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7. The anatomy, histology and development of the pharynx, larynx and thyroid gland.

7 Dec

7. The anatomy, histology and development of the pharynx, larynx and thyroid gland.

Anatomy of pharynx, larynx and thyroid gland.

Thyroid gland

Location = deep to sternothyroid, and sternohyoid m, @ level of C5-T1, isthmus located b/w 2nd & 3rd rings of trachea

2 lobes – L and R, w. isthmus (small linking piece) b/w the two

Covered by fibrous capsule, which sends CT septa into gland (more info in HISTO)

Function: endocrine gland, produces

  • thyroid hormone – controls rate of metabolism
  • calcitonin – controls calcium level

CLINICAL NOTE -Tumor of thyroid gland can cause excess weight gain, or excess resorption of Ca, causing bone fractures more likely

Blood supply:

  • sup thyroid a (ext carotid a)
  • inf thyroid a (thyrocervical trunk of subclavian)
  • a run b/w fibrous capsule, and pretracheal cervical fascia
  • both split into ant/post branches

Veins drain a venous plexus that covers ant surface of gland  –> sup thyroid v, middle, inf thyroid v

  • sup & middle –> IJV
  • inf thyroid –>brachiocephalic v

Lymph Drainage: network of lymph vessels –> prelaryngeal, pre tracheal, paratracheal l.n., inf deep cervical l.n. –> brachiocephalic lymph nodes, thoracic duct

SNS: post ggl fibers from sup/mid/inf cervical ggl –> run via cardiac arterial plexus  w/ thryroid a –> cause VC (vaso constriction)
*NOTE-  Thyroid reg. by hormones of hypophysis (pituitary)

Parathyroid glands:
Location: located just outside of fibrous capsule of thyroid gland

  • Sup glands = 1 cm above entry of inf thyroid a, @ level of inf border of cricoid cart
  • Inf glands = 1 cm below entry of inf thryoid a

Blood Supply: inf thryroid a, or br of esophageal, tracheal, laryngeal a
v –> venous plexus of thyroid gland

Lymph drainage: deep cervical and paratracheal l.n.

Nerves: see thyroid


Location: Bodies of C3 –> C6, leading from pharynx –> trachea

Function: regulate flow of air to and from lungs for vocalization = voice production, guard air passages, so food and liquids don’t enter it

Part of Larynx: Cartilages, Membranes, Ligaments, Joints, Muscles


  • Thyroid – single hyaline cartilage

    • forms the laryngeal prominence aka Adam’s apple,
    • has oblique line on lat surface of its lamina where inf pharyngeal constrictor, sternohyoid, thryrohyoid attaches,
    • has a superior/inf horn
      • superior horn attaches to hyoid bone = thyrohyoid membrane
      • medial part of membrane = median thyrohyoid ligament
      • lateral part of membrane = lateral thyrohyoid ligament
  • Cricoid cartilage – single hyaline cartilage
    • only one that encircles the entire larynx,
    • articulates w/ thryroid cartilage,
    • lower border of it marks end of pharynx/larynx
    • attaches to thyroid via median cricothyroid ligament
    • attaches to 1st ring of trachea via cricotracheal ligament
  • Arytenoid (2) – paired hyaline/elastic cartilages,
    • shaped like pyramids w/ attachment to vocal ligaments and vocalis m to  their vocal processes (ant part of cart)
  • Cunieform (2) – paired elastic cartilages that lie in aryepiglottic folds ant to corniculate cartilages
  • Corniculate (2) – paired elastic cart that lies on arytenoid cartilages, w/ aryepiglottic folds of mucous membrane
  • Epiglottis – single elastic cartilage, spoon shaped extension from sup/ant wall of larynx
    • Vallecula epiglottica – the areas b/w the the folds of the epiglottis, the median and lateral glossoepiglottic folds
    • attached to thyroid cartilage via thyroepiglottic lig

Membranes & Ligaments of Larynx:

  • Thyroid membrane – b/w hyoid and thyroid cart
  • Cricothyroid ligament – thryroid and cricoid cartilage
  • Quadrangular membrane – thyroid and arytenoid cart –> epiglottis, upper part of aryepiglottic folds, inf margin of this membrane makes the ventricular folds
  • Thryoepiglottic lig – epiglottis –> thyroid cart lamina
  • Hyoepiglottic lig – epiglottis –> hyoid bone
  • Conus elasticus –  elastic membrane from cricoid cart –> line interconnecting inner surface of thyroid cartilage and vocal process of arytenoid cartilage, closes off the laryngeal inlet, except for at rima glottidis


