The Development of the head

pharyngeal arches

INTRODUCTION

The development of the head and neck is by the pharyngeal arches, pouches and clefts.
These are structures in the primitive pharynx that give rise to structures in the head.

We're going to look at each of them with their structural derivatives.

PHARYNGEAL ARCHES

There are five pharyngeal arches. The followingare the arches with their derivatives:

FIRST ARCH

The cartilage of the first pharyngeal arch has a ventral and a dorsal part.
The dorsal part is also called the maxillary cartilage. This part further regresses but leaves two "children" as it does so. These are the mallus and incus. On further development it forms the pre-maxilla, then the maxilla, zygomatic bone and part of temporal bone. It does this through membranous ossification.

The ventral part forms the Merkel's or Mandibular cartilage. This forms the mandible through membranous ossification, except for the lingula which ossifies in cartilage.
Muscular derivatives of the first arch are muscles of mastication, mylohyoind musle, anterior belly of digrastic, tensor tympani and tensor palatini.
They are supplied by the mandibular nerve which is the nerve of the first arch.

SECOND (HYOID) ARCH

Skeletal derivatives:stapes, styloid process, stylohyoid ligament, lesser horn and upper part of body of hyoid bone.

Muscular derivatives: muscles of facial expression, posterior belly of digastric muscle, stylohyoid muscle, stapedius and auricular muscles.

Nerve supply: Facial nerve

THIRD ARCH

Skeletal derivatives: greater horn and lower part of body of hyoid bone.

Muscular derivatives:upper pharyngeal constrictors, stylopharyngeus.

Nerve supply: Glossopharyngeal nerve

FOURTH AND SIXTH ARCHES

Skeletal derivatives: cartilages of larynx

Muscular derivatives: cricothyroid, intrinsic muscles of larynx, muscles of pharynx.

Nerve supply: Nerve of 4th arch is superior pharyngeal and 6th arch is recurrent laryngeal, both are branches of vagus nerve.

NOTE:The 5th arch disppears during development without forming any structure.

PHARYNGEAL POUCHES

These are out-pouchings of the primitive pharynx.
Are in five pairs. They are:

FIRST POUCH

It elongates to form the Eustachian (pharyngotympanic) tube. The distal ends becomes dilated as the middle ear.
It also forms lining of tympanic membrane.

SECOND POUCH

Proliferates and us invaded by lymphoid tissues to form palatine tonsil and bed.

THIRD POUCH

Dorsal part of it gives rise to inferior parathyroid gland while ventral part forms the thymus.

FOURTH POUCH

Developes into superior parathyroid gland.

FIFTH POUCH

Also called the ultimobranchial pouch. It gets incorporated into the thyroid gland to form the parafollicular C cells that secrete calcitonin.

PHARYNGEAL CLEFTS

The first cleft is the only one to remain open. It forms the external auditory meatus.
In normal development, the tissue of first arch grows caudally and closes up the remaining clefts.

Please take a small quiz about this lecture here.

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