The Respiratory SystemThe Respiratory SystemThe lower respiratory organs (larynx,trachea,bronchi,lungs) begin to develop during 4th week, from laryngotracheal groove, which lies ventral to the caudal part of foregut (pharynx).

This groove develops in the floor of pharynx, caudal to 4th pair of pharyngeal pouches.

The epithelium & glands of larynx, trachea, bronchi & lungs are developed from the endoderm lining the laryngotracheal groove

The C.T,cartilage,and smooth muscle of respiratory system develop from splanchnic mesoderm surrounding the foregut.

B,sagittal S, illustrating L-T groove + rispiratory diverticulum.

C,Horizontal S, illustrating floor of primordial pharynx and location of L-T groove.

By the end of 4thweek, the laryngotracheal groove evaginated to form pouchlike respiratory diverticulum (lung bud),which is located ventral to the pharynx.

The distal end of diverticulum enlarges to form a globular tracheal bud.

Longitudinal tracheoesophageal folds develop and fuse together to form tracheo-esophageal septum.

This septum divides the foregut into a ventral part,the laryngo-tracheal tube (primordium of larynx,trachea,bronchi & lungs) + a dorsal part (primordium of oropharynx & esophagus).

Stages of development of tracheo- Stages of development of tracheo- esophageal septum during 4th & 5th weeks : esophageal septum during 4th & 5th weeks :

Stages of development of tracheo-Stages of development of tracheo-esophageal septum during 4esophageal septum during 4thth & 5 & 5thth weeks : weeks :

A,B,&C…. lateral views of caudal part of primordial pharynx showing the respiratory diverticulum and partitioning of foregut by the tracheo-esophageal septum into the esophagus +laryngo-tracheal tube by

D,E,& F… T.S illustrating formation of T-E septum to separate foregut into L-T tube & esophagus.

Development of larynxDevelopment of larynx

The epithelial lining of larynx develops from the endoderm of cranial (upper) end of L-T tube

Laryngeal epithelium proliferates, producing temporary occlusion of laryngeal lumen.

Recanalization of larynx occurs by 10th week, producing a pair of lateral recesses, the laryngeal ventricles, that are bounded by folds of m.m that forming vocal folds (cords) & vestibular folds.

Laryngeal cartilages (except epiglottis) & ms. develop from the mesenchyme of 4th & 6th pharyngeal arches that is derived from neural crest cells.

This mesenchyme proliferates, producing paired arytenoid swellings, converting the slitlike aperature into T-shaped laryngeal inlet.

4-weeks 5-weeks

6-weeks 10-weeks

Development of larynxDevelopment of larynx

The epiglottis develops from the caudal part of hypopharyngeal eminence which develops from mesenchyme in the venteral ends of 3rd &4th pharyngeal arches, but the cranial part of this eminence forms the posterior (pharyngeal)1/3 of tongue.

Laryngeal ms. develop from mesenchyme of 4th & 6th pairs of pharyngeal arches, therefore they are innervated by laryngeal branches of vagus nerves that supply these arches.

Growth of larynx and epiglottis is rapid during the first 3 years after birth, by this time the epiglottis taking its adult form.

Development of TracheaDevelopment of Trachea

A, 4-weeks,/ B,10weeks, /C,11weeks…. T.Ss of L-T tube, showing that endoderm of the lower part of L-T tube gives rise to epithelium (mucosa) & glands of trachea / and that splanchnic mesenchyme surrounding the tube forms C.T, muscle and cartilage.

D, T.S of developing trachea at 14 - weeks.

4-weeks L-T tube10 weeks

11 weeks 14 weeks

Development of Bronchi Development of Bronchi and Lungs :and Lungs :

A, 5weeks. / B, 6weeks, illustrating growth of the developing lungs into the splanchnic mesenchyme which lies medially to the pericardio-peritoneal canals. /also illustrating development of layers of the pleura at 6th w.

A,B,C lateral views of caudal part of pharynx during 4th & 5th weeks, illustrating division of tracheal bud into 2 primary bronchial buds at 5th w.

These bronchial buds grow laterally into pericardio-peritoneal canals (primordia of pleural cavities).

Together with the surrounding splanchnic mesenchyme, the bronchial buds differentiate into the Rt. & left main bronchi and their ramifications in the lungs.

