CONTENTS

OBJECTIVE INTRODUCTION FORMATION OF a) HEADb) BRANCHIAL ARCHESc) FACEd) PALATEe) TONGUEf) DEVELOPMENT OF MAXILLA & MANDIBLEg) TMJ ABNORMALITIES REFERENCE

OBJECTIVES

1.To explore the growth and formation of the head and neck

from embryological development

2.To have in-depth knowledge about the events in sequence

INTRODUCTION

The head and neck are one of the most complicated structures

that the embryo forms, with special intermediate structures (the

pharyngeal arches) and contributions from all 3 embryonic layers

(ectoderm, mesoderm, endoderm), and significantly, a major

contribution from the neural crest cells under the genetic control

of HOX genes.

WEEK 1 of GESTATION

Zygote undergoes series of mitotic divisions & transform into the ball of small cells termed blastomeres.

On 3 days, (after 4 mitotic divisions) the blastomeres become morula, a ball of 16 cells.

On further cell division, morula forms a 100-celled structure: BLASTOCYST

Within the blastocyst, a fluid filled cavity develops which divides it intoan inner layer (embryoblast) that becomes the embryo &an outer layer (trophoblasts) that forms the embryonic part of the placenta.

Source:http://emedicine.medscape.com/article/1289057-overview

TROPHOBLAST

EMBRYOBLAST

Morula

(Zygote)

At approximately day 5, the blastocyst attaches to the

endometrium in the posterior wall of the uterus.

by the end of the first week, the superficial implantation of the blastocyst is completed.

Source :http://what-when-how.com/nursing/normal-pregnancy-maternal-and-newborn-nursing-part-2/

WEEK 2 OF GESTATION

a rapid proliferation and differentiation of trophoblast into bilaminar structure i.e. syncytiotrophoblasts and cytotrophoblasts.

Amniotic cavity appears as a space between the cytotrophoblasts and the inner cell mass.

Inner cell mass is further differentiated into bilaminar embryonic disc i.e. EPIBLAST & HYPOBLAST.

The Epiblast, is related to the amniotic cavity & hypoblast with blastocyst cavity.

its localized thickening, the prochordal plate, becomes the future embryonic cranial region.

At approximately day 9, the blastocyst is embedded completely into the endometrial epithelium.

spaces appear in the syncytiotrophoblasts that form the lacunae network by fusion.

around 11-12 days, syncytiotrophoblasts

erode into the endometrial blood vessels.

Formation of uteroplacental circulation allows the maternal blood into the

syncytiotrophoblast lacunae networks.

Also in days 11 and 12, the primary yolk sac forms,

and part of the cytotrophoblast differentiates into extraembryonic mesoderm.

development of primary chorionic villi which provides maximum contact area with maternal blood.

On day 13,formation of extraembryonic

coelom occurs by the breakdown and coalescence of the fluid filled spaces in the

extraembryonic mesoderm.

eventually, it grows into the chorionic

cavity.

Cells from the hypoblast migrate to displace the extracoelomic cavity away from the embryo proper and encase a new space called secondary yolk sac.

The excoelomic cavity is reduced into a remnant called the EXOCOELOMIC cyst.

3rd WEEK of GESTATION

Formation of the trilaminar embryonic disc by the process of GASTRULATION.

Proliferation of the epiblast starts at the caudal end of the embryo leading to caudocranial groove formation called primitive streak.

Cranial limit of the primitive streak is marked by the primitive node.

Cells of epiblast layer proliferates & differentiates at primitive streak and migratebetween epiblast and hypoblast.

Give rise to 3rd layer called MESODERM.

At this stage embryo is termed as GASTRULA.

the folding process in vertebrate embryos, which includes the transformation of the neural plate into the neural tube called

NEURULATION.

The embryo at this stage is termed the

NEURULA.

As the embryo begins as a two-dimensional

planar structure.

A midsagittal groove appears as a result of invagination of the ectoderm centrally.

simultaneous elevation of ectodermal tissue alongside the groove to form the neural

folds.

