1. Pharyngeal apparatus It is formed during 4th week from
1- pharyngeal arches
2- pharyngeal pouches
3- pharyngeal grooves
4- pharyngeal membranes
These apparatus is contributed to the formation of the head & neck ( tongue, face, lips, jaws, palate, pharynx and neck . . Most congenital anomalies in these regions originate during transformation of the pharyngeal apparatus into its adult derivatives
Pharyngeal arches
begin to develop from neural crest cells that migrate into the future head & neck . By the end of the 4th week, 4 pairs of arches are visible externally. The 5th & the 6th arches are rudimentary and are not visible on the surface of the embryo. The arches are separated externally, from each other by fissures ( grooves or clefts ) covered by ectoderm. Also, the arches are separated internally by pouches ( balloonlike diverticula that are lined by endoderm ).

2. The first pair of arches ( the primordium of the jaws) appears as surface elevations lateral to the developing pharynex. The other arches appear as rounded ridges on each side of the future head & neck . .
The 1st arch is called mandibular arch , from it two prominences are developed
The maxillary prominence gives rise to the maxilla( upper jaw), Zygomatic bone ; Squamous part of the temporal bone .
The mandibular prominence forms the mandible ( lower jaw
The 1st pair plays a major role in facial development.
These arches support the lateral walls of the primordial pharynx, which is derived from the cranial part of the foregut

3. Squamous temporal bone
A 20 – week fetus illustrating the area of the face derived from the first pair of the pharyngeal arches.
Derivatives of maxillary prominence
Maxilla
Palatine bone
Zygomatic bone
Squamous temporal bone

4. The primordial mouth or Stomodeum appears as a slight depression of the surface ectoderm . It is separated from the cavity of the primordial pharynx by a bilaminar membrane (oropharyngeal membrane ) It is composed of ectoderm externally & endoderm internally. It ruptures at about 26 days , thus the primordial pharynx& foregut become communicating with the amniotic cavity.

5. Pharyngeal arch Components
Each arch consists of a core of mesenchyme and is covered externally by ectoderm & internally by endoderm. The original mesenchyme is derived from mesoderm in the third week. During the 4th week most of the mesenchyme is derived from neural crest cells which are the major source of the arches connective tissue ( bone, cartilage and ligaments) .
A typical pharyngeal arch contains
An aortic arch , an artery that arises from the truncus arteriosus of the primordial heart.
( vascular endothelial from the original mesenchyme ) .
A cartilaginous rod that forms the skeleton of the arch . ( from the neural crest ) .
A muscular component that differentiates into muscles of head & neck ( original mesen.)
A nerve that supplies the mucosa & muscles. It is derived from neuroectoderm of the Brain

6. An aortic arch that passes around the primordial pharynx to enter the dorsa aorta .

7. The 5th arch is rudimentary and has no derivatives.
Ventral parts of the 1st arch cartilage( mandibular process) form the horseshoe- shaped primordium of the mandible. The cartilage disappears as the mandible develops around it by intramembranous ossification of mesenchymal tissue surrounding it .
The middle part regresses, but its perichondrium forms the ligaments. The dorsal end is closely related to the developing ear , it ossifies and form two middle ear bones malleus& incus ).
The ventral end of the 2nd arch cartilage ossifies to form the lesser cornu and the superior part of the body of the hyoid bone The middle part regresses & its perichondrium forms styloid ligament The dorsal end, ossifies to form stapes& styloid process. If stylohyoid ligament ossifies, it may cause pain in palatine tonsil .
The ventral part of the 3rd arch cartilage, ossifies to form the greater cornu and the inferior part of the body of the hyoid bone.
The 4th & the 6th arch cartilages fuse to form the laryngeal cartilages ,except the epiglottis.
The 5th arch is rudimentary and has no derivatives.