5-weeks 6-weeks

4th week 5th week

Development of Bronchi Development of Bronchi and Lungs :and Lungs :

By 5th week the connection of each bronchial bud with the trachea enlarges to form the main bronchus.

The right main bronchus is larger than the left one.

The main bronchi subdivide into secondary bronchi (the right then forms 3 secondary bronchi, and the left 2 bronchi) that form lobar, segmental, & intrasegmental branches.

Development of Bronchi Development of Bronchi and Lungs :and Lungs :

On the right / superior lobar bronchus supplies the upper (superior)lobe of right lung, whereas the inferior lobar bronchus subdivides into 2 bronchi, one to the middle lobe, and the other to the lower (inferior) lobe.

On the left / the 2 lobar bronchi supply the upper & lower lobes of the lung.

Each lobar bronchus undergoes progressive branching.

Development of Bronchi Development of Bronchi and Lungs :and Lungs :

By the 7th week, The segmental bronchi (10 in the right and 8 or 9 in the left lung), begin to develop.

Each segmental bronchus with its surrounding mass of mesenchyme is the primordium of a broncho-pulmonary segment.

As the lungs develop, they acquire a layer of visceral pleura derived from the splanchnic mesenchyme, and the thoracic body wall becomes lined by a layer of parietal pleura, derived from the somatic mesoderm.

Maturation of the lungsMaturation of the lungsPseudoglandular period (6-16 weeks)

Canalicular period (16-26 weeks).

Terminal saccular period (26 weeks to birth).

Alveolar period (32 weeks to 8 years).

Maturation of the lungsMaturation of the lungsPseudoglandular Period :(6 to 16 weeks) :

All major elements of lung have formed, except those involved with gas exchange.

There are bronchi & terminal bronchioles but, No respiratory bronchioles or alveoli are present.

Respiration is not possible, so fetuses born unable to survive.

Maturation of the lungsMaturation of the lungsCanalicular Period : (16 to 26 weeks) :Enlagment of lumina of bronchi & terminal bronchioles.

Each terminal bronchiole gives rise to 2 or more respiratory bronchioles, each of which divides into 3 or 6 alveolar ducts.

At the end of this period, respiration is possible due to development of some thin-walled terminal saccules (primordial alveoli) at the end of respiratory bronchioles.

A fetus born at the end of this period may survive if given intesive care, but often dies because its respiratory system is still immature.

Maturation of the lungsMaturation of the lungsTerminal Saccular Period : (26 weeks to Birth) : Thin-walled many terminal saccules (primitive alveoli) are developed.

Capillaries (endothelial cells) begin to bulge into the epithelium of these primitive alveoli, forming the blood-air barrier, which permits gas exchange for survival of the premature fetus.

The terminal saccules are lined mainly by squamous epithelial cells of endodermal origin- type I alveolar cells or pneumocytes -for gas exchange.

scattered among squamous epithelial cells are rounded secretory epithelial cells- type II alveolar cells or pneumocytes- which secrete pulmonary surfactant (a mixture of phospho-lipids) on the internal walls of terminal saccules, lowering surface tension of alveoli.

Maturation of the lungsMaturation of the lungsTerminal Saccular Period : (26 weeks to Birth) :

The amount of surfactant increases during the last 2 weeks before birth.

The surfactant begins by 20 weeks but it is present only in small amounts in premature infants (by 26 to 28 weeks), they may suffer from respiratory distress because of surfactant deficiency.

Surfactant counteracts the surface tension forces and facilitates expansion of the terminal saccules (primordial alveoli).

Maturation of the lungsMaturation of the lungsAlveolar Period : (32 weeks to 8 years)It is the stage of lung maturation which continues after birth.

Mature alveoli do not develop until after birth, about 95% of alveoli develop postnatally.

The epithelial lining of terminal sacs becomes very thin squamous epithelium.

Type I alveolar cells become so thin that the adjacent capillaries bulge into the terminal saccules

By the late fetal period, the lungs are capable of respiration bescause the alveolo-capillary membrane is sufficiently thin to allow gas exchange, so lungs are capable of functioning as soon as the baby is born.

Maturation of the lungsMaturation of the lungsAlveolar Period : (32 weeks to 8 years)Before birth, Breathing movements can be detected by U.S, used in diagnosis of labor.

