Pharyngeal Arches Softchalk

Question 1

Describe the general development of branchial (pharyngeal) arches.

Answer 1

There are 5 pairs of pharyngeal arches numbered 1,2,3,4,6.
They from in craniocaudal succession during week 4.
They are separated externally by ectoderm-lined pharyngeal clefts (grooves) and internally by endoderm- lined pharyngeal pouches. Pharyngeal membranes appear in the floors of the pharyngeal grooves.

Question 2

Name the skeletal, ligamentous, muscular, nervous, and vascular derivatives of each branchial (pharyngeal) arch.

Answer 2

FIRST (MANDIBULAR)
Trigeminal (CN V)

Muscles of mastication
Mylohyoid and anterior belly of digastric
Tensor tympani
Tensor veli palatini
malleus, incus

Anterior ligament of malleus
Sphenomandibular ligament

Maxillary arteries

SECOND (HYOID)
Facial (CN VII)

Muscles of facial expression
Stapedius
Stylohyoid
Posterior belly of digastric

Reichert’s cartilage:

• Stapes
• Styloid process
• Lesser cornu of hyoid bone
• Upper part of body of hyoid bone

Stylohyoid ligament

Hyoid and stapedial arteries

THIRD
Glossopharyngeal (CN IX)

Stylopharyngeus

Greater cornu of hyoid bone
Lower part of body of hyoid bone

Common carotid and first part of the internal carotid arteries

FOURTH
Superior laryngeal branch of vagus (CN X)

Cricothyroid
Levator veli palatini
Constrictors of pharynx

Laryngeal cartilages
(the fourth and sixth pharyngeal arch cartilages fuse to form the laryngeal cartilages)

Left side: Arch of the aorta from the left common carotid to the left subclavian arteries
Right side: Right subclavian artery (proximal portion)

SIXTH
Recurrent laryngeal branch of vagus (CN X)

Intrinsic muscles of larynx
Striated muscles of esophagus

Laryngeal cartilages

Left side: Left pulmonary artery and ductus arteriosus
Right side: Right pulmonary artery

Question 3

Be familiar with the embryological origin of the thymus and parathyroid glands.

Answer 3

Thymus Basics: Develops from endoderm of the THIRD pair of pharyngeal pouches and surrounding mesenchyme. Neural crest cells have been shown to play a major role. Thymus growth is not complete at birth, but involution begins at puberty. Although the gland itself may become nearly unrecognizable, it is still functional throughout adult life.

Parathyroid Basics: Formed from the dorsal parts of the third and fourth pouches. Chief or principle cells differentiate during embryonic period and are active in fetal metabolism. The oxyphil cells differentiate 5-7 years after birth.

Question 4

Describe the embryological development of the thyroid gland.

Answer 4

The thyroid gland is the first endocrine gland to develop in the embryo. It is endodermal in origin and develops from the floor of the primordial pharynx. The thyroid gland descends into the neck and is connected to the tongue briefly by the thyroglossal duct. The primordial thyroid is hollow at first, but it grows into a mass of solid cells separated into lobes which are joined by the isthmus of the thyroid.

Question 5

cell precursor to pharyngeal arches and week of development

Answer 5

4th, 5th–neural crest cells

Question 6

lining of pharyngeal arches

Answer 6

inside-pouches

outside-grooves/clefts

Question 7

mesenchymal core for arches

Answer 7

paraxial and lateral plate mesoderm and neural crest cell-derivied ectomesenchyme

Question 8

dorsal part of first groove

Answer 8

external auditory meatus

Question 9

cervical sinus

Answer 9

second pharyngeal arch enlarging to overgrow the third and fourth arches

Question 10

Describe the general structure of the branchial (pharyngeal) arches, pouches, grooves, and membranes

Answer 10

• Pharyngeal grooves separate pharyngeal arches externally and are ectodermal
• Pharyngeal pouches separate the arches internally and are endodermal
• Pharyngeal membranes form in the floors of the pharygeal grooves, where the grooves and pouches approach each other
• Each pharyngeal arch is composed of (3)–outer ectoderm covering, inner endoderm lining, and core of mesenchyme

