PRENATAL DEVELOPMENT
OF THE JAWS
Many of the problems that result in craniofacial
anomalies arise in the early stage of the
embryonic development.
• in order to recognize the developmental
abnormality, as soon as possible and
• to assess the aetiology of malocclusions
the authorities are more considered with
developmental processes of the jaws and teeth.
DEVELOPMENT OF THE FACE
Development of the face
starts from ventral part of
the head process of an
embryo and the first
branchial arch.
The face develops mainly
between the 4th and 8th
weeks.
By the end of the embryonic
period ( eight weeks ) the
face has an unquestionably
human appearance.
DEVELOPMENT OF THE FACE
The first branchial arch develops two elevations called the
“ maxillary prominence” and “mandibular
prominence”.
In the 4th week the five facial primordia are around the
stomodeum ( primitive mouth ):
• The large “frontonasal prominence” constitutes the
cranial boundary of the stomodeum.
• The paired “maxillary prominences” of the firsth
branchial arch form the lateral boundaries and
• the paired “mandibular prominences” of the same
arch constitute the caudal boundary of the stomodeum.
Early formation of the face about 24 days after conception
Scanning electron micrographs of an embryo
Bilateral oval-shaped thickenings of the surface ectoderm, called
“nasal placodes”, develop on each side of the caudal part of
frontonasal elevation
Horseshoe-shaped “medial and lateral prominences” develop
at the margins of the nasal placodes. As a result, the nasal
placodes lie in depressions called “nasal pits”.
The maxillary prominences grow rapidly and soon approach
each other and the medial nasal prominences. During the 6th
and 7th weeks the medial nasal prominences merge with each
other and the maxillary prominences.
Diagrammatic representation of the structures at
the beginning of the 5th week when fusion is just
beginning.
Relationship at the beginning of the 6th week,
when the fusion is well – advanced.
Schematic representation of the contribution of the
embryonic facial processes to the structures of the
adult face
DEVELOPMENT OF THE FACE
As the medial nasal prominences
merge with each other, they form
an “intermaxillary segment” of
the maxilla. This segment give rise
to:
• the middle portion of the upper lip
called the philtrum
• the premaxillary part of the maxilla
and its associated gingiva
• the primary palate
The lateral parts of the upper lip, most
of the maxilla and the secondary
palate form from the maxillary
prominences.
These prominences merge laterally
with the mandibular prominences
DEVELOPMENT OF THE FACE
The mandibular prominences
merge with each other in the fourth
week and the groove between
them disappears before the end of
the fifth week.
The mandibular prominences give rise
to:
• the mandible
• lower lip and
• the inferior part of the face
The frontonasal prominence forms
the forehead and the dorsum and
apex of the nose.
The sides of the nose are derived
from the lateral nasal
prominences.
DEVELOPMENT OF THE PALATE
The palate develops from
the::
• the primary palate and
• the secondary palate
Although palatogenesis
begins toward the end
of the 5 week, fusion of
the palate´s parts is not
complete until the 12
week.
DEVELOPMENT OF THE PALATE
The primary palate – or
median palatine process,
develops at the end of
the fifth week from the
innermost part of the
intermaxillary segment
of the maxilla.
It forms a wedge-shaped
mass of mesoderm
between the maxillary
prominences of the
developing maxilla.
DEVELOPMENT OF THE PALATE
The secondary palate –
develops from two internal
projections from the
maxillary prominences,
called the lateral palatine
processes.
These shelflike structures
inicially project inferomedially
on each side of the tongue.
As the jaws develop, the
tongue moves inferiorly and
the lateral palatine processes
gradually grow toward each
other and fuse. They also
fuse with the primary palate
and nasal septum.
The fusion of the palatal processes begins anteriorly during
the 9th week and ends posteriorly in the region of the uvula
by the 12th week. The palatine raphe indicates the line of
fusion of the lateral palatine processes. The posterior portion of
the lateral palatine processes do not become ossified. They
extend beyond the nasal septum and form the soft palate and
uvula.
The failure of fusion of the
facial processes give
rise the group of
anomalies called clefts.
Bilateral cleft lip and
palate in an infant.
The separation of the
premaxilla from the
remainder of the maxilla
caused by failure of
fusion between medial
nasal prominences and
maxillary prominences.
Cleft of secondary
palate caused by
failure of fusion of
the palatal
shelves.
Unilateral cleft of lip,
the medial nasal and
maxillary
prominences fail to
fuse only on the right
side.
Table shows the
embryonic
development in
the time and the
related syndromes
that may arise in
these periods.
Fetal alcohol syndrom
Treacher Collins syndrome
(mandibulofacial dysostosis)
Hemifacial microsomia
Crouzon´s syndrome
DEVELOPMENT OF THE CRANIAL
BASE AND JAWS
The bones of the cranial
base are formed initially in
cartilage and are
transformed by
endochondral ossification
to bone.
Picture shows the
chondrocranium at 8 weeks
of intrauterine development.
A continuous plate of
cartilage extends from the
nasal capsule posteriorly all
the way to the foramen
magnum at the base of the
skull.