Neural folds

Meanwhile, cells of primitive streak proliferates in cranial direction to form a transient embryonic structure called

notochord (axial mesoderm) .

It represent the early midline axis of the embryo helping to establish the axial skeleton.

At 22nd day, as the folds fuse with each

other in the mid line, the neural tubeseparates from the overlying ectoderm from which it is derived.

A population of ectodermal cells adjacent to the neural fold and not included in the overlying surface ectoderm gives rise to the formation of the neural crest cells.

notochord

The neural crest cells are believed to migrate widely throughout the developing embryo in a relatively cell-

free enriched extracellular matrix.

Migration of NCC is under strict control of genes i.e.

Wnt gene for the induction of NCC ErbB4 gene for the migration of NCC

Most of the connective and skeletal tissues of the cranium and face ultimately come from the derivatives of

neural crest cells.

Migration of neural crest cells

Neural Crest Cells from:1. Anterior neural fold forms • much of epidermis of forehead and frontonasal regions, epithelium of

of primary palate and nasal cavities.

2. Posterior neural fold forms • epidermis of maxillary and mandibular regions, Secondary palate and

dorsum of tongue.

NCC migration was discovered via:

1. 3H labelled thymidine NCC2. cell mapping in chimeric embryos

Formation of Head

As the neural tube fuses cranially, closing off the anterior neuropore, a mesencephalic flexure occurs leading to the major divisions of the brain—

1. Prosencephalon (forebrain), 2. Mesencephalon (midbrain),3. Rhombencephalon (hindbrain)

During the 4th week,the primitive trisegmented brain further subdivides.

The prosencephalon (forebrain) divides into

1. Telencephalon with prominent lateral domes (cerebral hemispheres)

2. Diencephalon, which gives rise to the optic vesicles.

The mesencephalon remains undivided and in the flexed cephalic flexure.

The rhombencephalon divides into the

1. metencephalon (cerebellum, pons)2. myelencephalon (medulla).

By the end of the fourth week, the fundamental organization of the future brain is clearly identifiable.

BRANCHIAL ARCHES

The pharyngeal arches arises as outgrowths on the ventral surface of the embryo rostral to the foregut during 4th week of development.

It is lined externally by ectoderm and internally by endoderm.

The arches are laterally extending bands of tissue.

The gills of fish are formed embryologically when the branchial plates rupture, leaving slits between each of the branchial arches.

In humans, however, this rupture does not occur.

Pharyngeal membrane

The endoderm within the branchial pouches becomes specialized and eventually transforms into important structures of the neck and face.

The first branchial cleft, lined with ectoderm, will form the external ear canal.

Langman's 9th edition, 15.6.A

Cut section through the branchial arches in a developing embryo.

Each arch has its own neural ,vascular supply & cartilage.

The arches are grooved on the external surface by pharyngeal clefts

Whereas on the internal surface by pharyngeal pouches.

Pharyngeal membrane - ectoderm and endoderm contact regions(only first pair persist as tympanic membrane )

1. 1st Pharyngeal Arch (Mandibular Arch) has 2 prominences smaller upper(maxillary)which forms maxilla,

zygomatic bone and squamous part of temporal bone larger lower(mandibular) forms mandible

An abnormality in the first arch will cause congenital defects in the eyes, ears, palate and jaw

2. 2nd Pharyngeal Arch (Hyoid Arch)forms most of hyoid bone 3. Arch 3 and 4 are associated with neck structures.

Embryologically ,the arches are derived from the neural crest cells which migrates from specific segments of the

hindbrain(rhombomeres) with minor overlap between them.

NCC from rhombomere 1 & 2 together

with caudal midbrain derived crest cells, populate the 1st arch.

Crest cells from rhombomere 4 populates the 2nd arch.

While rhombomeres 6 & 7 contribute to 3rd,4th and 6th arches.

Rhombomeres 3 & 5 are depleted of crest cell population.

Each arch also has two nerves, one

sensory and one motor, that will innervate the muscle that is derived from that arch.

Schematic representation of nerve supply of each pharyngeal arches.