After birth, increase in the size of lungs results from increse in number of respiratory bronchioles & primordial alveoli rather than from increase in size of alveoli.

As these alveoli increase in size, they become mature alveoli, and the C.T. septa become secondary mature thin septa.

About 50 million alveoli, one –six of adult number, are present in lungs of a full-term newborn infant.

On chest radiographs, lungs of newborn infants are denser than adult lungs. By 8 years, about 300 million alveoli is present.

Maturation of the lungsMaturation of the lungsAlveolar Period : (32 weeks to 8 years)At birth, the lungs are half-filled with fluid from amniotic cavity, lungs & tracheal glands.

At birth, This fluid in lungs is cleared by : 1-via mouth & nose by pressure on fetal thorax during delivery. 2- Pulmonary capillaries. 3-Lymphatics & pulmonary arteries & veins.

At birth, lymphatics are more numerous than in adults.

Tracheo-esophageal FistulaTracheo-esophageal FistulaThese are the main varities of tracheo-esophageal fistula (TEF).

A, the common type, esophageal atresia (EA) is associated with TEF in more than 85% of cases.

B, fistula between trachea & esophagus.

C, air cannot enter the distal esophagus & stomach.

D, air can enter the distal esophagus & stomach, and esophageal & gastric contents may enter the trachea & lungs.

Tracheo-esophageal FistulaTracheo-esophageal FistulaIt is abnormal passage between the trachea & esophagus, males infants are most affected.

It is the most common anomaly of lower respiratory tract, and in more than 85% of cases it is associated with esophageal atresia (EA).

It is resulted from incomplete division of the of foregut into respiratory & esophageal parts during 4th week due to incomplete fusion of tracheo-esophageal folds ressults in a defective tracheo-esophageal septum and TEF.

it has 4 types, the most common type is for the superior part of esophagus to end blindly (A).

Polyhydramnios is often associated with esophageal atresia and TEF. This excess amniotic fluid develops because fluid cannot pass to stomach & intestine for absorption and transfering into placenta for disposal.

Gastric contents may reflux via the fistula into trachea & lungs to result in choking and pneumonitis.

The pharyngeal (Branchial) The pharyngeal (Branchial) apparatus :apparatus :

It consists of :It consists of : 1-pharyngeal arches. 1-pharyngeal arches. 2-pharyngeal pouches. 2-pharyngeal pouches. 3-pharyngeal grooves. 3-pharyngeal grooves. 4-pharyngeal membranes. 4-pharyngeal membranes.

These embryonic structures These embryonic structures contribute tocontribute to the development of the head & neckthe development of the head & neck regions.regions.

Drawings illustrating the human pharyngeal apparatus.A, dorsal view of cranial part of an embryo.

B to D, lateral views showing later development of pharyngeal arches.

E to G, ventral or facial views, illustrating the relationship of the first pharyngeal arch to the stomodeum. -Maxillary prominence of 1st arch lies lateral to stomodeum. -Mandibular prominence of 1st arch lies caudal to stomodeum.

H,horizontal S.of cranial part of emb.

I, horizontal S. illustrating the arch components + floor of pharynx.

J, sagittal S.illustrating the openings of pharyngeal pouches in the lateral wall of the primordial pharynx.

Pharyngeal archesPharyngeal archesThey begin to develop early in the 4th week as neural crest cells migrate into the future head & neck regions.

By the end of 4th week, 4 pairs of pharyngeal arches are visible externally

The 5th & 6th arches are rudimentary and not visible on surface of embryo.

Pharyngeal arches are separated externally by fissures- the pharyngeal grooves, which numbered cranio-caudally as the arches.

The arches appear along lateral wall of primordial pharynx.

The primordial mouth (stomodeum) is temporarily closed by oropharyngeal bilaminar membrane (ectoderm & endoderm), which ruptures at about 26 days.(during 4th week).

Fronto-nasal prominence

Pharyngeal archesPharyngeal arches

2- A ventral portion, the mandibular process, which gives rise the mandible ( lower jaw).

-1st pair of pharyngeal arches plays a major role in facial development.

2nd pharyngeal arch (hyoid arch) : contributes to the formation of hyoid bone.

1st pharyngeal arch (mandibular arch), the primordium of jaws consists of : 1- A dorsal portion, the maxillary process, which gives rise to the maxilla (upper jaw), zygomatic bone, and squamous part of temporal bone.

