EMBRYOLOGY OF THE HEAD & NECK - PART II

-Dr.RATNA DEEPIKA SESHAGIRI MDS PART I

CONTENTS:

• Development of Mandible• Development of Temporomandibular joint• Development of Palate• Development of Tongue• Development of Tonsils• Development of Salivary Glands• Development of muscle• Conclusion• References

DEVELOPMENT OF MANDIBLE:• The cartilages and bones of the mandibular skeleton

form from embryonic neural crest cells that originated in the mid and hindbrain of the neural folds.

• The first structure to develop in the region of the lower jaw is the mandibular division of the trigeminal nerve that precedes the ectomesenchymal condensation forming the first branchial arch.

• The mandible is derived from ossification of an osteogenic membrane formed from ectomesenchymal condensation at 36-38 days of development.

• A single ossification centre for each half of the mandible arises in the sixth week intra uterine life in the region of bifurcation of alveolar nerve and artery in to mental and incisive branches.

• Ossification membrane is lateral to Meckel’s cartilage and its accompanying neurovascular bundle.

• Ossification spreads from the primary centre below and around the inferior alveolar nerve and its incisive branches , and upwards to form a trough for the developing teeth.

• Spread of intramembranous ossification dorsally and ventrally forms the body and ramus of mandible.

• Ossifications stops dorsally at the site that will become mandibular lingula, from where Meckel’s cartilage continue in to the middle ear.

• Meckel’s cartilage diverges dorsally to end in the tympanic cavity of each middle ear, which is derived from the first pharyngeal pouch, and is surrounded by the forming petrous part of temporal bone.

• The dorsal end of the Meckel’s cartilage ossifies to form basis of two of the auditory ossicles

Malleus and Incus

• The third ossicle which is derived primarily from the cartilage of second branchial arch(Reichert’s cartilage)

Stapes

• Parts of Meckel’s cartilage transform in to

Sphenomandibular ligament

Malleolar ligament

• Meckel’s cartilage dorsal to the mental foramen undergoes resorption on its lateral surface at the same time as intramembranous bony trabeculae are forming immediately lateral to the resorbing cartilage.

Initial woven bone

Lamellar bone+ Haversian systems

5th month intrauterine

• Secondary accessory cartilages appear between the 10th and 14th week i.u to form

Head of condylePart of coronoid processMental protuberance

• The appearance of this secondary cartilages is dissociated from the primary branchial and chondrocranial cartilages.

• The secondary cartilage of coronoid process develops within the temporalis muscle as its predecessor.

• The coronoid accessory cartilage becomes incorporated in to the expanding intramembranous bone of the ramus and disappears before birth.

• In the mental region ,on either side of the symphysis, one or two small cartilages appear and ossify in the 7th month i.u to form variable number of mental ossicles in fibrous tissue of symphysis.

• The condylar secondary cartilage appears during the 10th week i.u as a cone shaped structure in the ramal region.

• Cartilage of condylar head increase by

Interstitial growth Appositional growth

• Condylar cartilage serves as a important centre of growth for ramus and body of the mandible.

• By14th week, first evidence of endochondral bone appear in condylar region.

• Change in mandibular position and form are related to direction and amount of condylar growth.

• The continuing presence of the cartilage provides a potential for continued growth ,which is realized in conditions of abnormal growth such as Acromegaly.

At birth At childhood

At Adult At Senility

Syndromes associated with mandible:

• Down’s syndrome • Marfan’s syndrome• Turners syndrome• Klinefelter’s syndrome• Pierre-robin syndrome• Treacher collins syndrome

Other Congenital anomalies:

• Agnathia• Micrognathia• Macrognathia• Facial Hemihypertrophy• Facial Atrophy

Development of Temporomandibular joint:

• The Temporomandibular joint is a secondary development ,both in its evolution and embryological history.

• The joint between malleus and incus that develops at the dorsal end of the Meckel’s cartilage is phylogenetically the primary jaw joint.

• When the temporomandibular joint forms at 10 week i.u., both the malleo-incudal and definitive jaw joints move in synchrony for about 8 weeks in fetal life .

• TMJ develops from 2 blastemas :

Condylar blastema Temporal blastema

• Between 10-12 weeks of I.U life

Condylar blastema develops from the secondary condylar cartilage

Temporal blastema arises from the otic capsule

• During the 10th week intrauterine two clefts develop in the interposed vascular fibrous connective tissue, forming the two joint cavities and thereby defining the intervening articular disk.

• Inferior compartment- 10th week

Separating the future disk from the developing condyle• Upper compartment – 11 ½ week.

After which cavitation occurs, due to muscle movement, by rupture of small spaces to coalesce into functional cavities.

1. Fibrous layer2. Cartilage3. Bone4. Bone marrow

• Condensation of mesenchyme forms the basis of joint capsule, which progressively isolates the joint from the surrounding tissues.

