Pharyngeal Apparatus

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  • Pharyngeal Apparatus (1st observed in week 4) –ventral side (initially)

    • Major contributor to head & neck development, especially the area around the pharynx; most congenital abnormalities in head & neck region as a result of mistakes in transformation of apparatus to adult derivatives

    • Pharyngeal Arches (1, 2, 3, 4, & 6)

      • Ultimately, 5 well-developed pairs of “pharyngeal arches” form in a cranial-to-caudal sequence; the arches are referred to as I, II, III, IV & VI (V is vestigial/rudimentary)

      • Arches consists mesenchymal core (see chart below for fate of these structrures)

        • Somitomeric Mesoderm (paraxial mesenchyme) – differentiates into muscles & arteries (week 3)

        • Neural crest cells – differentiate into bone & CT (week 4)

      • Each arch also has its own aortic arch vessel & its own cranial nerve (see chart below for fate of these structrures)

        • Aortic Arch – each runs around primordial pharynx & dumps into dorsal aorta

        • Cranial Nerve associated with each arch (supplies mucosa & mm in that arch)

      • Coverings:

    • Pharyngeal Pouches (1, 2, 3, & 4) – endodermal evagination lining the foregut (internal)

    • Pharyngeal Grooves (1, 2, 3, & 4) – ectodermal invagination located b/w each arch (ext.)

    • Pharyngeal Membranes (1, 2, 3, & 4) – it is the “skinny” section b/w ea. arch— structures consisting of ectoderm, intervening mesoderm & neural crest, and endoderm

    • NOTE: The components of the apparatus consist of endoderm, mesoderm, &/or ectoderm, t/f the apparatus tissues are trilaminar cell derivatives

    • Segmentation Control:

      • HOX gene (rhombomeres – region of segmented hindbrain) specify neural crest cell properties so that they migrate to specific arches

      • Retinoic acid (aka Vit A) play a key role in arch development; often used topically to treat acne

    • Fate of Apparatus – Summary (source: BRS):

Adult Derivative




(Mandibular Arch; Merckel’s Cartilage)


Mesoderm: Muscles of mastication (temporal, masseter, medial & lateral pterygoids), mylohyoid, anterior belly of digastric, tensor veli palatine, tensor tympani

Neural Crest: Maxilla, mandible, incus, malleus, zygomatic bone, squamous temporal bone, palatine bone, vomer, sphenomandibular ligament

1st Aortic Arch: Maxillary artery, external carotid artery (?)


(Hyoid Arch; Reichert’s Cartilage)


Mesoderm: Muscels of facial expression (buccinator, auricularis, frontalis, platysma, orbiuclaris oris & obicularis oculi), posterior belly of digastric, stylohyoid, stapedius

Neural Crest: Stapes, styloid process, stylohyoid ligament, lesser horn and upper body of hyoid bone

2nd Aortic Arch: Stapedius artery


(Glossal Pharyngeal Arch)


Mesoderm: Stylopharyngeus, common carotid arteries, internal carotid arteries

Neural Crest: Greater horn and lower body of hyoid bone

3rd Aortic Arch: Common carotid artery, internal carotid artery


CN X (superior laryngeal)

Mesoderm: Muscles of soft palate (except tensor veli palatine), muscels of the pharynx (except stylopharyngeus), cricothyroid, cricopharyngeus, laryngeal cartilages, rt. subclavian artery, arch of aorta

Neural crest: none

4th Aortic Arch: Arch of aorta (left side), right subclavian (right side)




  • Develops in fish not humans (branchia = gill)

  • Pharyngeal apparatus used to be called “branchial” apparatus


CN X (recurrent laryngeal

Mesoderm: Intrinsic muscles of larynx (except cricothyroid), upper muscles of the esophagus, laryngeal cartilages, pulmonary arteries, ductus arteriosus

Neural Crest: none

6th Aortic Arch: Left pulmonary artery (left side), ductus arteriosus (left side), right pulmonary artery (right side)



