Development Of Face Pharyngeal Apparatus And Foregut Notes

General Embryology And Development Of Face Oral And Paraoral Structures

‘Embryology’ deals with different stages of development of human body in the womb of the mother (intrauterine life) starting from the fertilization of the ovum till the birth of the baby.

Pharyngeal Apparatus Introduction

Foregut is the cephalic part of the yolk sac contained within the head fold of the embryo. The foregut is separated from the primitive mouth cavity by the buccopharyngeal membrane which ruptures in the 4th week of intrauterine life. The foregut now communicates with the exterior. At the junction of the future pharynx and oesophagus the laryngotracheal tube appears which later develops into larynx, trachea and lungs.

Read And Learn More: General Embryology Important Notes

Part of the foregut cranial to the laryngotracheal tube is called pre-laryngeal or pharyngeal part of the foregut. This develops into pharynx and a part of floor of the oral cavity and also contributes for the development of tongue, palatine tonsils, auditory tubes and middle ear.

Changes in the Pharyngeal Part of the Foregut:

The pharyngeal part of the foregut presents a funnel-shaped expansion which has

• A ventral wall
• A dorsal wall
• Two lateral walls

The mesenchyme in the walls of the pharyngeal part of the foregut undergoes segmentation in a cranio-caudal direction to differentiate into six pairs of mesodermal bars called pharyngeal arches/branchial arches.

Each Pharyngeal Arch consists of:

• An outer covering of ectoderm
• A central core of mesoderm
• An inner lining of endoderm

Components of Pharyngeal Apparatus:

During 4th and 5th weeks of intrauterine life, six pairs of pharyngeal arches are formed.

• A series of ectodermal grooves are seen on the external surface of the Pharyngeal part of foregut and are called pharyngeal clefts.
• The corresponding endodermal groves on the inner side between the arches are called pharyngeal pouches.
• These pharyngeal arches, clefts and pouches are numbered from the cranial to caudal end and together referred to as the pharyngeal apparatus which is responsible for the development of face, neck, mouth, tongue, pharynx and larynx.

Pharyngeal (branchial) Arches:

There are six pairs of pharyngeal arches. However, the fifth arch soon disappears without giving rise to any derivatives.

• Each pharyngeal arch meets the corresponding arch of the opposite side in the midline of the floor of primitive pharynx.
• The mesoderm of each arch consists of
  • A skeletal component
  • A striated muscular component
  • A cranial nerve
  • An aortic arterial arch

The various components of pharyngeal arches give rise to different structures in the head and neck regions.

Pharyngeal Apparatus

First Pharyngeal Arch and its derivatives or 1st Pharyngeal Arch :

• First pharyngeal arch is also called mandibular arch. It is differentiated into maxillary and mandibular processes.
• Maxillary process develops into the
  • Upper lip
  • Upper jaw
  • Palate
• The mandibular part contains the skeletal element of the arch called meckel’s cartilage which later gives rise to a part of the incus, malleus, Sphenomandibular ligament and anterior ligament of malleus.
• Mesenchymal tissue surrounding meckel’s cartilage develops into the mandible.
• The mesenchyme of the maxillary process gives rise to premaxilla, maxilla, zygomatic bone and a part of the temporal bone.
• The mesoderm of the first arch gives rise to mylohyoid, anterior belly of digastric, tensor tympani, tensor palati muscles and the muscles of mastication, namely masseter, temporalis, medial pterygoid and lateral pterygoid.
• Mandibular nerve is the nerve of the first pharyngeal arch.
• Artery of the first arch develops into maxillary artery.

Second Pharyngeal Arch and its derivatives:

Second pharyngeal arch is also called hyoid arch.

• Skeletal element of this arch is called reichert’s cartilage which develops into stapes, styloid process, lesser cornu of hyoid bone and upper part of body of hyoid bone (fig. 34.18).
• The mesoderm of the second arch develops into stapedius, stylohyoid, posterior belly of digastric muscle, auricular muscles and muscles of facial expression including the muscles of the scalp and the platysma.
• Facial nerve is the nerve of the second pharyngeal arch.
• Second arch artery develops into hyoid and stapedial arteries.