1. Cricothyroid Joint – tense vocal fold

  • b/w inf horn of thyroid cartilage and lamina of cricoid cart
  • fibrous capsule
  • Movement: ant/post tilting of cricoid rt – when cart tips ant, tenses the cords, and post, loosens cords, rotation & gliding of thyroid cartilage
  • Associated Muscles = vertical and oblique cricothryroid m

2.  Cricoarytenoid Joint

  • b/w cricoid and base of arytenoid
  • fibrous capsule
  • Movement:  Rotation, sliding, ant/post tilting – help tense and relax vocal folds
  • Associate muscles = transverse arytenoid, post cricoarytenoid, lat cricoarytenoid, oblique arytenoid, thryroarytenoid m vocalis m

Muscles of Larynx: – all innervated by recurrent laryngeal n, except cricothryroid = ext laryngeal n, both from CN X

Intrinsic Muscles: move the laryngeal parts, makes changes in the length and tension of the vocal folds


  1. Lat cricoarytenoid
  2. Oblique arytenoid
  3. Transv arytenoid
  4. Vocalis


  1. Post cricoarytenoid m

Tensors of vocal fold:

  1. Vertical and oblique cricothyroid m
  2. Vocalis m

Relaxers of vocal fold:

  1. Aryepiglottic
  2. Vocalis m
  3. Thyroarytenoid m

NOTE – ONLY 1 ABDUCTOR,  so if inf  laryngeal n is damaged, say bye to abducting – paralysis of larynx

Extrinsic Muscles: move the larynx as a whole
Elevators of Larynx: (Mainly Suprahyoid m)

  1. Mylohyoid
  2. Genioglossus
  3. Stylohyoid
  4. Stylopharyngeus
  5. Thyrohyoid

Depressers of Larynx: (mainly infrahyoid m)

  1. omohyoid
  2. sternohyoid
  3. sternohyoid

Cavity (Internal Structure) of Larynx:

3 parts:
1. Vestibule of larynx = laryngeal inlet –>the ventricular folds

  • ventricular folds = mucus membrane folds elevated by lower free edge of quadrangular membrane, run from thyroid cartilage above vocal ligament –> arytenoid cart
  • laryngeal inlet = upper border of epiglottis, ary epiglottic folds, interarytenoid notch

2. Ventricle of larynx = b/w ventricular and vocal folds

  • Vocal folds = from angle of thyroid –> vocal process of arytenoid cartilage,

    • IMPORTANT in voice production, b/c the control the stream of air thru rima glottidis
    • alter the shape and size of rima glottidis, by movement of artyenoids to faciliate respiration and phonation
    • Vocal folds made of:
      • vocal ligament = upper free edge of conus elasticus
      • vocal cords = fold of mucus membrane elevated by vocal lig
      • vocalis m = medial part of thyroarytenoid m

3. Subglottic (Infraglottic) space = rima glottidis –> lower border of cricoid cartilage

  • Rima glottidis – narrow space b/w vocal folds
    • During inspiration = wide, expiration = narrow, wedge shaped
  • Glottis =  Rima glottidis + vocal folds + vocal processes
  • Piriform recess = part of cavity of laryngopharynx on each side of laryngeal inlet

Breathing/Phonation (making noises):
The more tensed the vocal folds are, the more narrow the rima glottidis is.
Normal respiration – rima is narrow & wedge shaped
Forced respiration – wide and kite shaped
Phonation – narrow , slitlike
Whispering – almost not open

Blood Supply:
Sup laryngeal a – thru thyrohyoid membrane = internal surface of larynx
Inf laryngeal a – mucus mem and m. in inf part of larynx
enter w/ nerves of same name (Int br of sup laryngeal n/inf laryngeal n)

Veins: venis w/ arteries
Sup laryngeal v –> sup thyroid v –> IJV
Inf laryngeal v –> inf thyroid v, thyroid venous plexus –> L brachiocephalic v

Lymph Drainage:
Above vocal fold – upper deep cervical nodes
Below vocal fold – lower deep cervical nodes

Innervation: CN X

1. Superior Laryngeal n – splits into int laryngeal (SS, ANS) /ext laryngeal n (SM)

  • = mucus membrane above vocal fold, taste buds on epiglottis
  • enters w/ sup laryngeal a thru thryrohyoid mem
  • int laryngeal n – SS to laryngeal mucosa above vocal folds
  • ext laryngeal n – SM to inf constrictor m, cricothyroid m,  runs w/ sup thyroid a