Pharyngeal ArchesPharyngeal ArchesA, By the end of the 4th week (28days), 4 pairs of pharyngeal arches are developed.

B, Drawing showing pharyngeal pouches + aortic arches, that supplies the pharyngeal arches and arise from truncus arteriosus, then enter the dorsal aorta.

The derivatives of aortic arches : are the adult arteries of head & neck.

C, Horizontal section showing the floor of primordial pharynx and germ layer derivatives of the pharyngeal arch components : each arch consists of a core of mesenchyme, which is covered externally by ectoderm and internally by endoderm.

Pharyngeal Arch Pharyngeal Arch ComponentsComponentsEach arch consists of a core of mesenchyme that is

covered externally by ectoderm and internally by endoderm.

A core of mesenchyme derived from the original mesenchyme - mesoderm during 3rd week + neural crest cells (neuroecto-dermal origin) during 4th week that migrate into the arches.

Skeletal musculature & vascular endothelia are derived from the original mesenchyme in the arches.

A typical pharyngeal arch contains : 1-an aortic arch artery : arises from truncus arteriosus of primordial heart to enter the dorsal aorta. 2-a cartilaginous rod : that forms the skeleton of arch. 3-a muscular components : to give ms. of head & neck. 4-a nerve : to supply mucosa & ms. of arch.

Fate of Pharyngeal Fate of Pharyngeal ArchesArches A, lateral view of embryo at 5th week

(32 days) showing pharyngreal arches + cervical sinus.

B, section illustrating growth of 2nd arch over 3rd & 4rth arches.

C, embryo of about 33 days.

D, section illustrating early closure of the cervical sinus during 5th week.

E, embryo of 41 days (end of 6th w.).

F, section showing the transitory cystic remnant of cervical sinus.

G, drawing of 20-week fetus illustrating the area of face derived from the 1st pair of pharyngeal arches.

Fate of Pharyngeal ArchesFate of Pharyngeal ArchesThey contribute to the development of face, nasal cavities, mouth,larynx, pharynx, and neck.

During 5th week, 2nd arch enlarges to overgrow the 3rd & 4th arches, forming ectodermal depression- the cervical sinus. B,D

By the end of 7th week, 2nd to 4th pharyngeal grooves + the cervical sinus have disappeared, giving the smooth contour to the neck

Derivatives of Pharyngeal Derivatives of Pharyngeal Arch Cartilages :Arch Cartilages :

A, lateral view of 4-week embryo, illustrating location of cartilages in pharyngeal arches.

B, 24-week fetus illustrating adult dreivatives of arch cartilages : 1-Note development of mandible by intramembranous ossification of mesenchymal tissue surrounding the ventral part of 1st arch cartilage (Meckel’s cartilage) + development of malleus & incus (middle ear bones) dorsally + anterior ligament of malleus & sphenomandibular ligament in the middle part. 2-Note, the 2nd arch cartilage (hyoid or Reichert’s cartilage) gives rise dorsally stapes & styloid process, in the middle stylohyoid ligament, and ventrally lesser cornu + upper part of body of hyoid bone.

Derivatives of Pharyngeal Derivatives of Pharyngeal Arch Cartilages :Arch Cartilages :

B, 3- Note also derivatives of the 3rd arch cartilage which ossifies to form : greater cornu (horn) + inferior part of the body of hyoid bone. 4- Cartilaginous components of 4th & 6th pharyngeal arches fuse to form most of laryngeal cartilages (thyroid + cricoid + arytenoid + corniculate + cuneiform), EXCEPT the cartilage of epiglottis which develops from the mesenchyme of the caudal part of hypopharyngeal eminence lying in the floor of primordial pharynx and derived from 3rd & 4th pharyngeal arches.

The 5th arch is often absent, if present it is rudimentary and has No derivatives.

Derivatives of Pharyngeal Arch Derivatives of Pharyngeal Arch Muscles :Muscles :

A, lateral view of 4-week embryo showing the muscles derived from the pharyngeal arches, the arrow showing the pathway of myoblasts from the occipital myotomes to form the muscles of tongue.