8. The 1st arch forms muscles of mastication, tensor veli palatini, tensor veli tympani, mylohyoid and anterior belly of digastric.
The 2nd arch forms the muscles of facial expression & the auricular muscles. Occipitofrontalis, platysma, posterior belly of digastric, stylohyoid and stapedius.
The 3rd arch forms the stylopharyngeus..
The 4th arch forms cricothyroid, levator veli palatini, constrictor muscles of the pharynx and striated muscles of esophagus.
The 6th arch forms The other intrinsic muscles of the larynx .
Myoblasts from the occipital myotomes to form the tongue musculature

9. The derivatives of the 1st arch are supplied by the caudal two branches of trigeminal nerve , 5th, ( mandibular & maxillary ) . It is the motor nerve for muscles of mastication. It is sensory to the face, teeth, and mucous membranes of the nasal cavities, palate, mouth, and tongue.
The derivatives of the 2nd arch are supplied by the facial nerve
The derivatives of the 3rd arch are supplied by the glossopharyngeal nerve.
The derivatives of the 4th arch are supplied by the superior laryngeal nerve of the vagus .
The derivatives of the 6th arch are supplied by the recurrent laryngeal nerve of the vagus.
The nerves of the 2nd to 6th arches have little cutaneous distribution , they innervate the mucous membranes of the tongue, pharynx, and larynx.

10. Pharyngeal membranes
Appear in the floors of the pharyngeal grooves where ectoderm becomes nearer to the endoderm and few mesodermal cells lie inbetween .
The membranes disappear except for the 1st pair, which becomes the tympanic membranes ( eardrum ).

11. Development of the neck
Pharyngeal Grooves
The head & neck regions exhibit 4 pharyngeal grooves on each side during the 4th & 5th weeks . Only, the 1st pair persists as the external acoustic meatus. The other grooves lie in a slitlike depression the cervical sinus .
Development of the neck
During the 5th week , the 2nd arch enlarges or grows downwards or caudally. Also, the 6th arch elongates upwards or cranially. Thus the previous 2 arches over- grows the 3rd & 4th arches, forming an ectodermal depression ( cervical sinus ) . So , the 2nd , 3rd and the 4th pouches become hidden beneath the elongated two arches. By the end of the 7th week, the 2nd, 3rd ; 4th grooves & the cervical sinus have disappeared, giving the neck a smooth contour.

12. Pharyngeal Pouches
There are 4 well- defined pairs of pouches. The 5th pair is absent or rudiment
The 1st pouch It expands into an elongate tubotympanic recess The expanded distal part of this recess share in the formation of the tympanic membrane ( eardrum ). Its cavity gives rise to the tympanic cavity & mastoid antrum . The connection of the tubo-tympanic recess with the pharynx elongates to form the pharyngotympanic tube ( auditory tube ) .

13. The second pouch
It is obliterated by palatine tonsil Its proximal part remains as the tonsillar sinus or fossa Its endoderm proliferates & grows into the underlying mesenchyme.
The central parts of these buds break down, forming crypts. So, its endoderm forms the surface epithelium & the lining of the tonsillar crypts.
About 20 weeks mesenchyme around the crypts differentiates into lymphoid tissue which give rise to the lymphatic nodules of the palatine tonsil . .

14. The third Pharyngeal Pouch
It expands & proliferate during the 5th week and forms small nodules on the dorsal aspect . Then it develops a solid Dorsal bulbar part and a hollow , elongate ventral part
Its connection with the pharynx degenerates .By the 6th week the epithelium of each dorsal bulbar part begins to differentiate into an inferior parathyroid gland .
The epithelium of the elongate ventral parts proliferates& obliterating their cavities to form the thymus gland. These bilateral primordia come together in the median plane then descends into the superior mediastinum. The bilobed form remains throughout life The primordia of both glands lose their connections & migrate into neck. The parathyroid separate from the thymus & lie on the dorsal surface of the thyroid.

15. The Fourth Pharyngeal Pouch
It expands into dorsal bulbar and elongate ventral parts By the 6th week , each dorsal part develops into a superior parathyroid gland , which lie on the dorsal surface of the thyroid gland . The glands derived from the 3rd pouch descend with the thymus to a more inferior position than the glands derived from the 4th pouches.
The elongated ventral part of each pouch develops into an ultimopharyngeal body which fuses with the thyroid gland and its cells give rise to the parafollicular cells of the thyroid gland. These cells also called C cells which produce calcitonin, a hormone that regulate the calcium level in the body.
C cells differentiate from neural crest cells that migrate from the arches into the 4th pouches.

