Pharyngeal Apparatus & Development of Tongue

 PHARYNGEAL   APPARATUS

Pharyngeal / Branchial apparatus consists of..

• 6 Pharyngeal Arches - 5^th Arch disappears
• 4 Pharyngeal Clefts - only 1^st Cleft remains
• 4 Pharyngeal Pouches

DERIVATIVES  OF  PHARYNGEAL  ARCHES

FIRST (MANDIBULAR) ARCH  DERIVATIVES :-

Skeletal  Elements :-

• Malleus
• Incus
• Anterior ligament of malleus
• Spheno-mandibular ligament
• Body of the Mandible

Muscles :-

• Muscles of mastication
• Tensor tympani
• Tensor veli palatini
• Anterior belly of digastric
• Mylohyoid

Artery :-

• Maxillary Artery

Nerve :-

• Mandibular Nerve
• Chorda tympani nerve

SECOND (HYOID) ARCH  DERIVATIVES :-

Skeletal  Elements :-

• Stapes
• Styloid process
• Stylo-hyoid ligament
• Lesser cornu of Hyoid bone
• Upper part of body of Hyoid bone

Muscles :-

• Stapedius
• Stylohyoid
• Muscles of Facial expression
• Auricular muscles
• Epicranius muscle
• Posterior belly of Digastric
• Platysma

Artery :-

• Stapedial Artery

Nerve :-

• Facial Nerve

THIRD  ARCH  DERIVATIVES :-

Skeletal  Elements :-

• Greater cornu of Hyoid bone
• Lower part of body of Hyoid bone

Muscle :-

• Stylopharyngeus

Arteries :-

• Common Carotid Artery
• Internal Carotid Artery

Nerve :-

• Glossopharyngeal Nerve

FOURTH  ARCH  DERIVATIVES :-

Skeletal  Element :-

• Lamina of Thyroid cartilage

Muscle :-

• Cricothyroid

Arteries :-

• Right Subclavian Artery
• Arch of Aorta

Nerve :-

• Superior Laryngeal Nerve (External Laryngeal Nerve)

SIXTH  ARCH  DERIVATIVES :-

Skeletal  Elements :-

• Cricoid cartilage
• Arytenoid cartilage

Muscles :-

• Intrinsic muscles of Larynx (Except Cricothyroid)

Arteries :-

• Right & Left Pulmonary Arteries
• Ductus Arteriosus

Nerve :-

• Recurrent Laryngeal Nerve

DERIVATIVES  OF  PHARYNGEAL  CLEFTS

FIRST  CLEFT  DERIVATIVES :-

• External acoustic meatus
• Cuticular (outer) layer of tympanic membrane

Ectodermal cells at the dorsal ends of first, second & fourth clefts give rise to

Epibranchial placodes

 

Which form the ganglia of 7^th, 9^th & 10^th cranial nerves

DERIVATIVES  OF  PHARYNGEAL  POUCHES

• Ventral Derivatives
• Lateral Derivatives

VENTRAL  DERIVATIVES :-

• Tongue
• Thyroid gland
• Tracheo-laryngeal tube give rise to Respiratory system

LATERAL  DERIVATIVES :-

FROM  FIRST  POUCH :-

Tubotympanic recess -

• Auditory (Eustachian) tube
• Middle ear cavity
• Mastoid antrum & Mastoid air cells
• Tympanic membrane - mucosal layer

FROM  SECOND  POUCH :-

• Tubotympanic recess
• Tonsillar pits
• Tonsillar crypts
• Intra-tonsillar cleft

FROM  THIRD  POUCH :-

• Inferior Parathyroid gland
• Thymus

FROM  FOURTH  POUCH :-

Superior Parathyroid gland

Caudal Pharyngeal Complex -

• Thymic element - part of thymus
• Lateral Thyroid element - part of thyroid gland
• Ultimo-branchial body - Para-follicular (C) cells of thyroid gland

CONGENITAL  ANOMALIES

First Arch Syndrome :-

• It occurs due to the failure of migration of neural crest cells into the first arch.
• These neural crest cells form skeletal elements and connective tissue in the first arch.
• The first arch syndrome includes – Treacher Collins syndrome & Pierre Robin syndrome.

Treacher Collins syndrome –

• It is also called as mandibulofacial dysostosis.
• It is an autosomal dominant disorder due to mutation in TCOF1 gene located on chromosome 5.
• Incidence: 1:50,000 births.