Epithelial lining of auditory tube and middle ear cavity


Epithelial lining of palantine tonsil crypts


Inferior parathyroid gland, thymus


Superior parathyroid gland, ultimobranchial body♦



Epithelial lining of the external auditory meatus

2, 3, 4




Tympanic Membrane

2, 3, 4


♦Neural crest cells migrate into the ultimobranchial body to form parafollicular cells (C cells) of the thyroid
which secrete calcitonin

  • Anomalies of Pharyngeal Apparatus:Auricular pits/sinuses/cysts – remnants of 1st p. groove (s/b tympanic membrane)

    • Branchial cyst – failure of cervical sinus to obliterate, usually anterior to sternomastoid; may communicate with skin via external fistula or with pharynx via internal fistula

    • 1st Arch Syndrome – Spectrum of facial malformations due to insufficient neural crest migration into 1st arch; underdevelopment of lower face & mandible, cleft palate, abnormal external ears

    • DiGeorge Syndrome – Thymic/parathyroid aplasia, defects of the heart; failure of differentiation of pouches 3 & 4; abnormal migration of neural crest cells into arches 3 & 4

      • Chromosome 22 deletion abnormality

      • mnemonic: CATCH 22 (C –cardiac; A –abnormal facies; T --?; C--?; H--? 22 –chromosome)….he said in class, but I missed it.

    • Ectopic Parathyroid

  • Thryoid Gland – 1st endocrine gland to appear; 1st functional gland

  • Bilobed structure – median endodermal proliferation at foramen cecum (site persists – tongue) in floor of pharynx b/w arch I & II

  • Decends in neck at end of thyroglossal duct (later breaks down)

  • Anomalies:

    • Congenital hypothyroidism

    • Congenital Cretinism – thyroid gland absent or reduced; more severe

    • Lingual Thyroid – failure of descent; may cause dysphagia (difficulty swalling)

    • Remnants of thyroglossal duct – thyroglossal cysts & fistulae (median in position); common

  • Tongue

  • 1st Arch

    • Distal Tongue Buds

      • Lateral lingual swellings grow, merge (overgrow median tongue bud), & form anterior 2/3’s of the tongue fused at the median sulcus

    • Median Tongue Bud

      • Tuberculum impar in the floor of the pharynx & anterior to foreman secum; overgrown by distal tongue buds (no recognizable portion in adult tongue)

  • 2nd Arch

    • Copula

      • Develops caudal/posterior to foramen secum; eventually overgrown by hypobranchial eminence

  • 3rd & 4th Arch – Hypobranchial Eminence

    • 3rd Arch (cranial) – posterior 1/3 of tongue;

      • Terminal sulcus: fusion of anterior 2/3 & posterior 1/3 of tongue

    • 4th Arch (caudal)epiglottis

  • Occiptial Somites/Myotomes (base of skull)

    • Intrinsic tongue muscles – innervated by CN XII (hypoglossal nerve

  • Tongue Innervation – Summary:

    Nerve Function

    Anterior 2/3

    Posterior 1/3

    General Sensory


    IX + (sm. X contribution)


    VII (chorda tympani)



    XII (except X = palatoglossus)

    XII (except X = palatoglossus)

  • Tongue Anomalies:

    • Cysts/fistula – associated w/ thyroglossal duct

    • Ankyloglossia (tongue-tie) – frenulum is too long

    • Macroglossia – excessively large tongue, usually indicates more serious condition (cretinism or trisomy 21)

    • Microglossia

    • Bifid/cleft tongue – incomplete fusion of distal tongue buds

  • Salivary Glands (week 6-7)

  • Arise form oropharyngeal epithelium by epithelio-mesenchymal interaction

  • Pituitary

  • Ranthke’s Pouch (week 4) – dorsal ectodermal outpocketing of stomodeum in front of buccopharyngeal membrane