2. Recurrent Laryngeal n

  • (end branch of recurrent laryngeal n) all intrinsic m of larynx, except cricothyroid (ext laryngeal br of CN X)
  • SS = below vocal fold
  • terminates in int laryngeal n @ just above lower border of cricoid cart


funnel shaped fibromuscular tube from base of skull –> to inf border of cricoid cartilage
Layers of Pharyngeal wall:

  • Mucus membrane
  • Submucosa
  • Pharyngobasilar fascia
  • Muscular layer
  • Buccopharyngeal fascia

Has 3 parts:
Nasopharynx, Oropharnx, Laryngeopharynx

Nasopharynx: from base of occiput –> soft palate & isthmus of fauces
Structures w/in
1. Choana – post opening of nasal cavity
2. Pharyngeal fornix – angle b/w roof of nasopharynx and post wall of pharynx, location of pharyngeal tonsils* (aka adenoids)
3. Opening of auditory tubes
– roof of auditory tube covered w/ torus tubarius
– ant bordered by salpingopalatine fold
– post bordered by salpingopharyngeal fold – has salpingopharyngeus m, opens pharyngeal opening of auditory tube during swallowing
– around tube opening = tubal tonsils*
4. Pharyngeal recess – b/w salpingopharyngeal fold and post wall of pharynx

Oropharynx: b/w soft palate, root of tongue –> epiglottis
Structures w/in
1. Root of Tongue
2. Lingual tonsils*
3. Palatoglossal & palatopharyngeal folds – w/ tonsillar bed b/w,

  • that holds palatine tonsils*
  • tonsillar bed = formed by superior constrictor of pharynx, and pharyngobasilar fascia

Opening to Oropharynx = Faucial isthmus

  • Exit of oral cavity –> pharynx
  • bordered laterally by palatoglossal/palatopharyngeal folds

Isthmus of Pharynx (diff than Fauces)

  • Narrowest part of pharynx b/w soft palate & post wall of pharynx
  • b/w nasal and oral parts of pharynx
  • closed by elevation and tightening of soft palate and contraction of sup constrictor of pharynx & palatopharyngeus m
  • Prevents food -> nasopharynx

Act of Swallowing:
1. Bolus of food pushed back by elevating tongue (styloglossus) into fauces
2. Palatoglossus & palatopharyngeus m contract to squeeze the bolus backward into oropharynx. Tensor veli palatini & levator veli palatini eleavate soft palate & uvula to close entrance into nasopharynx
3. Wall of pharynx raised by palatopharyngeus & stylopharyngeus to receive food, Suprahyoid m elevate hyoid bone & laynx to close opening into larynx, passing over the epiglottis, prevent food from entering respiratory pathway
4.Action of sup,mid,inf constrictor move food through oropharynx and laryngopharynx –> esoph, where propelled by peristalsis

Blood Supply:

  • tonsillar br of facial a
  • asc pharyngeal a
  • asc palatine br of facial a
  • desc palatine a
  • pharyngeal br of maxillary a
  • br of sup/inf thyroid a


  • lies on middle pharyngeal constrictor
  • formed by pharyngeal br of CN IX, X, SNS br from sup cervical ggl
  • Vagal br = all m of pharynx, w/ exception of stylopharygeus m (CN IX), and tensor veli palatini (V2)
  • Glossopharyngeal br = sensory fibers of pharyngeal mucosa

Laryngopharynx: epiglottis –> cricoid cartilage (C4-C6)
1. Piriform recess – b/w larynx (aryepiglottic fold) & lat wall of pharynx (medial surface of thyroid cartilage & thyrohyoid membrane), contains sup laryngeal a, int laryngeal n
2. Med/Lat glossoepiglottic folds
3. Valleculae epiglottica – b/w med and lat glossoepiglottic folds on sup side of epiglottis
4. Post/lat walls = middle and inf constrictor m,
5. Internal wall = palatopharyngeus, stylopharyngeus m

Muscles: all innervated by Pharyngeal plexus, except stylopharyngeus m (IX)


  • Stylopharyngeus m
  • Palatopharyngeus m
  • Salpingopharyngeus m

Sup/middle/Inf constrictors
each one is thicker than the one above and cover the lower end of it

* To see Parapharyngeal space & Retropharyngeal space, refer to cards above

Histology: Pharynx, larynx and thyroid gland.

