Pharyngeal Arches

Question 1

When do the arches develop and from what?

Answer 1

The pharyngeal arches begin to develop early in the fourth week from mesenchyme derived from the neural crest;

the lateral plate mesoderm and the paraxial mesoderm also migrate to the head and neck region of the embryo

Question 2

How many are there?

What do they consist of?

Answer 2

Five pairs of pharyngeal arches, numbered 1, 2, 3, 4 and 6, form in craniocaudal sequence and by the end of the fifth week The fifth pharyngeal arch is often rudimentary and soon disappears.

Each pharyngeal arch consists of a core of mesenchyme, has an outer covering of ectoderm and is lined internally by endoderm

Question 3

What does ectoderm appear as?

Endoderm?

What do the first pair give rise to?

Answer 3

The ectoderm appears as the pharyngeal clefts (grooves) between the arches, and the endoderm as the pharyngeal pouches

The first pair of pharyngeal arches not only gives rise to the upper and lower jaw, but they also play a major role in the development of the face and palate.

Question 4

What does thier mesenchyme differentiate into?

What does each arch contain and where do they enter? (nerves)

Answer 4

The mesenchyme in each pharyngeal arch differentiates into a bar of cartilage, the associated muscle and an aortic arch artery .

Each pharyngeal arch also contains a cranial nerve (from nerves V, VII, IX and X) that enters it from the brainstem of the developing brain

Question 5

Give two examples of autosomal dominat genes causing first arch defects

Answer 5

These anomalies are grouped together and known as the first arch syndrome.

Treacher-Collins syndrome, the mandible and facial bones are small, and the external ears are malformed.

Pierre-Robin syndrome, the mandible is extremely small and the palate is cleft.

Question 6

What happens to muscles and their original nerves.

Answer 6

During further development many pharyngeal arch muscles migrate from their original site of origin to reach their final destination, but they retain their original innervation. Thus, the origin of each muscle can be determined from its nerve supply

So - move but keep original innervation