Third pharyngeal arch and its derivatives:

• The skeletal element of the arch gives rise to greater cornu and lower part of body of hyoid bone.
• Stylopharyngeus is the only muscle developing from the mesoderm of this arch which is supplied by the glossopharyngeal nerve which is the nerve of the third arch.
• Artery of the third arch develops into common and internal carotid arteries.

Fourth pharyngeal arch and its derivatives:

• Superior laryngeal branch of the vagus is the nerve of the fourth arch. Some authors believe that the fourth arch also gives rise to the constrictors of pharynx and levator palati muscle of soft palate.
• Arteries of the fourth arch have different derivatives on right and left sides. On the left side, it contributes to a part of arch of aorta and on the right side it develops into subclavian artery.

Pharyngeal Apparatus

Sixth Pharyngeal Arch and its derivatives:

• The nerve of the sixth arch is recurrent laryngeal nerve; a branch of vagus.
• Sixth arch artery on the left side develops into left pulmonary artery and ductus arteriosus and on the right side right pulmonary artery.

Summary of derivatives of the pharyngeal arches is given in table.

Summary of derivatives of pharyngeal arches:

Pharyngeal Clefts

As described earlier, ectodermal layer covers the external surface of the primitive,pharynx (pharyngeal apparatus).

• At about 5th week, embryo shows four ectodermal depressions between the Consecutive pharyngeal arches and are called pharyngeal clefts.
• At a later stage, except for the dorsal part of the first pharyngeal cleft, remaining clefts disappear. The dorsal part of the first pharyngeal cleft extends inwards towards the tubotympanic recess of the first pharyngeal pouch and develops into epithelium of external acoustic meatus and cuticular layer of tympanic membrane.
• Subsequently, due to proliferation of cells, the second arch sends a diverticulum which overlaps the third and fourth arches externally and merges caudally with epicardial ridge.
• As a result of this, second, third and fourth clefts get buried under this diverticulum leaving an ectodermal lined space; cervical sinus for a short period which soon disappears.
• The disappearance of the cervical sinus restores the smooth concavity of the side of the definitive neck.
• It may be noted that ectodermal cells covering the first pharyngeal arch form the skin over the lower jaw, (derived from the mandibular process) and upper jaw (derived from the maxillary process) up to the tragus of the auricle (derived from the dorsal end of first arch).
• Ectodermal cells of maxillary and mandibular processes also contribute for the formation of upper and lower lips, respectively. Along the edges of the lips, these cells proliferate into neighbouring mesenchyme which subsequently disintegrate to form alveolabial sulcus separating lips from the gums.
• The inner lining of the sulcus differentiates into dental lamina which later gives rise to enamels of teeth.

Pharyngeal Apparatus

Pharyngeal Pouches

• Pharyngeal pouches are series of endodermal lined depressions on the inner surface of the primitive pharynx lying between the consecutive pharyngeal arches.
• Total of five pharyngeal pouches are formed, but the fifth pouch remains Rudimentary.
• Ventral parts of pouches are obliterated by developing tongue.
• The dorsal parts of the pouches give rise to a number of structures in the head and neck regions.

Derivatives of the first pharyngeal pouch:

Tubotympanic Recess

• Derived from the dorsal part of the first pharyngeal pouch. Its medial part develops into auditory tube and lateral part dilates to form middle ear (tympanic) cavity, mastoid antrum and mastoid air cells.
• Distal end of tubotympanic recess reaches the first pharyngeal cleft separated from it by a thin layer of mesoderm. Here, endodermal cells lining the tympanic cavity form the mucous layer of tympanic membrane, thus making the tympanic membrane a derivative of all three germ layers (ectodermal cuticular layer, mesodermal fibrous layer and endodermal mucous layer).

Derivatives of Pharyngeal Pouches:

Derivatives of the Second Pharyngeal Pouch:

Endodermal cells of second pharyngeal pouch proliferate to form the palatine tonsil.

Derivatives of the Third Pharyngeal Pouch:

• Cells from the dorsal aspect of the third pharyngeal pouch differentiates into parathyroid iii (inferior parathyroid).
• The ventral aspect of the pouch gives rise to epithelioreticular cells and hassall’s corpuscles of thymus.
• Initially, both these structures are connected with each other. As a result of this, when the thymus migrates caudally, it drags the parathyroid III along the dorsal (posterior) surface of thyroid gland lobe where the parathyroid is disconnected from the thymus and thymus moves further down into the thorax.
• Thus in adults, parathyroid III is located at a lower level when compared to the location of parathyroid iv (from fourth pharyngeal pouch) and for the same reason, it is also called inferior parathyroid.