Slide #28  Larynx *H&E
Structures to Identify:

  • vestibule
  • true vocal folds
  • false vocal folds
  • cricoid cart
  • vocal ligaments
  • str columnar epith
  • epiglottis
  • rima glottidis
  • quadrangular membrane
  • vocalis m
  • str. sq epith
  • ventricles
  • thryoid cartilage
  • conus elasticus
  • psuedo str. epith

Lower power:

can see greater horn of hyoid bone
hyaline cartilages:histo-larynx
cricoid cartilages
thyroid cartilages

b/w cartilages = laryngeal musc – str. musc fibers
on outer side of thyroid cart = infrahyoid m can be seen

Vertical section thru larynx:
show 2 vocal folds, supporting cartilages & muscles
vestibule –> vestibular folds –> ventricle –> true vocal fold —> subglottic region

conducts air
origin of speech
helps in swallowing
sound production & resonance


false vocal fold = made by mucosa,
lined pseudostr. columnar epith w/ ciliae & goblet cells  = respiratory epithhisto-larynx-21

vocal fold
= str. squamous non  epith, more resistant to strain bacteria
vocal ligament located just deep to it
rich in a/v, esp capillaries

subglottic region = respiratory epith again

Lamina Propia
in LP = mixed glands (mostly mucus)
excretory ducts from glands, open @ epith
lymphatic nodules on ventricular side of fold
is thinner in area of vocalis m = no glands or a/v

Fibromuscular layer = quadrangular membrane
cartilages = become ossified to bone w/ age
cricoid cart = perichondrium + chondrons surrounded by matrix (PGs) + type II collagen fibers
ext pharyngeal m =  responsible to move & elevate larynx during swallowing
musc = thyroarytenoid m

– external layer of CT

Slide #34 Thyroid Gland *H&E

Structures to Identify:
CT septa
follicular cells
parafollicular cells

General Info:

unique exocrine gland b/c stores large amounts of secretory products extracellularly
has R & L lobe that are connected in middle via isthmus

CT septa separate thyroid gland into lobules
contain a/v, capillaries (from sup/inf thyroid a)

Follicular cells = prinicipal cells
follicular cells stores their secretory products in the cytoplasm
arranged into spherical shapes = follicles
surrounded by reticular fibers, and a/v – so thyroid hormones can enter blood
follicular epithelium = squamous, cuboidal, low columnar
structural and functional unit of thyroid gland

follicular cells can also release secretory products into lumen of follicles
stores their secretory products in the cytoplasm = colloid
made of thyroglobulin = GP w/ iodine acids
active thyroid hormones liberated from thyroglobulin
released into fenestrated capillaries that surround follicle

secretion of thyroid hormones = T3 + T4

  • regulate  cell & tissue base metabolism, heat production, influence body growth & development
  • inc fat/prot synthesis/degradation
  • inc rate of carb use absorption
  • inc heat production
  • regulated by TSH

Production of Thyroid Hormones:

Thyroxine (3,5,3′,5′-tetra­iodothyronine) is produced by follicular cells of the thyroid gland. It is produced as the precursor thyroglobulin (this is not the same as TBG), which is cleaved by enzymes to produce active T4.

Thyroxine is produced by attaching iodine atoms to the ring structures of tyrosine molecules. Thyroxine (T4) contains four iodine atoms. Triiodothyronine (T3) is identical to T4, but it has one less iodine atom per molecule.

Iodide is actively absorbed from the bloodstream by a process called ‘iodine trapping’ and concentrated in the thyroid follicles. (If there is a deficiency of dietary iodine, the thyroid enlarges in an attempt to trap more iodine, resulting in goitre.) Via a reaction with the enzyme thyroperoxidase, iodine is covalently bound to tyrosine residues in the thyroglobulin molecules, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT). Linking two moieties of DIT produces thyroxine. Combining one particle of MIT and one particle of DIT produces triiodothyronine.

* DIT + MIT → r-T3 (biologically inactive)
* MIT + DIT → triiodothyronine (usually referred to as T3)
* DIT + DIT → thyroxine (referred to as T4)

parafollicular cells
larger, pale staining cells
either peripherally, in follicular epith or w/in follicle
synthesize & secrete calcitonin =

  • reg Ca2+ metabolism, and decreases its level in blood
  • stimulate absorption of Ca2+ by bone
  • regulated by concentration of Ca in blood

Embryology: Development of the pharynx, larynx and thyroid gland.