Source:Langman's 9th edition 15.7

Diagrammatic representation of bones and cartilages of respective pharyngeal arches.

Pharyngeal Pouches

The pharyngeal pouches form as an outgrowths of the pharyngeal arches when they become lined by the pharynx.

There are 4 pairs of pouches, and similarly to the pharyngeal arches,

The 5th pair of pouches does not have any known developmental purpose.

http://missinglink.ucsf.edu/restricted/lm/CongenitalAnomalies/BranchialCleftCyst.html

Facial Formation

The differentiation of human face takes place between 4th & 7th weeks of IU.

The brain tissue exerts an organizing influence on the developing face.

The frontonasal process develops under the influence of the forebrain.

The forebrain establishes multiple signaling centers in the ectoderm under the control of SHH(Sonic hedgehog) gene.

This signaling ensures proper decent of FNP.

Around the 5th week of fetal development the face begins to take shape

nasal placodes develop bilaterally at the inferolateral corners of the

frontonasal process

Invagination of nasal placodes to nasal pits formation.

Also the development of nasal placodes in a medial direction in a horse shoe manner leads to the formation of medial & lateral nasal

process. http://embryology.med.unsw.edu.au/Notes/face2.htm

During the 6th and 7th week the nasal and maxillary processes begin to expand and fuse to form the upper lip

& cheeks.

The lower lip begins to form earlier when the mandibular swellings become continuous and the mandibular depression is filled in “by proliferation of mesenchyme”.

The maxillary processes also fuses with the respective mandibular process at 7-8 weeks forming angle of the mouth.

Complete fusion of the medial nasal prominences is important because this is where cleft lip can occur.

Next, the nasolacrimal groove and duct develop in the 7th week of gestation at the line of fusion between the lateral nasal process and maxillary process.

Formation of the Primary and Secondary Palate

The palate as a whole forms from two primordia which can be classified as the primary and secondary palate.

At around the 6th week of development the primary palate begins to take shape, arising from the medial nasal process.

Composed of mesoderm, this “wedge-shaped mass” will eventually extend to form the floor of the nasal cavity .

https://pocketdentistry.com/craniofacial-anatomy-and-embryology/

Around the 7 & 8th week of development the secondary palatebegins to develop from two lateral palatine processes & completed

around the 12th week .

As mandible develops, the tongue drops and the palatine processes grow medially and fuse in the midline.

They also fuse with the nasal septum and the primary palate.

Front view

Ossification occurs in an antero-

posterior direction.

The posterior portions of the lateral palatine processes do not become ossified, but extend past the nasal septum and fuse to form the soft

palate and uvula.

This is the last portion of the palate to form.

oral view

In order for the fusion of the two palatine processes to occur a significant amount of force is necessary, but the nature of this force is unknown.

One possible explanation is that the force is “generated by the progressive accumulation and hydration of hyaluronic acid”.

FORMATION OF TONGUE

Around the 4th week of embryonic development the structure that will become the tongue begins to form from the first, second, third and fourth pharyngeal arches.

Three swellings (one median tongue bud and two distal tongue buds), form and grow rapidly during the 5th week and merge with each other.

These form the anterior 2/3rd of the tongue.

Tuberculum impar forms no adult structure .

At same time 2 elevations develop caudal to foramen cecum:1. Copula: from 2nd arch2. Hypobranchial eminence: from 3rd & 4th arches

The hypobranchial eminence overgrows the copula which later disappears.

The post 1/3rd of the tongue is formed by the rostral part of the hypobranchial eminence (Arch 3)

Caudal part of hypobranchial eminence (Arch 4) forms the epiglottis.

Branchial mesenchyme forms the soft tissue, vascular and lymphatics of the tongue.Tongue muscles originate from the occipital somites.

Innervation to tongue:trigeminal nerve (V) - lingual branchfacial nerve (VII) - chorda tympani branchglossopharyngeal nerve (IX)hypoglossal nerve (XII) - motor components of innervated muscles

DEVELOPMENT OF MAXILLA

The mandibular arch gives off a bud from its dorsal end which grows ventro-medio-cranially called as MAXILLARY PROCESS.

