Introduction to Craniofacial Growth and Development

8/12/2019 Introduction to Craniofacial Growth and Development

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"It is not birth, marriage, or death, butgastrulation, which is truly the most

important time in your life."

Lewis Wolpert (1986)


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During the pre Embryonic period, the cellsthat will eventually give rise to all structures

of the body differentiate into three germlayers.

During this stage (Gastrulat ion), cell

movements result in a massivereorganization of the embryo from a simplespherical ball of cells, the blastula, into amulti-layered organism.

Many of the cells at or near the surface ofthe embryo move to a new, more interiorlocation.


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The cells of the embryo form three tissue

(germinal) layers:- Ectoderm (outside layer),

- Mesoderm (middle layer),

- Endoderm (inside layer).

This is one of the most crucial points indevelopment where a great deal ofdifferentiation occurs


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These specialized layers of cells are:

Ectoderm(forming all nerve and someepithelial tissue),

Mesoderm(forming all connective, muscle

and some epithelial tissue)

Endoderm(forming some epithelial tissue)

These cells undergo rapid regionaldevelopment in the embryo, producing theprecursors of adult structures


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At the end of Week 3, ectoderm differentiates into

neuroectodermand epidermis. The latter covers the outsideof the bodySignals from the underlying mesoderm cause the now neuralprecursor cells called the neural plateto invaginate and forma neural tube

Neuroectoderm forms neural tube and neural crest


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Hollow neural tube eventually becomes thecentral nervous system (top of tubebecoming the brain and farther down, thespinal cord)

When the neural tube forms, theintermediate cells between the tube andthe ectoderm become neural crest cel ls

These cells migrate out and become cellsof the peripheral nervous system


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Just beneath the neural tube lies another structure,the notochord.

This notochord functions as the midline segmentalorganiser for all of embryologic development.

As adults, only vestigial remnants of this oncedynamic notochord remain.

However a line of function, a midline that organisesour physiology, persists to guide us throughout life.


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As the development of the head progresses,neural crest cells (and lateral platemesoderm) both migrate into rapidly forming

pharyngeal arches,a series of bump-likestructures on both sides of the embryonichead

Neural crest cells, in addition to forming nervetissue, produce the bones of the cranium.

Within the pharyngeal arches, neural crestcells and lateral plate mesoderm give rise to

bones of the jaw and lower face, theviscerocranium

Lateral plate mesoderm also contributes to

the formation of the cartilages of the larynx


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The key to understanding craniofacialdevelopment are the PharyngealApparatus (PA)

Also known asBranchial Apparatus

Pharyngeal Apparatus include:

- Pharyngeal arches (mesoderm)- Pharyngeal clefts (ectoderm)

- Pharyngeal pouches (endoderm)

- Pharyngeal membranes


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This structure contributes extensively todevelopment of structures in the head and neck.

3 developmental layers come together to form thePA

In the 4thweeks of embryologicaldevelopment:

- swellings form at rostral end (head end)

- by end of 4thweek all swellings (PA) haveappeared from mesoderm

- 3 germinal layers are apparent:

1. Ectodermal lining: skin

2. Endodermal lining: internal surface

3. Mesodermal core


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The PA consists of 4 pharyngeal arches, pharyngeal pouchesand pharyngeal grooves.

Neural crest cells in the cranial region migrate to thepharyngeal apparatus

They cause enlargement of the arches and these cellscontribute to the development of bones and connectivetissues of the head and neck.

Pharyngeal arches develop into components of the face.

Arch mesoderm forms muscles of face


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6 arches develop. the 5tharch degeneratesand the 4thand 6tharches fuse to form one

Each pharyngeal arch has its own vascularsupply, cranial nerve innervation, muscularcomponents and skeletal components

(cartilage).

Grooves separate arches externally.Pouches separate arches internally.


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First arch splits giving rise to 2 regions:- Rostral part called Maxillary Process- Caudal region called Mandibular Process

Maxillary process gives rise to upper jaw (Maxilla)

Mandibular process gives rise to lower jaw(Mandible)

The first pharyngeal arch is often called themandibular arch. It is from this arch that the

jaws develop.


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Often called the hyoid archas part of the hyoidbone develops here.