  • Adenohypophysis = anterior pituitary (week 6) – Rathke’s pouch loses connection to stomodeum

  • Neurohypophysis = posterior pituitary (week 6) – downward extension of the diencephalons forms the infundibulum = neurohypophysis = posterior pituitary

  • Facial Development (b/w weeks 4 & 8): by “merging”

  • Five facial primodia appear as prominences around the stomodeum and consist primarily of neural crest derived mesenchyme

  • Frontal Nasal Prominence (mesenchyme ventral to forebrain)

    • Enlarges as brain develops

    • Nasal placodes invaginate as nasal pits; the mesenchyme around the pits form nasal prominences

    • Gives rise to:

      • Forehead

      • Dorsum of the nose:

        • Lateral Nasal Prominence (LNP)

          • Will merge w/ maxillary prominences

          • Will give rise to alae (the two outer/lateral 1/3 portion) of the nose

        • Medial Nasal Prominence (MNP) – merge  form intermaxillary segment & become

          • Philtrum (of upper lip)

          • Median Palantine Process (1o palate)

          • Nasal Septum

        • Nasal Cavities:

          • LNP & MNP surround nasal placodes

          • Placodes invaginate to form nasal pits (“sacs” separated by oronasal membrane

          • Oronasal membrane ruptures (week 6)  primitive choanae (openings from nasal to oral cavity)

          • Choanae move posteriorly w/ 2o palate formation

  • 2 x Maxillary prominences (1st arch) – enlarge & will form most of the upper lip, maxillae, cheeks and the 2o palate

    • Merge w/ LNP at nasolacrimal groove; ectoderm of groove floor invaginate underlying mesenchyme to form nasolacrimal duct

    • Merge w/ MNP to complete upper lip

    • Gives rise to lateral palantine processes (see Palate/Two Primorida/2o Palate)

  • 2 x Mandibular prominences (1st arch) – merge w/ ea other and give rise to the chin, mandible, & lower lip

  • Palate (week 5 – 12) by “fusion”

  • Two Primordia

    • Primary Palate – “fused” MNP’s (intermaxillary segment) = premaxilla (anterior to incisive foramen)

    • Secondary Palate

      • Lateral palantine processes (from maxillary prominences) first project medially & inferiorly below developing tongue

      • Lateral palantine processes ascend to horizontal position above the tongue and fuse, w/ ea. other, the nasal septum, & the 1o palate; completes hard palate, soft palate, & uvula; (“zip” from 1o palate to uvula)

  • Clinical Correlation:

    • Cleft Lipcleft of lip (w/ or w/out cleft of 1o palate)

      • Mainly genetic

      • Environmental factors include Drugs—Vitamin A, anti-acne drug, Acutane

      • Due to:

        • Inadequate mesenchyme in the maxillary processes so that the MNP & maxillary processes do not merge; cleft at philtrum may extend through alveolar part of maxilla

        • Insufficient migration of neural crest cells into maxillary prominence

    • Cleft Palatecleft of 2o palate (w/ or w/out cleft of 1o palate – no cleft lip)

      • Genetically diff’t anomaly than cleft lip

      • Failure of lateral palantine processes (maxillary prominence) to fuse to any one or more of the following:

        • Each other

        • The 1o palate

        • The nasal septum

      • Due to:

        • Inadequate growth

        • Failure of elevation

        • Excessively wide head

        • 2o rupture after fusion

    • Cleft Lip & Cleft Palate

      • May occur alone, but frequently combined

      • Found in many craniofacial syndromes

      • Due to insufficient quantity of neural crest cells which migrate into facial primordial

      • Multifactorial etiology: genetic + environmental

    • Incidence: multifactorial

      • Cleft lip: 1/1000 (more in males)

      • Cleft palate: 1/2500 (more in females)

  • Fetal Alcohol Syndrome – Alcohol during pregnancy can cause growth & mental retardation, and facial deformities including maxillary hypoplasia, short nose, and thin upper lip

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