B, sketch of head & neck of 20-week fetus, showing the 1st pharyngeal arch forms ms. of mastication & other ms./ 2nd arch forms ms. of facial expression, post.belly of digastric & other ms. / 3rd arch forms stylopharyngeus./ 4th arch forms cricothyroid, levator veli palatini + constrictors of pharynx. / 6th arch forms interinsic ms. of larynx + striated ms of esophagus.

Derivatives of Pharyngeal ArchesDerivatives of Pharyngeal Arches

Derivatives of Pharyngeal Arch Derivatives of Pharyngeal Arch NervesNerves A, lateral view of head &neck and thorax of

4-week embryo showing the cranial nerves : Trigeminal N.(V), Facial N.(VII), Glossopharyngeal N.(IX), and Vagus N.(X)—supplying 1st ,2nd , 3rd & caudal (4th + 6th ) pharyngeal arches, respectively.

B, sketch of head & neck of 20-week fetus showing the 2 branches of 1st arch nerve (CNV), maxillary & mandibular branches of trigeminal N

C, Sagittal section of fetal head & neck showing the sensory nerves of trigeminal N., which is the principal sensory N. of head & neck supplying face, teeth & m.m. of nasal cavity, palate, mouth & tongue, and is the motor N. for ms. of mastication.

C, The nerves from 2nd to 6th pharyngeal arches have little sensory nerves, however, they innervate m.m. of post.tongue, pharynx & larynx.

Pharyngeal PouchesPharyngeal PouchesThe endoderm of the pharynx lines the internal aspects of pharyngeal arches passing into balloonlike diverticula- the pharyngeal pouches. B & C

There are 4 pairs of pouches, the 5th is absent or rudimentary. B & C

The endoderm of pouches contacts the ectoderm of the pharyngeal grooves forming the double-layered pharyngeal membranes separate the pouches from the grooves. C

Pharyngeal PouchesPharyngeal PouchesA, 5-weeks embryo, showing that the 2nd pharyngeal arch grows over the 3rd & 4th arches, burying the 2nd to 4th grooves in the cervical sinus.

B, 6 -weeks. The endoderm lining the pharyngeal pouches gives rise to important organs in head & neck.

C, 7 –weeks. Note migration of the developing thymus, parathyroid, and thyroid glands into the neck.

11stst Pharyngeal Pouch Pharyngeal PouchIt expands into an elongate tubotympanic recess.

The distal part of this recess contacts the 1st pharyngeal groove to form the tympanic membrane (eardrum).

The cavity of tubotympanic recess gives rise to tympanic cavity + mastoid antrum. And its connection with the pharynx form pharyngotympanic tube (auditory tube).

22ndnd Pharyngeal Pouch Pharyngeal PouchThe endoderm of 2nd pouch proliferates and grows into the surrounding mesenchyme to form palatine tonsile.

At about 20 weeks (fifth months), the tonsile is infiltrated by lymphatic tissue forming lymphatic nodules of the palatine tonsile.

Part of the cavity of this pouch remains as the tonsillar sinus or fossa.

33rdrd Pharyngeal Pouch Pharyngeal PouchBy the 6th week, the endoderm of 3rd pouch gives rise dorsal part which differentiates into inferior parathyroid gland, and a ventral part which differentiates into the thymus.

Both gland primordia migrate into the neck. Later the parathyroid glands separate from thymus to lie on the dorsal surface of thyroid gland.

Histogenesis of ThymusHistogenesis of Thymus Its epithelial tubes and cordsIts epithelial tubes and cords are developed from are developed from

the endoderm of the 3the endoderm of the 3rdrd pair of pharyngeal pair of pharyngeal pouches,pouches, forming the forming the thymic corpuscles.thymic corpuscles.

Its lymphocytesIts lymphocytes are derived from the are derived from the hematopoietic hematopoietic cells.cells.

Its mesenchymeIts mesenchyme and a peculiar and a peculiar ms.cellms.cell in the in the medulla is derived from medulla is derived from the neural crest cells.the neural crest cells.

Growth and development of thymusGrowth and development of thymus are notare not complete at birth, but continue until puberty.complete at birth, but continue until puberty.

At pubertyAt puberty it begins to deminish in size it begins to deminish in size (i.e.,(i.e.,undergoes involutionundergoes involution).).

By adulthoodBy adulthood, it is , it is often scarcely (difficult) often scarcely (difficult) recognizable,recognizable, since it is since it is atrophied and replaced byatrophied and replaced by fatty tissuefatty tissue, however it is still functional for health., however it is still functional for health.

