Embryology of the Face 2

7/30/2019 Embryology of the Face 2

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EMBRYOLOGY OF THE FACE

INTRODUCTION

Human being comes into existence

as single fertilized ovum cell.

Fertilization involves the fusion of

the male and female gem cells (i.e.Spermatozoa and Ova) to form a

Zygote, which commences the

formation of a new individual. Pre-

natal development occupies 10-lunar

months. Pre-natal development isdivided into 3 successive phases.

The 1st 2 phases can be combined to

constitute the Embryonic stage of

development.

The 3rd phase is called the fetal

stage. The forming individual can be

described as an Embryo or Fetus

depending on its developmental

stage.

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1ST phase begins at fertilization and

spans the first 4 weeks of the

development. This phase involveslargely cellular proliferation and

migration with some differentiation

of cell populations.

Few congenital defects can resultfrom this period of development

because, if the disturbance is severe,

the embryo is lost but if it is less

severe the proliferative response can

compensate by a process of cellrenewal.

The second phase spans the next 4

weeks of development and it is

largely characterized by the

differentiation of all major externaland internal structures

(morphogenesis).

This is a particularly vulnerable

period of development for the

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embryo because it involves many

intricate embryologic processes and

it is during this period that manyrecognized congenital defects

develop.

From the end of the second phase to

third, further development is largelya matter of growth and maturation

and the embryo is now called fetus.

INDUCTION, COMPETENCE

AND DIFFERENTIATIONThese terms, induction, competence

and differentiation, describe

important concepts in Embryology.

All the cells of an individual stem

from the Zygote. These cells clearlydifferentiate into populations that

assume particular functions, shapes

and rates of turn over. These

populations of cells are said to be

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compartmentalized such that

successive generation of cells in a

given compartment may eitherremain constant or differentiate i.

e. change their characteristics and

establish a new population of cells.

The process that initiatesdifferentiation is termed induction.

An inducer is the agent that

persuades cells to be induced.

Furthermore, each compartment of

cells must be competent to respondto the induction process.

There is evidence that over time,populations of embryonic cells vary

their competence from no response

to maximum response and then back

to no response. In other word, there

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is a window of competence of

varying duration for different

population of cells.FORMATION OF THE 3-

LAYERED EMBRYO (i.e. the

differentiation of the Primary Germ

Layers)After fertilization, the mammalian

development involves a phase of

rapid proliferation and migration of

cells with little or no differentiation.

The proliferative phase lasts until 3germ layers are formed.

1st Phase of Development

After fertilization, the fertilized

ovum travels through the oviduct

and reaches the uterus. During thispassage, the cells undergo rapid

mitosis so that it forms a solid mass

of 16 cells, which at this stage is

called Morula.

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With the ovum in the uterine cavity,

fluid enters the morula and

accumulates in an eccentric location.The fluid-containing cavity is now

called Blastocoele and the embryo at

this stage is called Blastocyst.

Within the blastocyst, 2-cellpopulation can now be

distinguished. (1) Those lining the

cavity (Primary Yolk-sac) are called

Trophoblast cells and a small cluster

of cells within the cavity called innercell mass or Embryo blast.

The embryo blast cells form the

embryo proper whereas the

trophoblast cells are associated with

the implantation of the embryo andthe formation of the placenta.

As soon as the Blastocyst is

implanted into the Uterine

Endometrium, the embryo blast

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FORMATION OF THE NEURAL

CREST AND THE FATE OF

THE GERM LAYERS

It has been established that the

formation of the 3-layered embryo

occurs during the 3-4 weeks of

development. These initial events

involve cell proliferation andmigration.

In the next 3-4 weeks of

development, major tissues and

organs differentiate from the 3-

layered (Triploblastic) embryo. Thehead, face and tissues contributing to

the development of the teeth also

differentiate during this period.

