Head and neck

The pharyngeal arches

7 steps · 3 anomalies · Week 4 to Weeks 6 to 8

Read this first

The single highest-yield story in first-year embryology, and the one students try to memorise as a table. Do not. Learn it as a rule: every arch is a block of mesoderm with its own cartilage, its own muscles, its own nerve and its own artery, and whatever that arch makes keeps that nerve for life. That is why the muscles of mastication answer to the mandibular nerve and the muscles of facial expression to the facial nerve — they came from different arches.

Week 4

Step 1 of 7

What happens: Six pairs of arches appear in the wall of the primitive pharynx, each covered by ectoderm outside and endoderm inside, with mesoderm and neural crest between.
What it forms: A cartilage bar, a muscle mass, a nerve and an aortic arch artery in every arch.
What remains in the adult: The fifth arch is rudimentary and disappears, so the adult derivatives come from arches one, two, three, four and six.

Nothing in this set goes wrong at this step — it is here because the story does not make sense without it.

Every anomaly, and the step it blames

This is the table the examiner is working from. Read the clue column first and see whether you can name the step before you look.

Branchial cyst or fistula
You are told: A painless swelling at the front of the neck in a young adult, at the anterior border of sternocleidomastoid.
Failed step: The cervical sinus, the space left when the second arch overgrew the third and fourth clefts, is obliterated. (week 7)
The cervical sinus fails to disappear, leaving a cyst deep to the anterior border of sternocleidomastoid, or a fistula opening at its lower third.
DiGeorge syndrome
You are told: A neonate with hypocalcaemic tetany and repeated infections.
Failed step: The endodermal pouches between the arches differentiate. (week 6)
The third and fourth pouches fail to develop, so the thymus and both pairs of parathyroids are absent or hypoplastic.
First arch syndromes
You are told: Underdeveloped mandible and cheek bones with abnormal external and middle ear.
Failed step: The first, mandibular arch forms Meckel's cartilage and takes the mandibular nerve. (week 4)
Neural crest cells fail to migrate into the first arch, so its derivatives are hypoplastic.

Viva voce

1 of 5 · 0 answered

Say your answer out loud before you open it. That is the exam skill.

How many pharyngeal arches develop, and which one disappears?

Senior’s 30-second takeaway

The pharyngeal arches — the whole bench in four lines. Read this on the way in.

If you see this: A question naming an arch, a pouch or a cleft · A muscle whose nerve supply looks arbitrary · A neck swelling in a young adult
If asked to identify it: Each arch has its own cartilage, muscle, nerve and artery, and its derivatives keep that nerve for life.
If asked one more: The third pouch makes the inferior parathyroid because the thymus drags it past the fourth.
If asked why it matters: A persistent cervical sinus gives a branchial cyst at the anterior border of sternocleidomastoid.

Then stop. Let the examiner ask the next question.

🧬 Read this first

Every anomaly, and the step it blames

🎙️ Viva voce

Senior’s 30-second takeaway

Read this first

Viva voce