Pharyngeal Apparatus

Derivatives of the Fourth Pharyngeal Pouch:

• Cells from the dorsal aspect of the fourth pharyngeal pouch differentiates into parathyroid iv (superior parathyroid).
• It soon gets separated from the pharyngeal wall and attaches itself to caudally migrating thyroid gland.
• Thus, in adults, parathyroid iv is seen attached constantly to the lobe of the thyroid gland which is located above the parathyroid III. Hence, it is also called superior parathyroid.
• The ventral aspect of the fourth pouch gives rise to ultimobranchial body which is later incorporated into the developing thyroid gland in the form of parafollicular cells (c cells).

Development of the Tongue:

• Tongue develops from three different sources.
• Extrinsic and intrinsic muscles of the tongue develop from the occipital Myotomes.
• Connective tissue stroma of the tongue develops from the mesenchyme of the branchial arches.
• The mucous membrane (epithelium) of the tongue develops in relation with the pharyngeal arches from the endodermal cells covering the floor of the mouth which has been explained below.

Development of the Anterior Two-Thirds of the Tongue:

At about 4th week of Intrauterine Life:

• The medialmost parts of right and left first pharyngeal (mandibular) arches proliferate to form a pair of elevations in the floor of developing mouth called lingual swellings.
• Caudal to the lingual swellings, an unpaired median swelling derived from the first pharyngeal arch; called tuberculum impar is formed.
• The fusion of two lingual swellings and the tuberculum impar forms the Anterior two-thirds of the tongue. It is supplied by the mandibular (lingual nerve) and chorda tympani nerves which are nerves of the first arch.

Development of the Posterior One-Third of the Tongue:

• A midline elevation derived from the endodermal cells of medial ends ofMSecond, third and fourth arches called hypobranchial eminence is formed caudal to the tuberculum impar in the floor of the mouth. The cranial part of the hypobranchial eminence is derived from the second and third pharyngeal arches and is called copula ofhis.
• The posterior one-third develops from the copula. The third arch grows over the second arch to fuse with the first arch (the fusion is marked by the sulcus terminalis) and second arch gets buried under the surface.
• The posterior one-third of the tongue gets contribution only from the third arch and is supplied by the glossopharyngeal nerve which is the nerve of the third arch.

Summary of Development of Tongue:

Development of posteriormost part of the tongue:

• Posteriormost part of the tongue develops from the fourth pharyngeal arch and is supplied by superior laryngeal nerve (internal laryngeal nerve) which is the nerve of the fourth pharyngeal arch. The caudal part of the hypobranchial eminence which is derived from the fourth pharyngeal arch later develops into the epiglottis.

Development of Face

Development of Face Introduction:

During the 4th week of intrauterine life stomodeum (future mouth) is bounded

• Cranially by forebrain vesicle
• Caudally by ventral ends of right and left mandibular arches

Development of face takes place around the stomodeum by the contribution of five processes:

• A frontonasal process
• Right and left maxillary processes
• Right and left mandibular process

Formation of Frontonasal Process:

• During the 5th week, mesenchyme covering caudal surface of the forebrain proliferates and this proliferated mesenchyme along with its covering ectoderm is called frontonasal process.
• Two ectodermal elevations appear on the frontonasal process (one on each side) called the nasal placodes.
• Subsequently, depressions appear on the nasal placodes and are called nasal pits.
• The nasal pits communicate with the stomodeum and thus two different parts Of the frontonasal process can be recognized:
  • A pair of medial nasal processes which lie medial to corresponding Nasal pits.
  • A pair of lateral nasal processes lying lateral to each nasal pit

Formation of Maxillary and Mandibular Processes:

As the frontonasal process develops, a pair of triangular projections develop from the cranial aspect of the first pharyngeal (mandibular) arch called the maxillary processes.