Features-

• Underdeveloped lower jaw (mandibular hypoplasia)
• Underdeveloped zygomatic bone (malar hypoplasia)
• Down-slanting palpebral fissure
• External ear malformations (malformed pinna and meatal atresia)

Pierre Robin syndrome –

• It occurs due to anomalies of chromosome 2, 11 and 17; mostly mutation in SOXG gene of chromosome 17.
• Incidence: 1 in 10,000 births.

Features-

• Cleft palate
• Micro / Retrognathia (small mandible)
• Glossoptosis (backward displacement of the tongue)

Branchial Cyst :-

• It is a painless cystic swelling.
• Situated beneath the sternomastoid muscle.

It may be of 2 types –

Squamous-celled cyst –

• Lined by stratified squamous epithelium.
• Occurs due to failure of obliteration of cervical sinus .

Columnar-celled cyst –

• Lined by simple columnar epithelium.
• It may exist when some of endodermal pouch cells fail to regress properly.

Branchial Fistula :-

• It may be classified under 3 types – external, internal & complete fistula.

External Fistula –

• It shows an external communication.
• It is seen along the anterior border of sternomastoid muscle, above the sterno-clavicular joint.
• It occurs due to incomplete closure of cervical sinus.

Internal Fistula –

• It shows an internal communication into the pharynx.
• It occurs due to failure of normal closure of pharyngeal pouches.
• It affects tonsillar fossa.
• Extends outwards as lateral pharyngeal diverticulum.

Complete Fistula –

• It shows a complete communication between the pharynx & exterior.

DEVELOPMENT  OF  TONGUE

• Development of tongue is considered under the following headings-
• Development of mucous membrane
• Development of muscles 
• Development of fibro-areolar stroma

Development of Mucous Membrane :-

• It is entirely derived from endoderm of fore gut.
• 3 endodermal elevations appear, which are produced by the proliferation of underlying mesenchyme, they are –
• A pair of lingual swellings at the ventral ends of 1^st arches.
• A median elevation tuberculum impar between the 1t & 2^nd arches.

Lingual swellings fuse with each other

 

Then join with the tuberculum impar & give rise to ventral 2/3^rd of tongue.

Along the sides of developing tongue an endodermal alveolo-lingual sulcus starts developing

It separates the tongue from floor of mouth

Behind the tuberculum impar a median thyroid diverticulum extends caudally as thyroglossal duct

The duct disappears after the formation of thyroid gland & its commencement is represented by foramen caecum of tongue. 

• Dorsal to the tuberculum impar, another median endodermal elevation appears, known as hypobranchial eminence or Copula of His.
• It is contributed by ventral ends of 2^nd 3^rd & 4^th arches.

Dorsal part of the hypobranchial eminence form epiglottis & its mucous lining

Ventral part forms dorsal 1/3^rd of the tongue

This part grows forwards & fuses in V-shaped manner with the ventral 2/3^rd of the tongue

The line of fusion is represented by sulcus terminalis.

Nerve supply -

Mucous membrane of anterior 2/3^rd of the tongue - (excluding vallate papillae)

• General sensations are carried through lingual nerve (post-trematic nerve of first arch).
• Special sensations are carried through chorda tympani nerve (pre-trematic nerve of first arch).

Mucous membrane of posterior 1/3^rd of the tongue – (including vallate papillae)

• General & special sensations are carried through glossopharyngeal nerve (nerve of third arch).

Mucous membrane of valleculae –

• General & special sensations are carried through internal laryngeal branch of superior laryngeal nerve (nerve of fourth arch). 

Development of Muscles :-

• The tongue muscles are derived from 3 or 4 occipital myotomes.
• They migrate ventrally along the epipericardial ridge.
• The invade the tongue in the floor of the mouth.
• Such migration of occipital myotomes drags the hypoglossal nerve.

Development of Fibro-areolar Stroma :-

• It binds the tongue muscles.
• It is derived from the mesenchyme of branchial arches.

CONGENITAL  ANOMALIES

Aglossia :-

• Complete agenesis of the tongue rudiments.

Hemi-glossia :-

• Suppression of one of the lingual swellings.

Bifid tongue :-

• Anterior part of the tongue splits into two.
• Caused by failure of fusion of two lingual swellings.

Ankylo-glossia or Tongue-tie :-

• Alveolo-lingual sulcus separates the tongue imperfectly from floor of the mouth.
• It is manifested by –
• Shortening of the frenulum linguae.
• Movements of the tongue are restricted.
• Lingual speech is disturbed.

Lingual Thyroid :-

• Thyroglossal duct fails to migrate caudally form foramen caecum of the tongue.
• Eventually, thyroid gland develops within the tongue producing a lingual tumour.