• The joint capsule composed of fibrous tissue, recognizable by the 11th week i.u., forms lateral ligaments

• The Temporomandibular joint of the new born child is comparatively lax structure, with stability solely dependant upon the capsule.

• At birth – Mandibular fossa is almost flat and bears no articular tubercle.

• At 7 years –After the eruption of permanent teeth, Articular tubercle begin to become prominent.

• After 12 years- the disk becomes S-shaped, more compact, more collagenous and less cellular.

• Mature disk- Avascular and Aneural in its central portion,but is filled with vessels, nerves and elastic fibres posteriorly.

Development of Palate:

Early palate formation:

• The primitive stomodeum that forms a wide central shallow depression in the face is limited in its depth by the oropharyngeal membrane .

• The characteristically deep oral cavity is formed by ventral growth of prominences surrounding the stomodeum .

• The stomodeum establishes as an oronasalpharyngeal chamber and entrance to the gut on the 28th day, when the dividing oropharyngeal membrane disintegrates, providing continuity of passage between the mouth and pharynx.

• The stomodeal chamber divides into separate oral and nasal cavities when the frontonasal and maxillary prominences develop horizontal extensions into the chamber.

• Frontonasal prominence

Single median primary palate• Maxillary prominences

Two lateral palatal shelves

• The shelves elevate unevenly with the anterior third “flipping up” followed by an oozing “flow” of the posterior two-thirds.

• Elevation of shelves enables their mutual contact in the midline, the primary palate anteriorly and the nasal septum superiorly.

• The shelves also fuse with the nasal septum , except posteriorly, where the soft palate and uvula remain unattached.

• Ossification provides the basis for the anterior bony hard palate. The posterior third of palate remains unossified.

SECONDARY PALATE FORMATION:• The three elements that make up the

secondary palate formation

Two lateral maxillary palatal shelves

Primary palate

• Palatine shelves ascend to attain a position above the tongue and fuse; forming Secondary palate.

• Incisive foramen is the midline landmark between primary & secondary palate.

Developmental anomalies of palate:

• Cleft palate: Unilateral cleft palate and

lip Bilateral cleft palate and lip Bifid uvula

• Epstein pearls• Bohn’s nodules• Nasopalatine duct cyst

Syndromes associated with cleft palate:

• Achondroplasia• Cleiodocranial dysplasia• Treacher Collins syndrome• Fetal alcohol syndrome• Goltz-Gorlin syndrome(Focal dermal hypoplasia syndrome)• Marfan syndrome• Pierre Robin syndrome• Van-der Woude syndrome

• Stickler syndrome(Pierre Robin sequence)• Oro-facial Digital syndrome• Apert syndrome• Goldenher syndrome• Ehler danlos syndrome• Lowry -Miller syndrome• Velo Cardiofacial syndrome(DiGeorge syndrome)

Syndromes associated with cleft lip:

• Achondroplasia• Fetal alcohol syndrome• Goldenher syndrome• Goltz-Gorlin syndrome (Focal dermal hypoplasia syndrome)• Van- der Woude syndrome• Median facial dysplasia syndrome

DEVELOPMENT OF TONGUE:

• The tongue arises in the ventral wall of the primitive oropharynx from the inner lining of the four branchial arches.

• During the 4th week i.u., paired lateral thickenings of mesenchyme appear on the internal aspect of the first branchial arch to form the lingual swellings.

• Between the swellings a median eminence appears ,the tuberculum impar (unpaired tubercle) whose caudal border is marked by a blind pit.

• This pit, the Foramen caecum, marks the site of origin of the thyroid diverticulum, an endodermal duct that appears during the somite period.

• The diverticulum migrates caudally ventral to the pharynx ,as the thyroglossal duct, which bifurcates and subdivides to form the thyroid gland.

• The lingual swellings grow and fuse with each other, emcompassing the tuberculum impar,to provide ectodermal derived body of the mucosa of the tongue.

• The ventral bases of the second, third and fourth branchial arches elevate in to a united, single midventral prominence known as the Copula.

• A posterior subdivision of this prominence is identified as the hypobranchial eminence.

• A V-shaped sulcus terminalis ,whose apex is the foramen caecum, demarcates the mobile body of the tongue from its fixed root.

• The line of the sulcus terminalis is marked by 8-12 large circumvallate papillae that develop at 2-5 months i.u.

• Fungiform papillae develop on the dorsal surface of the tongue-11 weeks i.u.

• Development of Filiform papillae is not complete till postnatally.

• Gustatory cells – 7 th week i.u.