• Each Maxillary Process:
  • Grows medially below the developing eye to meet the corresponding Lateral nasal process.
• It further grows medially below the nasal pit to fuse with Corresponding medial nasal process. This growth of the maxillary process separates the nasal pits from the stomodeum.
• Mandibular process is the main part of the first pharyngeal arch that lies caudal to stomodeum and grows ventrally to meet its counterpart from the opposite side.

Summary of development of the face:

• Development of Mouth: From the stomodeum after the rupture of the Buccopharyngeal membrane.
• Development of Lower Lip: From fused right and left mandibular processes
• Development of Upper Lip:
  • Lateral parts of the lip are formed by corresponding (right and left) Maxillary processes.
  • Median part of lip (philtrum) is formed by the two medial nasal
    Processes which are the parts of frontonasal process.
• Development of Cheek: The maxillary and the mandibular processes forming the upper and lower lips fuse with each other lateral to the developing mouth to form the cheeks. Thus, the upper part of cheek develops from maxillary process and the lower part of cheek develops from mandibular process.
• These different developmental components of face explain the basis for the cutaneous innervation of the face, which is shown.

Summary of development of face:

Development of the Nose

The nasal pits are separated from the stomodeum as the maxillary processes grow medially and fuse with the medial nasal process.

• The two nasal pits develop into anterior nasal apertures (anterior nares).
• During the 6th week, nasal pits deepen to form nasal sacs which further grow towards the developing forebrain to form the nasal cavity which after the formation of  the secondary palate and definitive choanae communicates with nasopharynx.
• Paired lateral nasal processes which lie lateral to the nasal pits develop into alae of the nose.
• The right and left nasal pits (anterior nasal apertures) gradually come close to each other due to the progressive thinning of frontonasal process. Deeper part of thin medial frontonasal process develops into nasal septum.
• The lateral walls of the nose are formed by the right and left lateral nasal processes from which later elevations project into the nasal cavity to form nasal conchae.
• As these processes differentiate into different components of the nose, their mesoderm proliferates and forms a bulge in the midline to form the prominence of the nose. Thus, the anterior nasal apertures (nasal pits) open downwards.

 

 

Summary of development of nose and paranasal air sinuses:

When maxillary process grows medially, it fuses with lateral nasal process. To begin with, they are separated by a groove called nasolacrimal groove. Ectoderm lining the groove forms a solid cord of cells which soon gets canalized to form nasolacrimal duct. The cranial end of the duct gets dilated to form lacrimal sac.

Development of the Primary Epithelial Band:

The epithelium covering the facial processes differentiates into odontogenic epithelium or the tooth forming epithelium. The epithelium proliferates and thickens to form the primary epithelial bands on two surfaces:

1. Inferior surface of the maxillary process
2. Medal nasal process
3. Superior surface of the mandible

On the 37th day, a single band of epithelium is observed.

Development of Paranasal Air Sinuses:

From the lateral wall of the nasal cavity, extensions (diverticula) invade into the neighbouring bones like maxilla, sphenoid, frontal and ethmoid to form air-filled spaces in these bones called paranasal air sinuses. All the paranasal air sinuses except frontal sinus begin to develop before birth and maxillary sinus is the first sinus to develop.

Development of Palate

Developmentally there are two stages of formation of palate:

• Formation of primary palate
• Formation of secondary palate The primary palate is a derivative of frontonasal process and the secondary palate is a derivative of maxillary process.

Summary of Development of Hard and Soft Palates:

Formation of primary palate:

• During the 6th week of intrauterine life, right and left maxillary processes grow medially compressing the medial nasal process towards the midline.
• This triangular part of frontonasal process bearing four incisor teeth is called primary palate which later ossifies to form pre-maxilla.

Formation of Secondary Palate

• As primary palate is formed, during the 6th week, bilateral shelf-like projections called palatine processes grow medially from the inner surface of the right and left maxillary processes.
• The palatine processes grow medially and obliquely downwards below the Developing tongue.
• During the 7th week, the palatine processes attain horizontal position and the tongue withdraws between the two processes due to the growth of the jaw.
• The palatine processes fuse with each other to form secondary palate.

