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

Arch

Nerve




1

(Mandibular Arch; Merckel’s Cartilage)



CN V2
CN V3

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 (?)

2

(Hyoid Arch; Reichert’s Cartilage)



CN VII

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

3

(Glossal Pharyngeal Arch)



CN IX

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

4

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)

5

n/a

Rudimentary:

  • Develops in fish not humans (branchia = gill)

  • Pharyngeal apparatus used to be called “branchial” apparatus

6

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)

Pouch







1




Epithelial lining of auditory tube and middle ear cavity

2




Epithelial lining of palantine tonsil crypts

3




Inferior parathyroid gland, thymus

4




Superior parathyroid gland, ultimobranchial body♦

Groove







1




Epithelial lining of the external auditory meatus

2, 3, 4




Obliterated

Membrane







1




Tympanic Membrane

2, 3, 4




Obliterated

♦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

    V3

    IX + (sm. X contribution)

    Taste

    VII (chorda tympani)

    IX

    Muscles

    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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