Question 11

Derivatives of 1st pharyngeal pouch

Answer 11

Timpanic (middle ear) cavity and Auditory (eustachian) tube

Question 12

Derivatives of 2nd pharyngeal pouch

Answer 12

Palatine tonsils and Tonsillar fossa

Question 13

Derivatives of 3rd pharyngeal pouch

Answer 13

Inferior parathyroid gland and the Thymus

Question 14

Derivatives of the 4th pharyngeal pouch

Answer 14

Dorsal part of 4th pouch = superior parathyroid gland

Ventral part of 4th pouch = Ultimobranchial body (parafollicular C cells of the thyroid gland)

Question 15

Name the derivatives of the pharyngeal grooves

Answer 15

Only one pair of grooves has derivatives! The dorsal part of the first groove will give rise to the external auditory meatus (ear canal)

The 2nd - 4th pharyngeal grooves (and the cervical sinus) have disappeared by end of 7th week

Question 16

Name the derivatives of pharyngeal membranes

Answer 16

Only one pair of membranes turns into adult structures! The 1st pharyngeal membrane–along with the intervening layer of mesenchyme–becomes the tympanic membrane.

Question 17

Name the 2 main congenital anomalies related to pharyngeal apparatus development

Answer 17

1) First Arch Syndrome, has two manifestations: Treacher Collins syndrome and Pierre Robin sequence
2) Digeorge syndrome

Question 18

Embryological origin and main clinical characteristics of First Arch Syndrome

Answer 18

This syndrome is believed to result from insufficient migration of neural crest cells into the 1st arch. Has two manifestations:

• Treacher Collins syndrome (mandibular dysotosis): caused by autosomal dominant gene. Sx = malar hypoplasia (underdevelopment of Zyg bone) with down-slanting palpebral fissures, mandibular hypoplasia, defects of lower eyelids and external ears
• Pierre Robin sequence: caused by autosomal recessive disorder. Main sx = small mandible (micrognathia) and bilateral cleft palate

Question 19

Embryological origin and main clinical characteristics of DiGeorge syndrome

Answer 19

Deletion of 22q11.2 part of chromosome. It is a developmental defect of neural crest-derived tissue and is a NEUROCRISTOPATHY. Has triad of malformations:

1) Minor craniofacial defects - micrognathia, low-set ears, cleft palate
2) total or partial agenesis of derivatives of 3rd and 4th pharyngeal pouches = thymus, parathyroid glands, and parafollicular cells
3) cardiovascular anomalies

Question 20

Embryological origin and main clinical characteristics of Congenital Hypothyroidism. Compare with agenesis of the thyroid gland.

Answer 20

Congenital Hypothroidism (CH) is a condition of thyroid hormone deficiency. Can be caused by severe iodine deficiency, thyroid agenesis, or abnormalities in biosynthesis of the hormone. CH may show no symptoms or mild signs (dry, rough skin, excessive sleeping, low or hoarse cry).
However, if deficiency is caused by agenesis of the thryoid gland, sx can include: larger anterior fontanel, persistence of posterior fontanel, umbilical hernia, and a large tongue (macroglossia)

Question 21

Embryological origin and most common presentation of an ectopic thyroid gland

Answer 21

An ectopic thyroid gland is an infrequent congenital anomaly and is usually located along the course of the thyroglossal duct. It is caused by incomplete descent of the thyroid gland. Lingual thyroid tissue is the most common of ectopic thyroid tissues.)
***As a rule, an ectopic thyroid is the only thyroid tissue present

Question 22

Embryological origin and most common presentation of thyroglossal duct cysts and sinuses

Answer 22

Caused by persistence of the thyroglossal duct, which forms a cyst in the tongue or in the anterior part of the neck, usually just inferior to the hyoid bone. Unless these lesions become infected, most of them are asymptomatic. Swelling of cyst usually presents as painless, enlarging, moveable mass.
If cyst becomes infected, perforation of the skin occurs and forms a Thyroglossa Duct Sinus that usually opens in the median plane of the neck, anterior to the laryngeal cartilages.

Question 23

Main difference in clinical presentation of Branchial v. Thyroglossal duct cyst

Answer 23

Thyroglossal is on midline of the neck, while the branchial cysts are on either side of the neck

Question 24

In what condition does cleft palate develop as a secondary anomaly?

Answer 24

Pierre Robin sequence (palate can’t fuse together because tongue is in the way)

Question 25

Most common cause for congenital cataracts

Answer 25

Rubella