Formation of hard and soft Palates

• Two secondary palates anteriorly fuse with the primary palate at a y-shaped suture.
• As the primary and secondary palates fuse, a median nasal septum from the frontonasal process grows downwards to fuse with the upper surface of Developing palate.
• Anterior most part of the hard palate bearing four incisor teeth is formed by the primary palate and major part of the hard palate and whole of the soft palate develop from the two secondary palates.
• The anterior part undergoes intramembranous ossification extending from Developing maxilla and palatine bones to form hard palate.
• The posterior part of the palate which extends beyond the nasal septum fails to ossify and forms the soft palate.
• In adults, incisive fossa is considered as the junction between the primary and secondary palates.

Development Of The Mandible

Mandible develops initially by intramembranous ossification and further development is due to the secondary cartilages of the mandible.

The mandible develops from the mandibular process of the first pharyngeal arch. The events in the development of the mandible are

1. Appearance of the meckel’s cartilage
2. Condensation of the mesenchyme and beginning of first point of ossification
3. Formation of the rudimentary mandible
4. Development of the secondary cartilages of the mandible
5. Formation of the alveolus

Appearance of the Meckel’s Cartilage

• At the 6th week of intrauterine life, before the development of the mandible meckel’s cartilage appears. It is a cartilaginous rod belonging to the first pharyngeal arch.
• It extends from the otic capsule in the region of the developing ear to the symphysis menti in the midline.
• Meckel’s cartilage is not directly involved in the formation of the mandible but provides a framework around which the mandible is formed.

Condensation of the Mesenchyme and First Point of Ossification:

• The initial mandible formed is a band of dense fibrous tissue on the Anterolateral part of meckel’s cartilage.
• At the 7th week of intrauterine life, condensation of mesenchyme is seen near the site close to the future mental foramen. The first point of ossification begins in this condensation at the point where the inferior alveolar nerve branches into the mental and incisive nerves.

Formation of the Rudimentary Mandible:

• From this point of ossification, the formation of bone rapidly spreads Backwards, forwards and upwards to enclose the inferior alveolar nerve and its terminal branches within bone.
• Further spread of ossification in a forward and backward directions forms bone on the lateral aspect of meckel’s cartilage which forms the future body of the mandible. This extends till the midline as all the above-mentioned events also occur on the opposite side. The bone does not fuse in the midline and are separated by fibrous tissue.
• The alveolar processes develop due to increased bone formation and enclose the developing tooth germs.
• The ramus of the mandible is formed due to ossification that occurs posteriorly in a direction away from meckel’s cartilage.
• By the 10th week, a rudimentary mandible is formed.

Development of the Secondary Cartilages of the Mandible:

• Between the 10th and 14th weeks, three secondary cartilages of the mandible develop:
  • Condylar cartilage – forms the condyle
  • Coronoid cartilage – forms the coronoid process
  • Symphyseal cartilage – forms the symphysis menti

Formation of the Alveolus:

• The developing bone gets close to the developing teeth as they reach the bud stage to form the alveolus. The size of the alveolus depends on the size of the tooth germ it encloses.
• Resorption of bone occurs on the inner wall and deposition of bone occurs on the outer wall of the alveolus and the teeth are enclosed within bony trough. The development of the interdental septa separates the teeth from each other.

Fate of Meckel’s Cartilage:

• Meckel’s cartilage ossifies to form the incus and malleus.
• Its sheath forms the sphenomandibular and sphenomalleolar ligaments.

Development Of The Maxilla

Maxilla is formed through intramembranous bone formation. The growth and Development of the maxilla are influenced by the growth of the orbital, nasal and oral cavities due to their proximity with the maxilla.

Ossification centre and spread of ossification:

• On the 8th week of intrauterine life, a centre of ossification develops near the site of the developing deciduous canine.
• The ossification then spreads into the different processes of the developing maxilla, i.e. Palatine, zygomatic, frontal and alveolar.
• Bone remodelling and sutural growth lead to the growth of the maxilla.
  • The growth of the eyeballs, nasal septum and the orbital pad of fat Exert forces to separate the maxilla from the adjacent bones and permits sutural growth.

Formation of the alveolar process of the maxilla:

• The increase in the height of the maxilla is due to the development of the alveolar process. This occurs due to deposition of bone and also bodily displacement of the maxilla. The growth of the nasal cavity causes resorption of bone on the upper portion of the palatine process and the deposition of bone on the lower surface.