16. Histogenesis of Parathyroid Glands
The fifth Pharyngeal pouch When it develops it helps to form the ultimopharyngeal body
Histogenesis of Parathyroid Glands
The chief or principal cells differentiate during the embryonic period . They regulate fetal calcium metabolism The oxyphil cells differentiate 5 to 7 years after birth.

17. Histogenesis of Thymus
Epithelial tubes ( endoderm ) grow within the mesenchyme. These tubes become solid cords that proliferate and give rise to side branches . Each side branch becomes the core of a lobule. Some cells of the epithelial cords become the THYMIC CORPUSCLES ( Hassall ) (endodermal ). Other of the epithelial cords form the epithelial reticular cells ( endodermal ) . The lymphocytes are derived from hematopoietic stem cells ( mesodermal ) . A thin layer of mesenchyme surround the gland to form the capsule .
The mesenchyme & the macrophages and the muscle cells ( smooth muscles of vessel ) are derived from neural crest cells ( mesoderrmal ) .

18. Branchial Sinuses 1- external It opens along the anterior border of the sternocleidomastoid muscle in the inferior third of the neck. It is uncommon. It results from failure of the 2nd groove & the cervical sinus to oblitrate. Anomalies of the other grooves ( 1st, 3rd; 4th )occure in about 5%. It is detected during infancy by discharge of mucous material from them. These bilateral cervical sinuses are in about 10 % of cases and commonly associated with auricular sinuses.
2- Internal open into the pharynx & are very rare. They usually open into the tonsillar sinus or near the palatopharyngeal arch. They result from persistence of the proximal part of the 2nd pouch.

19. Branchial Fistula Branchial Cysts
It is an abnormal canal that internally open into the tonsillar sinus & externally in the side of the neck. It results from persistence of parts of the 2nd groove & 2nd pouch. It passes between the internal and external carotid arteries.
Branchial Cysts
Remnants of parts of the cervical sinus and or the 2nd groove persist and form cyst. They are produce as a slowly , enlarging ; painless swelling in the neck. They enlarge due to accumulation of fluid & cellular debris derived from desquamation of their epithelial linings. They often lie inferior to the angle of the mandible or anywhere along the anterior border of the sternocleidomastoid muscle. They observed also in the parathyroid glands.

20. Development of the thyroid gland
It is the first endocrine gland to develop in the embryo . It begins to form about 24 days after fertilization from a median endodermal thickening in the floor of the primordial pharynx. From this thickening a small outpouching is formed.
As the embryo & tongue grow , It descends in the neck passing ventral to the developing hyoid bone & laryngeal cartilages.
It is connected to the tongue by a thyroglossal duct
At first its primordium is hollow then it become solid & divides into right and left lobes which are connected by the isthmus, which lie anterior to the developing 2nd & 3rd tracheal rings By 7th weeks , it has its definitive shape & reached its final site in the neck. Also, at this time the thyroglossal duct degenerate & disappear
The proximal opening of the duct persists as a small pit in the tongue ( the foramen cecum )
copula
Tuberculum impar

21. A pyramidal lobe extends superiorly from the isthmus in a about 50 % of people . It may be attached to the hyoid bone by fibrous tissue and or smooth muscle ( levator of thyroid gland ) . These structures represent a persistent part of the distal end of the thyroglossal duct.

22. Histogenesis of the thyroid gland
The thyroid primordium consists of a solid mass of endodermal cells
Later, this cellular aggregation breaks up into a network of epithelial cords . This is due to its invasion by the surrounding vascular mesenchyme.
By the 10th week , the cords have divided into small cellular groups.
In each cell group ( cluster ) , the cells become arranged in a single layer around a lumen.
During the 11th week colloid begins to appear in it . So , the thyroid follicles is formed. Also, the synthesis of thyroid hormones can be demonstrated.
Note that the follicular cells arise from endoderm & the parafollicular cells arise from mesoderm.

23. Thyroglossal Duct Cysts
It is formed anywhere along the course followed by the thyroglossal duct during descend of the primordial thyroid gland from the tongue.
It is formed by a remnant of the thyroglossal duct. Which persist..
Usually, it is formed inferior to the hyoid bone in the midline or in the tongue.
Most are observed by the age of 5 years. It is a painless, progressively enlarging movable mass. It may contain some thyroid tissue.
Following infection of a cyst, a perforation of the skin occurs, forming a thyroglossal duct sinus that opens in the median plane of the neck, anterior to the laryngeal cartilages.