DEVELOPMENT OF THE CRANIAL
BASE AND JAWS
•
Cartilage is a nearly avascular tissue
whose internal cells are supplied by
diffusion through the outer layers. This
means, of course, that the cartilage
must be thin. At early stages in
development, the extremely small size
of the embryo makes a
chondroskeleton feasible, but with
further growth, it is no longer possible
without an internal blood supply.
•
During the fourth month in utero,
there is an ingrowth of blood
vascular elements into various points
of the chondrocranium (and the other
parts of the early cartilaginous
skeleton). These areas become
centers of ossification, at which
cartilage is transformed into bone, and
islands of bone appear in the sea of
surrounding cartilage
DEVELOPMENT OF THE CRANIAL
BASE AND JAWS
• The cartilage continues to grow
rapidly but is replaced by bone with
equal rapidity. Eventually, the old
chondrocranium is represented
only by small areas of cartilage
interposed between large sections
of bone.
• Not all bones of the adult skeleton
were represented in the embryonic
cartilaginous model, and it is
possible for bone to form by
secretion of bone matrix directly
within connective tissues. Bone
formation of this type is called
intramembranous bone
formation. This type of ossification
occurs in the cranial vault and
both jaws
DEVELOPMENT OF THE CRANIAL
BASE AND JAWS
The mandible of higher animals
develops in the same area as the
cartilage of the first pharyngeal
arch-Meckel's cartilage.
It would seem that the mandible
should be a bony replacement for
this cartilage. In fact, development
of the mandible begins as a
condensation of mesenchyme just
lateral to Meckel's cartilage and
proceeds entirely as an
intramembranous bone formation.
Meckel's cartilage largely
disappears as the bony mandible
develops. Rests of this cartilage
are transformed into a portion of
two of the small bones of the
middle ear. Its perichondrium
persists as the sphenomandibular
ligament
DEVELOPMENT OF THE CRANIAL
BASE AND JAWS
The condylar cartilage develops
initially as an independent
secondary cartilage, which is
separated by a considerable gap
from the body of the mandible
Early in fetal life it fuses with the
developing mandibular ramus.
A Separate areas of mesenchymal
condensation, at 8 weeks.
B Fusion of the cartilage with the
mandibular body, at 4 months
C Situation at birth.
DEVELOPMENT OF THE CRANIAL
BASE AND JAWS
The maxilla forms initially from a center of mesenchymal condensation
in the maxillary process.
This area is located on the lateral surface of the nasal capsule, the
most anterior part of the chondrocranium.
Although the growth of the chondrocranium contributes to lengthening
of the head and anterior displacement of the maxilla, it does not
contribute directly to formation of the maxillary bone.
In term of orthodontics the most important thing is
the different growth pattern of the maxilla and
mandible during the antenatal life. As a result
the antero-posterior relationship of jaws is
changing.
• The tongue and mandible of the 68 week-old embryo are posteriorly
positioned relative to the maxilla. It
is “1th embryo mandibular
retrognathism”.
• Within the next month mandible
tends to grow faster then maxilla.
This change allows depression of
the tongue from common oral and
nasal cavity to oral cavity. The
tongue is loosing the contact with
the nasal septum and the oral and
nasal cavities can be isolated by
the connecting palatal proceses.
This state is called the “embryo
mandibular prognatism”.
• During the last period of
antenatal development
the accelerated growth of
cranial cavity and the
upper facial skeleton
causes the “2th embryo
mandibular
retrognathism”.
AT BIRTH
•
At birth the mandible is in its
physiological rest position. The
relative lack of growth of the lower
jaw prenatally also makes birth
easier, since a prominent bony
chin at the time of birth would be
a considerable problem in
passage through the birth canal.
• By the age of 6 month mandible
will have reached the right
position relative to the maxilla. It
is probably due to the muscle
function during the sucking.
AT BIRTH
•
Appearance of the jaw relief
reflects their complicated
development.
• The alveolar arches, also
called the gum-pads are
horse-shoe shaped in the
maxilla and U-shaped in the
mandible.
• They are low and the vault of
the palate is very shallow.
• The alveolar part is
separated on its palatal side
from the hard palate by a
continuous horizontal groove
known as the "dental or
gingival groove".
AT BIRTH
• In the frontal region the
groove is bridged by incisal
papilla and the gum pad
along with anterior part of
palate form the "incisal
plateau".
• At birth the maxillary and
mandibular gum-pads have
20 segmented elevations
corresponding to the
unerupted deciduous teeth.
• The groove that marks the
distal margin of the canine
segment continues into the
buccal sulcus and is called
the “lateral sulcus”.
AT BIRTH
•
The gum-pads rarely come into occlusion. They are
separated verticaly when at rest. When pressed against
each other they are mostly still separated anteriorly, and
the space sometimes extends backwards as far as the
canine region. Transversally, the upper alveolar arch is
slightly wider than the lower one.
AT BIRTH
•
If the incisal plateau is horizontal, there is no overlaping of the gum pads
and the lower gum pad occlude with incisal plateau about 2-8 mm
posteriorly to its border because of mandibular rest position.
•
The other extrem is the very sheer plateau witch completely overlaps the
lower gum pad. It´s usually a predecessor of later Class II Division 2 of
Angle´s classification (deep bite and retroclination of upper incisors
because of theirs sheer position). The majority cases are betwen these two
extremes.