Paraxial mesoderm - musculature

Lateral plate mesoderm - musculature

Neural crest cells- skeletal portions of each arch 

Ectodermal placodes- neurons of cranial sensory ganglia

The 2nd pharyngeal arch enlarges by week 6.

The space b/w the 2nd arch and the other arches is called the cervical sinus.

It is lined by ectoderm. 

Cervical cysts are caused by a persistent cervical sinu and may form cervical fistulas.

Complete cervical fistula: fistula opens to both the pharynx and an external surface. Often seen on the anterior border of the sternocleidomastoid.

Incomplete cervical fistula: fistula opens to only pharynx or external surface (i.e. sternocleidomastoid) 

The first branchial cleft forms the lining of the external auditory meatus and the external surface of the tympanic membrane.

What is the tubotympanic recess dervied from and what structures does it give rise to? 

The 1^st  pharyngeal pouch forms a diverticulum, the tubotympanic recess.             

The recess is drawn out into-

[image]      the auditory (Eustachian) tube

[image]      the tympanic cavity  (middle ear cavity)

[image]      the internal lining of the tympanic membrane (ear drum)

[image]      the lining of the mastoid air cells.

The 2^nd  pharyngeal pouch

 It forms the wall of the pharynx at the level of the palatine tonsils as tonsillar fossa in the adult. Palatine tonsil is formed in it by the collection of lymphocytes

The 3rd pharyngeal pouch gives rise to two important diverticula-

From the dorsal part of the 3rd pharyngeal pouch arises parathyroid III which becomes the inferior parathyroid gland.

From the ventral part of the pouch arises thymus III which becomes the thymus. 

During the 7th week, both of these diverticula separate from the pouch and move caudally. As they migrate, the thymus pulls the inferior parathyroid with it.

Clinical correlates:

 Ectopic thymus: The thymus may leave thymic tissue along its path as it descends.

Ectopic parathyroid: Occasionally parathyroid III or accessory parathyroid tissue will be carried into the mediastinum by the migrating thymus.

The 4^th   pharyngeal pouch forms in the wall of the pharynx at the level of the laryngeal inlet.

From the dorsal portion of the 4th pharyngeal pouch Parathyroid IV arises; it separates from the pouch and migrates caudally, and attaches itself to the dorsal surface of the caudally migrating thyroid gland. It ultimately becomes the superior parathyroid since it does not migrate as far caudally as parathyroid III.

Ventral portion of the 4th pouch may give rise to small amount of thymic tissue called thymus IV.

The 5th pharyngeal pouch, which may appear as pouch off of the 4th pouch, gives rise to the Ultimobranchial body. These lose their attachment to the pharynx and become incorporated into the thyroid which has moved caudally.

The ultimobranchial bodies give rise to the parafollicular cells of the thyroid. These cells produce calcitonin hormone.

Malleus and Incus, mandible, maxilla, and temporal bone regions, sphenomandibular ligament

Maxillary artery 

Stapes, styloid process, lesser cornu and upper half of body of hyoid bone and stylohoid ligament.

Stapedial artery (embryological)

Facial nerve

Greater cornu and lower half of body of styloid.

Common carotid artery, internal carotid artery and root of external carotid artery. 

Glossopharyngeal nerve

Superior laryngeal nerve (branch of Vagus)

Arch of aorta and right subclavian artery

Constrictors of the pharynx

Laryngeal cartilages 

Laryngeal cartilage

Ductus arteriosus

Recurent laryngeal branch of Vagus nerve

Intrinsic muscles of larynx and soft palate 

The tuberculum impar appears as a single median elevation behind the lingual swellings at the caudal level of the first arch, forms part of the body of the tongue.

The root of the tongue develops from a median swelling - the hypobranchial eminence (copula) – formed by the mesoderm of the second, third and fourth pharyngeal arches.

Posterior 1/3 or the root of the tongue originates from the hypobranchial eminence. The third arch part overgrows the 2^nd. Hence, this part is supplied by the Glossopharyngeal nerve. Glossopharyngeal nerve supplies both general sensory as well as taste fibers to the posterior 1/3.

Epiglottis and the extreme posterior part of the tongue are developed from the 4^th arch and hence are supplied by the superior laryngeal nerve.

Sulcus terminalis separates the anterior 2/3 from the posterior 1/3. Just in front of this, there is a row of circumvallate papillae. The mucous membrane over this row is pulled from the 3^rd arch. Hence, the mucous membrane over the vallate papillae is supplied by the glossopharyngeal nerve.

False:

The muscles of the tongue develop from Occipital somites and migrate to the tongue. They carry their nerve supply with them which is hypoglossal nerve (12^th Cranial N).

Bifid Tongue - A midline split in the anterior two-thirds of the tongue due to improper fusion of the lateral lingual swellings.

It arises from the 5th pouch

The Parafollicular cells (or ‘C’ cells - produce Calcitonin) of the Thyroid gland develop from the Ultimobranchial body 

, the thyroid gland continues to retain a connection with the pharyngeal lumen. This connection is known as the thyroglossal duct (because it connects the thyroid with that part of the pharyngeal floor which is organizing into the tongue). This duct later disappears.

Retention of all or part of the thyroid duct anywhere along its migratory path gives rise to cysts known as thyroglossal cysts.

Thyroglossal fistula – The thyroglossal cyst may be connected to the outside by a canal.

Retention and enlargement of that portion of the thyroglossal duct in contact with the thyroid results in the formation of a pyramidal lobe.

The neural crest

Since neural crest contributes to the formation of craniofacial region and the heart, many infants with craniofacial defects also have cardiac abnormalities (conotruncal defects). These cells are very susceptible to damage by alcohol and retinoic acid.

Treacher Collins syndrome (mandibulofacial dysostosis) - hypoplasia of the bones of the face and malformed ears, is an autosomal dominant trait. It may also be caused by the teratogens.

DiGeorge Syndrome:

Infants born without a thymus and parathyroid glands. Characterized by hypoparathyroidism, increased susceptibility to infections, anomalies of the mouth (fish mouth), conotruncal anomalies.

DiGeorge syndrome occurs because the 3rd & 4th pharyngeal pouches fail to differentiate into thymus and parathyroids. 

It may occur as part of a spectrum called CATCH 22 (Cardiac defects, abnormal facies, thymic hypoplasia, cleft palate, hypocalcemia, & deletion on the long arm of chromosome 22 (22q11).