44thth Pharyngeal Pouch Pharyngeal PouchBy 6th week, the epithelium of the dorsal part of 4th pouch forms the superior parathyroid gland, while of the ventral part forms the ultimopharyngeal body.

The inferior parathyroid glands derived from the 3rd pouch are located inferior to those from 4th one

5th pharyngeal PouchIf it develops, this rudimentary pouch becomes part of 4th pouch and contributes to form the ultimopharyngeal body, which which fusesfuses with thyroid gland to give rise with thyroid gland to give rise to to parafollicular cells ofparafollicular cells of thyroid thyroid gland,gland, ( (C –cellsC –cells ), which produces ), which produces calcitonin hormonecalcitonin hormone to regulate the to regulate the normal calcium level.normal calcium level.

Histogenesis of Parathyroid Histogenesis of Parathyroid Glands :Glands :

The epithelium of the dorsal parts of The epithelium of the dorsal parts of 33rdrd & 4 & 4thth pouches pouches proliferates during 5proliferates during 5thth week week to form smallto form small nodules on the dorsall nodules on the dorsall aspect of each pouch as aspect of each pouch as the superior & the superior & inferior parathyroid glands.inferior parathyroid glands.

The chief or principle cellsThe chief or principle cells are formed are formed during the during the embryonic periodembryonic period to regulate to regulate fetal calcium metabolism.fetal calcium metabolism.

The oxyphil cellsThe oxyphil cells are formed are formed 5 to 7 5 to 7 years after birth (childhood period).years after birth (childhood period).

Pharyngeal GroovesPharyngeal Grooves

There are 4 pharyngeal grooves (clefts) on each side, that are developed during the 4th & 5th weeks.

These grooves separate the pharyngeal arches externally.

Only one pair of grooves contributes to postnatal structures, the 1st pair groove persists as the external acoustic meatus.

The other grooves lie in a slitlike depression- the cervical sinus.

These grooves normally oblitrated with the sinus as the neck develops.

Pharyngeal membranesPharyngeal membranesThey are formed where the epithelia of the grooves (ectoderm) & the epithelia of pouches (endoderm) approach each other.

Only The 1st pharyngeal membrane gives rise the tympanic membrane (eardrum).

Branchial Branchial sinusessinuses : :Branchial sinus : -it is uncommon, it results from failure of 2nd pharyngeal groove & cervical sinus to oblitrate. -it is commonly detected during infancy discharging mucous material. -External branchial sinus : -it opens externally in the inferior part of lateral side of neck along ant.border of sternocliedomastoid ( lateral cervical sinuses). (C,D) -Internal branchial sinus : -Very rare to open internally into pharynx and usually open into tonsillar sinus, resulting from persistence of proximal part of 2nd pharyngeal pouch. (D)

Branchial Branchial FistulaFistula, & , & CystsCysts : :Branchial fistula : -it results from persistence of parts of 2nd pharyngeal groove & pouch. -it passes between the int. & ext. carotid arteries to opens internally into tonsillar sinus and it opens externally into subcutaneous tissue of the lateral side of the neck. (E)

Branchial cyst : -it develops from remnants of the cervical sinus and /or the 2nd pharyngeal groove. -it often lies free in the neck ,just inferior to the angle of mandible. -it may develop along anterior border of sternomastoid or in parathyroid gl. -it is not visible until in late childhood. -it contains fluid & cellular debris of desquamated epithelium. (F)

Branchial Vestige :Branchial Vestige :

It results from cartilaginous or bony remnants of pharyngeal arch cartilages appear under skin in the side of neck, Usually anterior to the inferior part of sternocliedo- mastoid ms. (F)

Development of Development of

Thyroid Gland :Thyroid Gland :A, B, & C, sagittal sections of 4, 5, & 6 weeks embryos, illustrating successive stages in the development of thyroid gland.

D, section, showing the path of thyroid gland during its embryonic descend (indicated by the former tract of thyroglossal duct).

Development of Development of Thyroid Gland :Thyroid Gland :

It is the first endocrine gland to develop.

