Chapter 49. The Branchial Apparatus: The Branchial (pharyngeal) Arches - Review of Medical Embryology Book

The branchial apparatus (branchial, meaning gills) consists of the branchial arches, the pharyngeal pouches, the branchial grooves, and the branchial membranes. Modeling and development of the cephalic end and face of the embryo result in a complex form for the pharyngeal segment of the digestive tract. The lateral walls of the developing pharyngeal gut form the branchial system or apparatus
Branchial or pharyngeal arches are masses of mesoderm covered by ectoderm and lined by entoderm. Within these masses, muscular and skeletal components develop, as well as aortic arches and nerve networks. The arches are separated by grooves, visible on the surface of the embryo as pharyngeal clefts and in the interior as the pharyngeal pouches

THE BRANCHIAL SYSTEM is only transitory. Continuous modification of the cephalic end of the embryo, during brain development, also affects the basic architecture. The arches give rise to skeletal structures from predominantly cartilaginous precursors; the pouches and clefts either are effaced or persist as ducts or canals. Some give rise to important glandular structures. The branchial system is formed at the 5 mm embryo stage, in week 4, with its 4 arches, 4 pharyngeal clefts, and 5 pharyngeal pouches

The mandibular arch (arch I) is centered on Meckel's cartilage. The malleus and incus, ossicles of the middle ear, develop from its posterior portion. The mandible and muscles of mastication form from tissues (mesenchyme and invading neural crest cells) surrounding Meckel's cartilage

It is supplied by aortic arch I (facial artery) and the mandibular division of the trigeminal nerve (V3)
The hyoid arch (arch II) is centered on Reichert's cartilage. It gives rise to the stapes of the middle ear, the styloid process of the temporal bone, the stylohyoid ligament, the lesser horns and upper part of the body of the hyoid bone, the platysma muscle, and the muscles of facial expression

It is supplied by aortic arch II (external carotid artery) and the facial nerve (VII)

The thyrohyoid arch (arch III) produces the body and greater horns of the hyoid bone and the stylopharyngeus muscle

It is supplied by aortic arch III (internal carotid artery) and the glossopharyngeal nerve (IX)

The (unnamed) arch IV is much less clearly differentiate It gives rise to the cartilages of the larynx and the cricothyroid muscle

It is supplied by aortic arch IV and the external branch of the superior laryngeal nerve of the vagus nerve (X)

Pharyngeal arches V and VI (unnamed) are never seen in humans. Their corresponding aortic arches, however, do occur. The muscle mass of arch V forms some of the intrinsic muscles of the larynx, which are supplied by the recurrent or inferior laryngeal branch of the vagus nerve (X)

The cartilages of arches IV and V become the framework of the larynx
The thyroid cartilage originates from both arches IV and V
The cricoid, arytenoids, the rings of the trachea, and the bronchi are formed from arch VI
Arch VI is distinctly differentiated, whereas V is transitory

AT 8 mm, ARCH II develops more rapidly than the others and overlaps the other arches caudally. This process is further accentuated by the flexion of the head at this stage of development
AT ABOUT 13 mm (about 34 days), ARCH II has entirely overlapped arches III and IV. It also has closed the second, third, and fourth pharyngeal clefts to form the cervical sinus. Only the first pharyngeal cleft persists, and by day 42, it is seen as the external auditory meatus, the only exterior evidence of the pharyngeal system