Pharyngeal Arches Softchalk Flashcards
Describe the general development of branchial (pharyngeal) arches.
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.
Name the skeletal, ligamentous, muscular, nervous, and vascular derivatives of each branchial (pharyngeal) arch.
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
Be familiar with the embryological origin of the thymus and parathyroid glands.
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.
Describe the embryological development of the thyroid gland.
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.
cell precursor to pharyngeal arches and week of development
4th, 5th–neural crest cells
lining of pharyngeal arches
inside-pouches
outside-grooves/clefts
mesenchymal core for arches
paraxial and lateral plate mesoderm and neural crest cell-derivied ectomesenchyme
dorsal part of first groove
external auditory meatus
cervical sinus
second pharyngeal arch enlarging to overgrow the third and fourth arches
Describe the general structure of the branchial (pharyngeal) arches, pouches, grooves, and membranes
- 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
Derivatives of 1st pharyngeal pouch
Timpanic (middle ear) cavity and Auditory (eustachian) tube
Derivatives of 2nd pharyngeal pouch
Palatine tonsils and Tonsillar fossa
Derivatives of 3rd pharyngeal pouch
Inferior parathyroid gland and the Thymus
Derivatives of the 4th pharyngeal pouch
Dorsal part of 4th pouch = superior parathyroid gland
Ventral part of 4th pouch = Ultimobranchial body (parafollicular C cells of the thyroid gland)
Name the derivatives of the pharyngeal grooves
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
Name the derivatives of pharyngeal membranes
Only one pair of membranes turns into adult structures! The 1st pharyngeal membrane–along with the intervening layer of mesenchyme–becomes the tympanic membrane.
Name the 2 main congenital anomalies related to pharyngeal apparatus development
1) First Arch Syndrome, has two manifestations: Treacher Collins syndrome and Pierre Robin sequence
2) Digeorge syndrome
Embryological origin and main clinical characteristics of First Arch Syndrome
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
Embryological origin and main clinical characteristics of DiGeorge syndrome
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
Embryological origin and main clinical characteristics of Congenital Hypothyroidism. Compare with agenesis of the thyroid gland.
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)
Embryological origin and most common presentation of an ectopic thyroid gland
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
Embryological origin and most common presentation of thyroglossal duct cysts and sinuses
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.
Main difference in clinical presentation of Branchial v. Thyroglossal duct cyst
Thyroglossal is on midline of the neck, while the branchial cysts are on either side of the neck
In what condition does cleft palate develop as a secondary anomaly?
Pierre Robin sequence (palate can’t fuse together because tongue is in the way)
Most common cause for congenital cataracts
Rubella
