Robin sequence !

Malformation

Defects of the jaw, cheekbones and ears because the first arch did not grow properly. They can make it hard for a newborn to breathe.

Updated 2026-11-01

See it in 3D: Mandible · Maxillae · Zygomatic bones · Malleus · Incus · Auricle · Ear canal · Tongue · Soft palate

What it is

First-arch syndromes are defects of the structures the first pharyngeal arch builds: mandible, maxilla, zygoma, ossicles and outer ear. The two most tested are Treacher Collins syndrome, a genetic mandibulofacial dysostosis, and the Pierre Robin sequence, in which a small mandible sets off the other findings. In both, the root problem is the neural crest that fills the first arch.

Key facts

Treacher Collins: about 1 in 50,000 people (MedlinePlus).
Isolated Pierre Robin: between 1 in 8,500 and 1 in 14,000 (MedlinePlus).
'Sequence' means one first defect (the mandible) produces the others in a chain.

Embryological cause

Treacher Collins: variants in TCOF1 (most cases; treacle protein), POLR1D (both autosomal dominant, about 60% new) or POLR1C (recessive).
These genes help make ribosomal RNA; their failure causes excess death of 1st- and 2nd-arch neural crest cells, although migration is normal.
Pierre Robin: the first defect is a small mandible (micrognathia), which leaves the tongue back (glossoptosis), between the palatal shelves, and blocks palate closure (weeks 7–10).
Pierre Robin can be isolated (changes near SOX9), part of a syndrome (Stickler, 22q11.2) or a deformation from chin compression (oligohydramnios).
Teratogens such as isotretinoin and alcohol can injure first-arch neural crest and give similar pictures.

How it presents

Treacher Collins: underdeveloped cheekbones and mandible, down-slanting palpebral fissures, lower eyelid coloboma and sparse lashes.
Treacher Collins: small or absent ears, ear canal atresia and conductive hearing loss in about half; intelligence usually normal.
Pierre Robin: triad of micrognathia, glossoptosis and airway obstruction; most have a U-shaped cleft palate.
Both can cause breathing and feeding problems from birth.
Differential: hemifacial microsomia (oculo-auriculo-vertebral spectrum), usually asymmetric.

Diagnosis

Prenatal ultrasound: small, receding mandible on the fetal profile, sometimes with polyhydramnios from poor swallowing.
At birth: airway and feeding assessment; sleep study if there is obstruction.
Early hearing assessment, especially in Treacher Collins.
Genetic testing (TCOF1, POLR1C, POLR1D; look for Stickler or 22q11.2 in Pierre Robin).

Test yourself

Think of the answer, then tap the question to see it.

What is the first defect in the Pierre Robin sequence?

The small mandible (micrognathia), which pushes the tongue back and blocks palate closure.

Which gene is most often altered in Treacher Collins syndrome?

TCOF1, which encodes the treacle protein; inheritance is autosomal dominant.

Why is there conductive hearing loss in Treacher Collins syndrome?

Because 1st- and 2nd-arch derivatives are affected: ossicles, external ear canal and auricle.

Which embryonic cells fail in both conditions?

The neural crest cells that form the mesenchyme of the first arch.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

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Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: Treacher Collins syndrome: MedlinePlus Genetics (National Library of Medicine (MedlinePlus), 2022); Isolated Pierre Robin sequence: MedlinePlus Genetics (National Library of Medicine (MedlinePlus), 2016); Embryology, Branchial Arches - StatPearls - NCBI Bookshelf (StatPearls Publishing (NCBI Bookshelf), 2023); Langman's Medical Embryology, 13th ed., ch. 17: Head and Neck (Wolters Kluwer, 2015).

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Representative model, for educational purposes only.