Components of branchial/pharyngeal apparatus:

  • Pharyngeal arches
  • Pharyngeal pouches
  • Pharyngeal clefts/grooves

Pharyngeal (branchial) arches:

  • Derived from neural crest cells
  • Resemble fish gills (branchia)
  • Begin to develop early in the 4th week
  • By end of 4th week, four pairs of arches are visible on the surface (not 5th and 6th ) and a buccopharyngeal membrane ruptures forming communication between primitive oral cavity and foregut
  • Contribute to the formation of the neck as well as the face.

Pharyngeal Arch Derivatives:

Each arch has a core of mesenchymal tissue (mesoderm) covered by surface ectoderm (outside) and by endodermal epithelium (inside)

  • Ectoderm -> skeletal
  • Mesoderm -> muscles with accompanying nerve
  • Arterial component (aortic arches) – each pharyngeal arch has an aortic arch running w. it
  • Therefore, each arch carries nerve, muscle, bone and blood supply

First pharyngeal arch:

  • Maxillary process (dorsal): Premaxilla, maxilla, zygomatic bone, portion of temporal bone
  • Mandibular process (ventral): Contains Meckel’s cartilage which disappears except for dorsal end (incus & malleus) and mandible
  • Muscles of mastication, digastric (ant belly), mylohyoid, tensor tympani and tensor palatini
  • Therefore, the accompanying motor nerve is the mandibular branch of trigeminal (V2) and sensory are V1, V2, and V3
  • 1st aortic arch practically disappears but forms the maxillary artery

Second pharyngeal arch:

  • Reichert’s cartilage – stapes, styloid process, stylohyoid ligament, lesser horn and upper part of the hyoid
  • Muscles include: stapedius, stylohyoid, digastric (post belly), auricular, and those of facial expression
  • Facial nerve (CN VII)
  • 2nd aortic arch – stapedial & hyoid arteries

Third pharyngeal arch:

  • Cartilaginous contributions include greater horn and lower part of hyoid
  • Sole muscle: stylopharyngeus
  • CN IX (Glossopharyngeal nerve)
  • 3rd aortic arch (quite large): common carotid, 1st portion of internal carotid (remainder dorsal aorta), and external carotid

Fourth & sixth pharyngeal arch:

  • Cartilaginous contributions to larynx derived from fusion: thyroid, cricoid, arytenoid, corniculate, and cuneiform
  • Muscles of 4th: cricothyroid, levator palatini, and pharyngeal constrictors are innervated by SLN (CN X)
  • Muscles of 6th: intrinsics of larynx are innervated by RLN (CN X)
  • 4th aortic arch: L->arch of aorta & R->subclavian
  • 6th aortic arch: L & R pulmonary with ductus arteriosus on left

Pharyngeal pouches (5):

  • 1st:tubotympanic recess-> middle ear & eustacian tube -> TM
  • 2nd palatine tonsil/fossa
  • 3rd: inferior parathyroid (dorsal), thymus (ventral)
  • 4th: superior parathyroid
  • 5th: ultimobranchial body  -> calcitonin producing C cells (parafollicular)

Pharyngeal clefts/grooves (4):

  • 1st: external auditory meatus
  • 2nd-4th : epicardial ridge and cervical sinus (disappears)


  • Int lining from endoderm, as well as the laryngeal epithelium & glands
  • musc & cartilage from 4th & 6th pharyngeal arch = thyroid, cricoid, arytenoid cartilages – therefore innervated by CN X
    • superior laryngeal n – above the vocal fold
    • recurrent laryngeal n = below the vocal fold
  • @ wk 4, on the ventral side of the primitive gut, a pocket forms that bulges out from the gut = laryngotracheal diverticulum
  • distal end of diverticulum to form lung bud
  • then, 2 folds of tracheo-esophageal folds, push medially and push together to midline to form a wall “septum”
    • ant (ventrally) = laryngealtracheal tube
    • post (dorsally) = esophageal tube
  • opening of laryngeal diverticulum = orifice

Thyroid gland:

  • endoderm lining @ midline of floor in pharynx – forms pocket = diverticulum
  • @ 7th wk = this pocket moves downward – and passes ventral to hyoid & laryngeal cart
  • remain connected to tongue by thyroglossal duct — remains foramen cecum
  • functional @ 3rd month