25. Development of the tongue
Near the end of the 4th week , a median triangular elevation appears in the floor of the primordial pharynx just rostral to the foramen cecum , it is called the median tongue bud ( Tuberculum impar ). Then two oval distal tongue buds ( lateral lingual swellings ) develop on each side of the median tongue bud.
These three lingual buds result from the proliferation of mesenchyme in the ventromedial parts of the first pair of pharyngeal arches.
The distal tongue buds rapidly increase in size , merge with each other, & overgrow the median tongue bud. So, the distal tongue buds form the anterior two – thirds ( Oral part ) of the tongue The median tongue bud forms no recognizable part of the adult tongue
Distal tongue duds

26. Formation of the posterior third ( pharyngeal part )of the tongue is indicated by two elevations that develop caudal to the foramen cecum.
1- The copula forms by fusion of the ventromedial parts of the 2nd pair of pharyngeal arches.
2- The hypopharyngeal eminence develops caudal to the copula from mesenchyme in the ventromedial parts of the 3rd & 4th pairs of arches.
As the tongue develops copula disappears and gradually the hypopharyngeal eminens overgrown it So, the pharyngeal part of the tongue is developed from the rostral part of the hypopharyngeal eminence.

27. The distal part of the hypopharyngeal eminence gives rise to the epiglottis .

28. The arch mesenchyme forms the connective tissue & the vasculature of the tongue.
Most of the tongue muscles are derived from myoblasts that migrate from the occipital myotomes.
The hypoglossal nerve accompanies the myoblasts during migration & innervates the tongue muscle, except palatoglossus which is supplied by pharyngeal plexus by fibers arising from the vagus nerve.
The entier tongue is within the mouth at birth & its posterior third descends into the oropharynx by the 4 years of age.

29. Fusion of the distal tongue buds is indicated externally by a middle groove, ( the median sulcus of the tongue). & internally by the fibrous lingual septum.
The line of fusion of the oral & pharyngeal parts of the tongue is roughly indicated by a V- shaped groove ( the terminal sulcus ) .
At first , the tongue is adherent to floor of the mouth. Later , its oral part separates from the floor of the mouth and the lower gum by a horseshoe groove called alveolo-lingual groove.
So, its inferior surface is connected to the floor of the mouth by a frenulum ( mucous memberane fold.

31. Papillae & Taste Buds
Lingual papillae appear near the end of the 8th week.
The vallate & foliate appear first , close to terminal branches of the glossopharyngeal nerve.
The most common papillae is the filiform, they are threadlike shape . They develop from the 10 th to 11th weeks.( early fetal period ). They contain afferent nerve endings that are sensitive to touch.
The fungiform appear later near termination of the chorda tympani ( from facial nerve ).
Taste buds develop during weeks 11 to 13 by interaction between the epithelial cells of the tongue & the invading gustatory nerve cells .
These gustatory nerve cells are from the chorda tympani, glossopharyngeal, and vagus nerves
Most taste buds form on the dorsal surface of the tongue. Also, they develop on the palatoglossal arches, palate, posterior surface of the epiglottis, and the posterior surface of the oropharynx At 26 to 28 weeks, there is a reflex pathways between taste buds & facial muscles by bitter- tasting substances.

32. Nerve Supply of the tongue
The sensory supply to the mucosa of the oral part is from the lingual branch of the mandibular division of the trigeminal nerve.
The 2nd arch does not share in the formation of the tongue mucosa, But , the corda tympani ( branch from facial nerve ) supplies the taste buds in the oral part. , except for the vallate papillae.which are innervated by the glossopharyngeal nerve of the 3rd arch. The reason is that the mucosa of the posterior third is pulled anteriorly as the tongue develops.
The pharyngeal part is innervated by the glossopharyngeal nerve .
The superior laryngeal branch of the vagus nerve of the 4th arch supplies a small area of the tongue anterior to the epiglottis ( root of the tongue )