Pharyngeal Apparatus and Their Derivatives Flashcards
State the components of the pharyngeal apparatus.
- Pharyngeal arches
- Pharyngeal pouches
- Pharyngeal clefts/grooves
- Pharyngeal membrane
- [Diagram: components of the pharyngeal apparatus]
What are pharyngeal arches? When do they develop? What do they resemble in fish?
- Pharyngeal arches are paired bars of mesenchymal tissue around embryonic pharynx.
- They develop in the 4th week as neural crest cells migrate into the head and neck region.
- They resemble gills in fish (branchia), hence also called the branchial arches.
When they develop, at first what pairs of arches are visible externally by week 5? Which ones are not visible? In humans, which arch involutes? At that time, how do the pharyngeal arches appear?
- the first 4 pairs are visible by week 5.
- 5th and 6th pairs are not visible.
- 6th arch merged with 4th arch.
- 5th pairs involutes/disappears in humans.
- these arches appear as surface elevations in front and lateral to the developing pharynx.
The arches are separated by clefts and pouches. What are these clefts and pouches? What is the pharyngeal membrane?
- The clefts are external, and are lined by ectoderm
- The pouches are internal, and are lined by endoderm
- The tissue interphase between one arch and the next is called pharyngeal membrane.
- [Diagram]
NB: Pharyngeal membrane consists of all the three layers of the gastrula i.e.?
State the components of a pharyngeal arch. (5)
(1) A core of mesenchyme: it has dual sources i.e. neural crest cells and intraembryonic mesoderm.
(2) A cartilaginous bar: offers structural support to the components of the pharyngeal arch.
(3) Aortic arch artery: to supply the components of the arch. (we have 1st, 2nd, 3rd, 4th, and 6th aortic arches. They come from the aortic sac, supply the branchial arches and then terminate in the primitive dorsal aorta.)
(4) A branch of a cranial nerve: to supply the muscles in the arch, and to supply the mucosa of the pharyngeal arch.
(5) A muscular component.
NB: Musculature develops from the mesoderm, skeletal components derived from neural crest cells
[Diagram 1] [Diagram 2]
State the fate of the pharyngeal arches.
- Some of the cartilage involutes, but the parts of it that remain form the skeleton of the arch.
- The muscular components differentiate into some muscles of the head and neck region.
- The nerve supplies the mucosa and muscles derived from the arch.
- The aortic arch artery supplies the components of the arch, but most of them disappear. A few of them persist to supply the components of the arch.
1st pharyngeal arch (Mandibular arch)
(a) Cranial nerve
(b) Muscular derivatives
(c) Arch cartilage and skeletal derivatives
(d) The arch artery persists as?
(a) Cranial nerve: Sensory: CN V2, Motor: CN V3
(b) Muscular derivatives
➤ Muscles of mastication: masseter, medial pterygoid, lateral pterygoid, temporalis
(Remember that the masseter and medial pterygoid form part of the parotid bed? They were ____________ relations of the parotid gland.)
[Diagram: Medial and lateral pterygoid muscles.]
➤ Anterior belly of digastric muscle
➤ Tensor veli palatini
➤ Tensor tympani
➤ mylohyoid muscle
(c) Arch cartilage and skeletal derivatives: Meckel’s cartilage [usually disappears as mandible forms around it]
➤ Malleus
➤ Incus
➤ Anterior ligament of malleus
➤ Spine of sphenoid
➤ Sphenomandibular ligament
➤ Genial tubercle of mandible
➤ Part of the future maxilla, mandible, zygomatic bone, temporal bone
(d) Artery: Maxillary artery
2nd pharyngeal arch (Hyoid arch)
(a) Cranial nerve
(b) Muscular derivatives
(c) Arch cartilage and skeletal derivatives
(d) The arch artery persists as?
(a) Cranial nerve: CN VII
(b) Muscular derivatives
➤ Muscles of facial expression
➤ Auricular muscles; auricularis superior, auricularis posterior, auricularis anterior
➤ Stylohyoid muscle
➤ Platysma
➤ Posterior belly of digastric muscle
➤ Stapedius
(c) Arch cartilage and skeletal derivatives: Reichert’s cartilage
➤ lesser horn/cornu and upper part of body of hyoid bone (ventral end of cartilage)
➤ styloid process (dorsal end)
➤ stapes (dorsal end)
➤ perichondrium of cartilage forms stylohyoid ligament
[Diagram: Hyoid bone]
(d) Artery: Ascending pharyngeal artery (this is the ventral part of the aortic arch artery remnant) and stapedial artery (this is the dorsal part of the aortic arch artery remnant).