Where is the hyo id bone located and how is i t

impor tant to communicat ion?

Hyoid bone acts as a movable base for tongue. Itis an attachment point for neck muscles that raiseand lower larynx during swallowing and speech

The remaining parts of the hyoid bone develop inthe third pharyngeal arch and the fourthpharyngeal arch and contributes to developmentof laryngeal cartilages.


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View of a 24-week foetus illustrating the adult derivatives of the

arch cartilages


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Sketch of the head and neck regions of a 20-week foetus, dissected toshow the muscles derived from the pharyngeal arches


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A human embryo has five pairs of pharyngealpouches although only 4 develop.

Composed of Endoderm

Only the first pharyngeal pouch develops intostructures that will have an impact oncommunication.

The first pharyngeal pouch develops into atubotympanic recess that subsequently developsinto the Middle ear cavity, the Tympanicmembraneand the Eustachian tube


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Pharyngeal

Pouch

Derivative

1 Epithelium of tympanic cavity

and auditory tube

2 Epithelium of tonsil

3 Inferior parathyroid glands

and epithelium of thymus

4 Superior parathyroid glands.


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Schematic sagittal section of a head, neck and upper thoracic regions of a 20-weekfetus, showing the adult derivatives of the pharyngeal pouches


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located between arches

these are spaces, thus contain no germ layercomponents

initially 4 clefts of which only one developsas 2nd arch grows over all other clefts fillingthem in

1stcleft forms the External Auditory Meatus


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sites on bottom of arches

where ectoderm is joined to endoderm

4 membranes initially

as most clefts are filled in, only first membrane

develops.

this lies close to external auditory meatus anddevelops into the Tympanic membrane


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Differential growth among various rudiments determinesdevelopment of a normal or abnormal face.

2 factors determine morphogenesis:

- Genetic

- Environment

Differential growth controlled by differences in cell behaviour:

- cell death

- cell migration

- change in shape

- change in size etc If any of these are perturbed, likely outcome is some type of birth

defect

Of the approximately 5,000 known human inherited conditions,

over 700 are craniofacial abnormalities


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Regulation of cell behaviour conducted by secretedchemical factors (growth factors etc)

These molecules are diffusable within the embryo

They bind to receptors on cells and initiate signallingpathways within cells

This results in regulation of gene expression

(stimulate or suppress transcription factors)

One class of transcription factors involved inpatterning of craniofacial region:

- Homeot icor Hoxgenes


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A series of homeotic genes known as Hox

genes are known to play an important role incraniofacial development.

These sequences of genes act as regulatorsof other genes; when they are affected, thefunction of other genes are affected.

These genes expressed in hindbrain region. Neural crest cells migrate out and carry

pattern with them to Pharyngeal Arches

Hox genes encode transcription factors,

including Msx-1, Msx-2, Dlx1-6, and Barx-1. Thus a defect in Hox genes results in a

defect in neural crest cells and this affectsthe craniofacial region.


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Proper craniofacial development requires the orchestrated

integration of multiple specialized tissue interactions.

Recent analyses suggest that craniofacial development is not

dependent upon neural crest pre-programming as previously

thought but is regulated by a more complex integration of cell and

tissue interactions.

In the absence of neural crest cells it is still possible to obtain

normal arch patterning indicating that neural crest is not

responsible for patterning all of arch development.

The mesoderm, endoderm and surface ectoderm tissues play a

role in the patterning of the branchial arches, and there is now

strong evidence that Hoxa2 acts as a selector gene for the

pathways that govern second arch structures


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Relatively uncommon

First Arch Syndrome: malformation of theeyes, ears due to an insufficient migrationof neural crest cells into 1stpharyngealarch

Treacher Collins Syndrome: defects oflower eyelids, deformed external ears


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Cleft Lip/Palate The lip usually closes by 5 to 6 weeks after

conception, and the palate by 10 weeks.

The lip or the lip and palate together fail toclose in approximately 1 in every 1,000babies born.

Cleft lip/palate occurs more often amongAsians (about 1.7 per 1,000 births) and

among certain groups of American Indians(more than 3.6 per 1,000 births). Males are affected more frequently than

females


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Thank you..

Documents

Introduction to Craniofacial Growth and Development