Developmental anomalies of tongue:

• Ankyloglossia(Tongue tie)• Bifid tongue• Microglossia• Macroglossia• Aglossia

Syndromes associated with tongue:

• Oro-facial digital syndrome- Cleft tongue

• Down’s syndrome• Stickler syndrome• Hurler syndrome• Beckwith-wiedemann syndrome• Hanhart Syndrome

Development of tonsils:

Development of tonsils:

• The endoderm lining the second pharyngeal pouch between the tongue and soft palate invades the surrounding mesenchyme as a solid group of buds.

• Central parts of these buds degenerate to form TONSILLAR CRYPTS .

Invading lymphoid cells surround the crypts

To group as lymphoid follicles

• Lymphoid tissue invades into Palatine region Posterior region Pharyngeal region Lingual region Auditory region• These lymphoid masses encircle the

oropharynx to form Waldayers ring .• It is a ring of immunodefensive

tissues that grows postnatally in the oropharynx .

• Palatine tonsils arise at site of second pharyngeal arch.

• Pharyngeal and Lingual tonsils develop in mucosa of posterior wall of the pharynx and roof of the tongue.

• Tubal tonsils are formed by the lateral extensions of the lymphoid tissue posterior to the openings of the auditory tubes.

Development of salivary glands:

• During fetal life, each salivary gland is formed at a specific location in the oral cavity through the growth of the bud of the oral epithelium into the underlying mesenchyme.

• The three major sets of salivary glands-

Parotid gland Submandibular gland Sublingual gland

Developmental stages:

• Stage 1: Induction of oral epithelium by underlying mesenchyme.

• Stage 2: Formation and growth of epithelial cord.

• Stage 3: Initiation of branching in terminal parts of epithelial cord and continuation of glandular differentiation.

• Stage 4: Repetitive branching of epithelial cord and lobule formation.

• Stage5: Canalization of presumptive ducts.

• Stage 6: Cytodifferentiation.

Cells of salivary glands:

• The lining epithelium of the ducts, tubules and acini differentiate both morphologically and functionally.

• Inner epithelial layer - secretory cells (serous and mucous)

• Outer epithelial layer- myoepithelial cells (derived from neural crest)

Major Salivary Glands:

• Parotid Gland: Purely serous. First to appear, 6th week i.u. Appears on the inner cheek near the

angle of the mouth and then grows back to the ear.

Stenson’s duct opens in buccal mucosa opposite maxillary 2nd molar.

• Submandibular Gland: Mixed serous and mucus. Appear late in the 6th week prenatally. Appears bilaterally in the floor of the

mouth. Wharton’s duct opens in the floor of the

mouth on either side of the lingual frenum.• Sublingual Gland: Predominantly mucus. Appears around 8th week i.u. Appears lateral to the submandibular gland. Bartholin’s duct opens into the Wharton’s

duct and drains through the sublingual caruncle.

Development of Muscle:• Craniofacial voluntary muscles develop from paraxial mesoderm that

condenses rostrally as incompletely segmented somitomeres and segmented somitomeres of occipital and rostral cervical regions.

• Myomeres of somitomeres +Myotomes of the somites

Myoblasts

divide and fuse

Myotubes

Cease further mitosis Myocytes

• The mesenchymal component of the pharyngeal arches gives rise to special visceral (striated) musculature, which is voluntary in nature.

MESENCHYME MUSCLES1st arch Muscles of Mastication

2nd arch Muscles of Facial Expression

3rd arch Stylopharyngeus4th arch Pharyngeal muscles,

Palatopharyngeus, Levator veli palatini, Uvular

muscles6th arch Laryngeal muscles

Occipital Somites Muscles of the tongue

Schematic description of the embryonic origins of (clockwise from upper right) the ocular,masticatory, facial, pharyngeal, neck, and tongue muscles.

Timeline for muscle development

Orofacial musculature:1. First to develop in the body.2. Genioglossus and Geniohyoid- at 32 to 36 days i.u.3. Mylohyoid & Anterior Belly of Digastric-first to develop from the 1st arch.Palatal musculature:1. Tensor Veli Palatini- 40 days post conception.2. Levator Veli Palatini, Palatopharyngeus- around 45 days.3. Uvular muscles-when palatal shelves fuse.4. Palatoglossus is the last to develop.

Masticatory musculature:1. Develop as individual entities from the 1st arch mesenchyme.2. The muscles need constant reattachment due to remodelling of the mandible during growth phase.

MUSCLES OF FACIAL EXPRESSIONS

MUSCLES OF MASTICATION

Conclusion:

• Embryology helps us to understand the normal growth pattern of a variety of structures and the time at which they complete their growth.

• So embryology is important for providing correct treatment modalities at an appropriate time.

References:

• CRANIOFACIAL EMBRYOLOGY : GEOFFERY SPERBER

• TEN CATE’S ORAL HISTOLOGY

• LANGMAN’S MEDICAL EMBRYOLOGY

• TEXTBOOK OF HUMAN EMBRYOLOGY INDERBIR SINGH