2005 zillmusom

I. DEVELOPMENT OF BRANCHIAL ARCHES ‑ structures which develop in embryo; are comparable to gills of fish; reflect fact that ontogeny (development of individual) resembles phylogeny (evolution of species); are important in understanding final structure and innervation of head and neck.


A. Week 4 ‑ Neural crest cells invade future head and neck region of embryo; cells form ridges on side of head and neck (Sadler Fig. 15.3) located lateral to rostral part of the foregut; will form branchial arch components. Terminology confusing (Note: Branchial Arch = Pharyngeal Arch, Branchial Arch Artery = Aortic Arch, Cleft = Groove)


B. Branchial apparatus (Sadler Fig. 15.6A, 15.7) ‑ Composed of 4 elements (including branchial arches):


1. Branchial arch ‑ components ‑ arches are covered by ectoderm externally; lined internally by endoderm; core of arch formed by mesenchyme; mesenchyme will form muscles, arteries, connective tissue, cartilage and parts of skeleton; each arch has a specific nerve that innervates the muscles that develop from that arch; some arteries will form adult vessels (considered earlier as Aortic Arches).


2. Branchial groove (Pharyngeal cleft) ‑ ectodermal (external) cleft between adjacent arches (Sadler Fig. 15.8)


3. Branchial pouch ‑ endodermal outpocketing of rostral part of foregut; pouches are located between adjacent branchial arches. (Sadler Figs. 15.4, 15.6, 15.10)


4. Branchial membrane ‑ site of contact of ectoderm of branchial groove with endoderm of pharyngeal pouch (see Sadler Fig. 15.10, 15.11 for pouch 1).


D. Branchial apparatus of embryo is reshaped into new structures; structures can disappear or form vestigial remnants by the end of the embryonic period.


II. FATE OF BRANCHIAL ARCHES ‑ contribute to formation of face, neck, mouth, larynx, and pharynx see chart (also Sadler Figs. 15.7-15.9)


A. Branchial Arch Cartilages form skeletal elements (bones, cartilages and ligaments)

B. Branchial Arch Nerves are cranial nerves (SVE component) First arch = Trigeminal (V), Second arch = Facial N. (VII); Third arch = Glossopharyngeal N. (IX); Fourth arch = Vagus (X); Sixth arch (caudal) = Accessory N. (XI)


C. Branchial arch muscles many (see chart); each muscle migrates but continues to be innervated by the cranial nerve to the arch from which the muscle is derived.










First (V)

1) Malleus

2) Incus

1) Ant. ligament of malleus

2) Sphenomandibular ligament

1) Muscles of Mastication

2) Tensor tympani

3) Tensor palati

4) Mylohyoid

5) Ant. belly of Digastric

1) Auditory tube

2) Tympanic cavity

Second (VII)

1) Stapes

2) Styloid process

3) Hyoid bone - lesser horn, upper half of body

Stylohyoid ligament

1) Muscles of Facial Expression

2) Stapedius

3) Stylohyoid

4) Post. belly of Digastric

Lining (crypts) of palatine tonsils

Third (IX)

Hyoid bone - greater horn, lower half of body



1) Inferior parathyroid gland

2) Thymus

Fourth (X)

Cartilages of Larynx


1) All muscles of Larynx

2) All muscles of Pharynx (except Stylopharyngeus)

3) All muscles of Soft Palate (except Tensor palati)

1) Superior parathyroid gland

2) C-cells of Thyroid

Sixth (XI)



1) Sternocleidomastoid

2) Trapezius



Note: First Branchial Groove (Cleft) becomes External Auditory Meatus

First Branchial Membrane becomes Tympanic Membrane







groove (cleft)





III. FATE OF BRANCHIAL POUCHES (Sadler Fig. 15.10, 15.11)


A. Pouch 1 ‑ elongates into tubotympanic recess; forms Auditory tube and tympanic cavity.


B. Pouch 2 ‑ forms epithelial lining of Crypts (spaces) of the Palatine tonsils.


C. Pouch 3 ‑ Upper part forms Inferior Parathyroid gland; lower part forms Thymus gland


D. Pouch 4 ‑ Expands into superior and inferior parts.


1. Superior part ‑ forms superior parathyroid gland;


2. Inferior part ‑ source of C cells (produce hormone calcitonin) which fuse with thyroid gland.


NOTE: Superior parathyroid gland develops from Pouch 4 and Inferior parathyroid gland from Pouch 3; final position occurs because elements from Pouch 3 migrate caudal to Pouch 4.




A. Four branchial grooves separate the branchial arches externally on each side; only one pair of branchial grooves forms a structure in the adult; the first branchial groove forms the External auditory meatus, the first branchial membrane forms the Tympanic Membrane. (Sadler Fig. 15.10, 15.11)


B. The other branchial grooves develop to lie in a larger depression called the cervical sinus; this sinus is normally obliterated during development


Note: Cervical sinus can persist as a Branchial sinus (blind pouch off pharynx) or a Branchial Cyst Fistula (channel connecting pharynx to skin); when present are found anterior to Sternocleidomastoid. (Sadler Fig. 15.14, 15.15)


Note: Branchial fistula (channel) ‑ when present often extends from 2nd pharyngeal pouch and passes between Internal and External Carotid arteries and exits to skin Anterior to the sternocleidomastoid muscle; can become infected.




V. DEVELOPMENT OF THYROID GLAND ‑ (Sadler Figs. 15.17 - 15.20)


A. Initial stage ‑ a median endodermal thickening forms in floor of primitive pharynx at site of junction of future anterior 2/3's and posterior 1/3 of tongue.


B. Later ‑ thickening elongates into floor of pharynx as the Thyroid diverticulum; opening of diverticulum on surface of developing tongue called the Foramen Cecum.


C. Developing Thyroid diverticulum descends in the neck anterior to the hyoid bone and larynx; as diverticulum (developing gland) elongates into neck, a Thyroglossal duct connects diverticulum with foramen cecum.


D. Developing thyroid gland reaches final site in neck (anterior to upper rings of trachea); thyroglossal duct disintegrates; foramen cecum remains as a vestigial pit on the tongue.


E. Congenital malformations (Atlas Fig. 8.17, Sadler Figs. 15.19 - 15.20)


1. Persistent thyroglossal duct remnants ‑ part of duct can remain and form thyroglossal cysts anywhere from foramen cecum of tongue to thyroid gland in neck; cysts found in midline of neck and can be located anterior to hyoid bone or larynx.


2. Pyramidal lobe ‑ present in 50 percent of people; represents persistent part of thyroglossal duct, which can contain some thyroid tissue; lobe can be attached to hyoid bone by fibrous strand; usually no associated clinical problems.