3rd pharyngeal arch (carotid arch)
(a) Cranial nerve
(b) Muscular derivatives
(c) Skeletal derivatives
(d) The arch artery persists as?
(a) Cranial nerve: CN IX
(b) Muscular derivatives: Stylopharyngeus muscle (it elevates the pharynx)
(c) Skeletal derivatives: lower part of body and greater horn of hyoid bone
(d) Artery: Common carotid artery and the initial segments of internal carotid artery
4th pharyngeal arches
(a) Cranial nerve
(b) Muscular derivatives
(c) Skeletal derivatives
(d) The arch artery persists as?
(a) Cranial nerve: superior laryngeal branch of X
(b) Muscular derivatives: All pharyngeal muscles except stylopharyngeus (a longitudinal muscle of the pharynx):
➤ longitudinal muscles of the pharynx other than stylopharyngeus: palatopharyngeus, salpingopharyngeus
➤ circular muscles of the pharynx: superior, middle and inferior pharyngeal constrictors
NOTE that the inferior pharyngeal constrictor has two parts; the upper part from the 4th arch called thyropharyngeus, and the lower part from the 6th arch called cricopharyngeus.
[Diagram]: Thyropharyngeus and Cricopharyngeus
➤ palatoglossus [only tongue muscle derived from a pharyngeal arch: other tongue muscles are derived from occipital myotomes]
➤ levator palatini
➤ cricothyroid
(c) Skeletal derivatives: Laryngeal cartilages; thyroid, cricoid etc.
(d) Artery: The right 4th aortic arch artery becomes right subclavian artery. The left 4th aortic arch becomes arch of aorta (recurrent laryngeal nerve recurs under them)
6th pharyngeal arches
(a) Cranial nerve
(b) Muscular derivatives
(c) Arch cartilage and skeletal derivatives
(d) The arch artery persists as?
(a) recurrent laryngeal nerve of X
(b) intrinsic laryngeal musculature (control the: tension of vocal cord, opening of the glottis and the entry into the larynx)
[All are innervated by recurrent laryngeal nerve of X except cricothyroid which is innervated by superior laryngeal nerve of X]
(c) Laryngeal cartilages; thyroid, cricoid etc.
(d) The right 6th arch artery becomes the right pulmonary artery. The left 6th arch artery becomes the left pulmonary artery and the ductus arteriosus.
Why do 1st arch syndromes arise? How do they manifest? List these syndromes.
☛ They arise due to some level of deficiency of mesenchyme in the 1st pharyngeal arch (embryological basis).
☛ They manifest as abnormal skeletal makeup i.e. mandibulofacial dystosis (anatomical basis).
☛ They are:
1. Treacher-Collins syndrome
[Description]
2. Pierre-Robin syndrome
[Description]
What pharyngeal arch anomaly involves both 1st and 2nd arch derivatives?
Goldenhar syndrome: Facial asymmetry due to hemifacial microsomia [embryological basis]
State the fate of pharyngeal clefts.
☛ The 1st cleft forms the external auditory meatus
☛ The 2nd – 4th clefts fuse & obliterate
☛ Persistence = branchial cysts, sinus or fistula.
☛ These malformations occur anterior to the sternocleidomastoid muscle.
☛ Reminder:
cyst: an enclosed space without any drainage
sinus: a tract with a blind end (in this case, could be opening to the skin)
fistula: abnormal channel connecting 2 epithelial surfaces (in this case, connecting the skin to the mucosal pharynx)
☛ [Image: Branchial cysts]
☛ [Image: Branchial sinus]
☛ [Diagram: Branchial fistula]
There are 4 pharyngeal pouches. What happened to the 5th pouch?
There are 5 arches, so there are 4 pouches. However, when the 5th pharyngeal arch involuted, what was supposed to be the 5th pharyngeal pouch then merged with the 4th pharyngeal pouch and hence what was to be the 5th pouch became the ultimobranchial body.
Further notes:
The ultimobranchial body (UBB) is an outpocketing of the fourth pharyngeal pouch that fuses with the thyroid diverticulum, giving rise to calcitonin-producing C-cells.