It begins to develop during 4th week (26 days after fertilization) from a median endodermal thickening in the floor of primordial pharynx, which form a small outpouching – thyroid primordium. (A & B)

For a short time the thyroid gland is connected to the tongue by a narrow tube, the thyroglossal duct. (B & C)

The developing thyroid gland descends in the neck, passing ventral to the developing hyoid bone & laryngeal cartilages. (C & D)

Development of Development of Thyroid Gland :Thyroid Gland :

By 7th weeks, thyroid gland has assumed its definitive shape and has usually reached its final site in the neck. (D)

By7th weeks, the thyroglossal duct has normally degenerated and disappeared. The proximal opening of this duct persists as a small pit in the tongue – the foramen cecum. (D)

Development of Pyramidal Development of Pyramidal Lobe of Thyroid Gland :Lobe of Thyroid Gland :

Photograph of anterior surface of adult thyroid gland, showing persistence of the thyroglossal duct.

Note the pyramidal lobe ascending superiorly from the isthmus. It represents a persistent part of the inferior end of thyroglossal duct.

Development of Pyramidal Development of Pyramidal Lobe of Thyroid Gland :Lobe of Thyroid Gland :

At first the thyroid primordium is hollow but it soon becomes solid and divides into right & left lobes, which are connected by the isthmus, which lies anterior to the developing 2nd & 3rd tracheal rings.

A pyramidal lobe extends superiorly from the isthmus in about 50% of people. It may be attached to hyoid bone by fibrous tissue and /or smooth ms.-the levator of thyroid gland. A pyramidal lobe + the associated smooth ms. arise from a persistent part of distal end of thyroglossal duct.

Histogenesis of Thyroid Histogenesis of Thyroid Gland :Gland :

The thyroid primordiumThe thyroid primordium consists of a consists of a solid of solid of endodermal cells,endodermal cells, which form a which form a network of network of epithelial cords.epithelial cords.

During 11During 11thth week, week, colloid colloid begins to begins to appear in appear in the thyroid follicles the thyroid follicles as a sourceas a source of thyroxine hormone.of thyroxine hormone.

Parafollicular cellsParafollicular cells – – C cellsC cells derived fromderived from the ultimobranchial bodythe ultimobranchial body as a source of as a source of calcitonin.calcitonin.

Thyroglossal Duct Cysts & Thyroglossal Duct Cysts & sinuses :sinuses :

A, showing the possible locations of thyroglossal duct cysts + thyroglossal duct sinus. The broken line indicates the course of thyroglossal duct during descent of the developing thyroid gland from the foramen cecum to its final position in anterior part of the neck.

B, illustrating lingual & cervical cysts. Most cervical cysts are located just inferior to the hyoid bone.

Thyroglossal Duct Cysts & Thyroglossal Duct Cysts & sinuses :sinuses :

Normally the thyroglossal duct atrophies and disappears, but a remnant of it may persist forming a cyst in the tongue (lingual) or in the anterior part of the neck, usually just inferior to hyoid bone (cervical). Most cysts are seen by the age of 5 years. (B)

Cysts may form anywhere along the course of thyroglossal duct during descent of the thyroid gland from tongue into the neck. Unless this cyst becomes infected, most of them are asymptomatic, it usually painless, enlarging, movable mass, may contain some thyroid tissue.

Infected cyst may perforate, forming thyroglossal duct sinus that usually opens in the median plane of neck anterior to laryngeal cartilages. (A)

Arch Derivatives Arch Derivatives of the Tongue :of the Tongue :

A & B, horizontal sections of pharynx, showing successive stages in the development of tongue during 4th & 5th weeks.

C, drawing of adult tongue showing the pharyngeal arch derivatives of the nerve supply of its mucosa.

Development Development of the Tongue :of the Tongue :

By the end of 4th week, there are 3 swellings develop : 1-a median triangular elevation – the median tongue bud (tuberculum impar) appears in the floor of primordial pharynx, just rostral to foramen cecum and / from 2-the distal tongue buds (lateral lingual swellings), on each side of median tongue bud. (A)

The 3 lingual buds arise from proliferation of mesenchyme in ventromedial parts of 1st pair of pharyngeal arches. (A)

Development Development of the Tongue :of the Tongue :

The distal tongue buds merge with each other and overgrow the median tongue bud, which disappears.

The merged distal tongue buds form the anterior 2/3 (oral part) of the tongue.

Fusion of the diatal tongue buds is indicated by a middle groove – the median

sulcus of the tongue.