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The major key events that occur

during this period involve: (a) the

differentiation of the nervous systemand neural crest tissue from the

Ectoderm, (b) the differentiation of

mesoderm and (c) the folding of the

embryo in 2-planes along the cauda-cephalic (tail-head) axis and lateral

axis.

The nervous system develops as a

thickening within the ectodermal

layer at the head - end of theembryo. This thickening constitutes

the neural plate, which rapidly form

raised margins, i.e. the neural folds.

These folds in turn encompass and

delineate an increased midlinedepression i.e. the neural groove.

The folds eventually fuse so that a

neural tube now separates from the

ectoderm forming the floor of the

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amniotic cavity with mesoderm

intervening. From the neural tube,

the brain and the spinal corddevelop. In the mammalian embryo,

a group of cells separates from the

lateral aspect of the neural plate and

not from the crest but thename/term, neural crest cells, has

been entrenched and retained for

these groups of cells.

These groups of cells (neural crest

cells) have the capacity todifferentiate extensively within the

developing embryo giving rise to a

number of structure, e.g. the sensory

ganglia, sympathetic neurons,

Schwann cells, pigment cells,meninges and the cartilage of the

branchial arches. They also form

most of the embryonic connective

tissue in the facial region. The

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contribution of the neural crest cells

is so extensive and their contribution

so significant that they have beenconsidered as a fourth (4th) germ

layer.

In a dental context, the proper

migration of neural crest cells isessential for the development of the

face and the teeth. In Treacher

Collins Syndrome (mandibulofacial

dysostosis) full facial development

is prevented by interference in themigration of neural crest cells to the

facial region. All the tissues of the

teeth, except enamel, and its

supporting apparatus are derived

directly from neural crest; theirdepletion will prevent proper dental

development.

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Derivatives from the Mesoderm

Paraxial Mesoderm: This forms

segmental blocks somites and eachsomite contributes to 2 adjacent

vertebrae and their disks, a

segmental mass of muscles and the

dermis of the associated skin.Intermediate Mesoderm: It gives rise

to the urogenital system.

Lateral Plate Mesoderm: forms the

connective tissue of the muscles

and viscera, the serous membraneof the pleura, pericardium and

peritoneum, blood and lymph

cells; and the cardiovascular and

lymphatic system as well as

spleen and adrenal cortex.As a result of lateral folding, the

ectoderm of the floor of the amniotic

cavity now encapsulates the embryo

and forms the surface epithelium.

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The paraxial mesoderm remains

adjacent to the neural tube and

notochord.The lateral plate mesoderm cavitates

to form a space (coelom) and the

mesoderm bounding this cavity now

lines the body wall and gut. Theintermediate mesoderm becomes

relocated to a position on the dorsal

wall of the coelom. The Endoderm

forms the gut.

One of the derivatives of mesodermis mesenchyme i.e. the embryonic

connective tissue.

The neural crest cells also give rise

to mesenchyme, which is termed

ectomesenchyme to distinguish itfrom other mesodermal

mesenchyme.

In summary, early stages of

embryonic development involve

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rapid cell proliferation and some

histodifferentiation leading quickly

to the formation of a trilaminar diskconsisting of 3 layers which are the

ectoderm, mesoderm and endoderm

with a head to tail axis fold

established. The nervous systemdevelops from the ectoderm of this

disk through the development of

folds. These folds also spin off

neural crest cells that contribute

extensively to many tissues of theembryo including most of the dental

tissues. All the tissues of the embryo

arise from these 4-cell populations.

The shape of the embryo is

determined by the folding of the diskin 2 planes along the caudacephalic

and lateral axes with the head fold

being crucial to the establishment of

the primitive mouth.

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Environmental factors have

relatively little impact on the

production of congenital defectsduring the early stages of

development because their effects

are masked. If damage is slight,

proliferative activity is so great thatthe damage can easily be

compensated for by the replacement

of new cells. If the damage is severe,

the embryo dies. Environmental

effects are however significant in thelater stages of embryonic

development.

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Embryology of the Face 2

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