Thus at this stage the primitive mouth or

stomodeum is overlapped from above by the frontal process, below by the mandibular process and on either side by the maxillary process.

The maxilla also develops from a center of ossification in the mesenchyme of the maxillary process of the first arch.

No arch cartilage or primary cartilage exists in the maxillary process. But the center of ossification is associated closely with the cartilage of the nasal capsule.

stomodeum

From this center, bone formation spreads posteriorly below the orbit toward the developing zygoma and anteriorly toward the future incisor region .

Ossification also spreads superiorly to form the frontal process.As a result of this pattern of bone deposition ,a bony trough forms for the infraorbital nerve.

From this trough a downward extension of bone forms the lateral alveolar plate for maxillary tooth germ.

Ossification also spreads into the palatine process to form the hard palate.

A secondary cartilage also contributes to the development of the maxilla. A zygomatic, or malar cartilage appears in the developing zygomatic

process and for a short time adds considerably to the development of the maxilla.

The body of the maxilla is relatively small because the maxillary sinus has

not developed. This sinus forms during the sixteenth week as a shallow groove on the nasal aspect of the developing maxilla.

At birth the sinus is still a rudimentary structure about the size of a small pea.

DEVELOPMENT OF MANDIBLE

At the 6th week of development a cartilaginous rod forms, also known as

Merkel’s cartilage from the region around the ear to the midline mandibular processes.

The mandibular nerve splits at this time into the lingual and inferior

alveolar branches and will line the cartilage.

During 7th week the first ossification center forms and from this point bone develops out from the midline.

Ossification takes place in the membrane covering the outer surface of Meckel's cartilage

and each half of the bone is formed from a single center which appears,

in the region of the bifurcation

of the mental and incisive branches, about the 6th week of fetal life.

A canal also forms backward to contain the alveolar nerve. Medial and lateral alveolar plates form so that tooth germs can form in a trough between them.

The bone of the mandible will to continue to form after the teeth have developed to support them.

Around 10th weeks the mandible is recognizable and much of the bone has formed.After this point there will be a strong dependence for formation on 3

cartilages: 1. the condylar cartilage,2. the coronoid cartilage and 3. the symphyseal cartilage.

The condylar cartilage will be converted almost entirely to bone, but the small portion of cartilage that remains at the articular end is necessary for the continuous growth of the mandible.

The coronoid and midline symphyseal cartilages are also important for growth and development, but disappear before birth and in the year after respectively.

The TMJ is an articulation between two bones initially formed from membranous centers of ossification during the 10th week of life.

before the condylar cartilage forms, a broad band of undifferentiated mesenchyme exists between the developing ramus of the mandible and the developing squamous tympanic bone.

With formation of the condylar cartilage, this band is reduced rapidly in width and is converted into a dense strip of mesenchyme.

The mesenchyme immediately adjacent to this strip breaks down to form the joint cavity, and the strip becomes the articular disk of the joint.

Source:F.H.Netter 4th ed.

Temporomandibular Joint

Abnormalitiesstages Time (Post Fertilization) Related syndrome

Germ layer formation Day 17 Fetal alcohol syndrome

Neural tube formation Day 18-23 Anencephaly

Origin, migration and interaction of cell population

Day 19-28 Hemi facial microsomia,Treacher colins’ syndrome

Formation of organ system Day 28-38 Cleft lip and/or palate, other facial clefts

Secondary palate Day 42-55(6-9wks) Cleft palate

Final differentiation of tissue Day 50-birth Achondroplasia, Crouzon's,Apert’s syndrome etc.

References

Ten cate’s oral Histology 7th edition O.P.Kharbanda 2nd edition I.B Singh Textbook of embryology F.H.Netter 4th edition Source:Langman's 9th edition

https://embryology.med.unsw.edu.au/embryology/index.php/Lecture_-_Head_Development# http://emedicine.medscape.com/article/1289057-overview http://missinglink.ucsf.edu/restricted/lm/CongenitalAnomalies/BranchialCleftCyst.html

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