The median tongue bud forms No recognizable part of adult tongue.

Development Development of the Tongue :of the Tongue :

There are 2 elevations that develop caudal to foramen cecum : 1- The copula arises from the mesenchyme of the ventromedial parts of 2nd pair of pharyngeal arches. 2- The hypopharyngeal (hypobranchial) eminence develops caudal to copula from mesenchyme of ventromedial parts of 3rd & 4th pairs of arches.

The copula is gradually overgrown by hypopharyngeal eminence and disappears.

So, the posterior (pharyngeal) part of tongue develops from the rostral part of hypopharyngeal eminence. (C)

Development Development of the Tongue :of the Tongue :

The line of fusion of anterior & posterior parts of the tongue is indicated by a V- shaped groove – the terminal sulcus.

Most of tongue ms. are derived from myoblasts migrating from the occipital myotomes.

The C.T + vasculature of tongue are derived from the pharyngeal arch mesenchyme.

The hypoglossal N. accompanies the myoblasts during their migration to innervate most of the tongue ms.

At birth, the entire tongue is within the mouth, its post. 1/3 descends into oropharynx by 4 years of age.

Papillae & Taste Buds and / Papillae & Taste Buds and / nerve supply of Tongue :nerve supply of Tongue :

The types of lingual papillae in the mucosa covering anterior 2/3 of tongue The types of lingual papillae in the mucosa covering anterior 2/3 of tongue areare : vallate, foliate, fungiform & filiform ( filiform is the : vallate, foliate, fungiform & filiform ( filiform is the most common typemost common type which develops in which develops in early fetal period,10-11weeksearly fetal period,10-11weeks).).

All the mucosa of of anterior 2/3 of tongue (oral part) for general sensation All the mucosa of of anterior 2/3 of tongue (oral part) for general sensation EXCEPTEXCEPT the vallate papillae : the vallate papillae : is supplied by is supplied by lingual N.lingual N. (it is branch of (it is branch of mandibular, from mandibular, from trigeminal N. ,N. of 1trigeminal N. ,N. of 1stst pharyngeal arch). pharyngeal arch).

Taste buds develop during 11-13 weeks.Taste buds develop during 11-13 weeks. Taste sensation of anterior 2/3 of tongue (oral part) :Taste sensation of anterior 2/3 of tongue (oral part) : is supplied by is supplied by chorda chorda

tympanitympani branch of branch of facial N. (N.of 2facial N. (N.of 2ndnd arch). arch).

mucosa

Papillae & Taste Buds and / Papillae & Taste Buds and / nerve supply of Tongue :nerve supply of Tongue :

General sensation + taste sensation of posterior 1/3 of tongue, including vallate General sensation + taste sensation of posterior 1/3 of tongue, including vallate papillae :papillae : are supplied by are supplied by glossopharyngeal N. (N.of 3glossopharyngeal N. (N.of 3rdrd arch). arch).

a smalla small area of tongue anterior to epiglottis (root of tongue) area of tongue anterior to epiglottis (root of tongue) : : is supplied byis supplied by tthe superior laryngealhe superior laryngeal branch of branch of vagus N. (N. of 4vagus N. (N. of 4thth arch) . arch) . Because the 2Because the 2ndnd arch mesenchyme forming the copula, is overgrown by 3 arch mesenchyme forming the copula, is overgrown by 3 rdrd arch so, arch so,

the facial N. of 2the facial N. of 2rdrd arch arch does not supply any of tongue mucosa,does not supply any of tongue mucosa, EXCEPT EXCEPT for for taste taste buds of oral part of tongue ( anterior 2/3).buds of oral part of tongue ( anterior 2/3).

mucosa

Development of Salivary Development of Salivary Gland :Gland : They develop during 6-8th weeks, their

parenchymal (secretory tissue) derived from the epithelial buds of primordial oral cavity and their C.T. derived from neural crest cells.

Parotid glands : develop early in 6th week, from buds arising from oral ectodermal lining, near angle of stomodeum (primordial mouth). Secretion begins at 18th weeks.

Submandibular glands : develop late in 6th week, from endodermal buds in floor of stomodeum. Secretion begins at 16th weeks. Their growth continues after birth.

Sublingual glands : develop later in 8th week, from endodermal epithelial buds in the floor of stomodeum