BDS [Orthodontics] course

1. Pre Natal Growth And Development
PROF (Dr.) SAIBEL FARISHTA

2. During this period, the height increases by almost 5000 times as
compared to only a threefold increase during the post natal period. It is
divided into 3 periods –
Period of the Ovum – This period extends for two weeks from the
time of fertilization. During this period the cleavage of the ovum and
attachment of the ovum to the intra uterine wall occurs.
Period of the Embryo – This period extends from the 14th day to the
56th day of intra uterine life. Major part of the development of the facial
and cranial region occurs during this time.
Period of the Foetus – This phase extends between the 56th day of
intrauterine life till birth. In this period, accelerated growth of the cranio-
facial structures occurs resulting in an increase in their size. There is also
a change in proportion between the various structures.

3. Pre Natal Growth Of Cranial Base
The earliest evidence of formation of the cranial base is seen in the
post/late somitic period [4th – 6th week of intra uterine life]. During this
period mesenchymal tissue derived from primitive streak, neural crest
and occipital sclerotomes condense around the developing brain and
form a capsule which is known as Ectomenix or Ectomeningeal capsule. A
portion of this capsule gives rise to the future cranial base.
Evidence of skull formation is seen late after development of other cranial
structures.
From around 40th day onwards, the Ectomeningeal capsule is slowly
converted into cartilage. This is the onset of cranial base formation. The
conversion of mesenchymal cells into cartilage or chondrification occurs
in 4 regions –
• Parachordal
• Hypophyseal
• Nasal
• Otic

4. • Parachordal – The chondrification centers which form around the
cranial end of the notochord are called Parachordal cartilages.
• Hypophyseal – Cranial to the termination of notochord, the
Hypophyseal pouch develops which gives rise to the anterior lobe of the
pituitary gland.
[i] On both sides of hypophyseal stem two post sphenoid cartilages
develop, which fuse together and form the posterior part of the body of
sphenoid.
[ii] Cranial to pituitary gland, two trabecular cartilages develop which
fuse together and form the anterior part of body of sphenoid.
[iii] Lateral to pituitary gland, chondrification centers are seen which
form the lesser wing and greater wing of sphenoid.

5. • Nasal – Initially a capsule is seen around the nasal sense organ. This
capsule chondrifies and forms the cartilages of the nostrils, which later
fuses with the cartilages of the cranial base.
• Otic – Initially a capsule is seen around the vestibulo-cochlear sense
organs which condrifies and later ossifies to give rise to the mastoid and
petrous portions of the temporal bone. The otic cartilages also fuse with
the cartilages of the cranial base.
The initially separate centres of cartilage formation in the cranial base
fuse together into a single irregular and perforated cranial base.
Numerous nerves, blood vessels result in perforations or foramina in the
developing cranial base. The ossifying chondro-cranium meets the
ossifying cranial vault to form the neurocranium.

7. Chondro – Cranial Ossification
The cranial base which is in a cartilagenous form undergoes ossification.
The bones of the cranial base undergo both endochondral and
intramembranous ossification.
• Occipital Bone – It shows both types of ossification. Seven ossification
centres are seen, 2 intra membranous and 5 endochondrial.
• Temporal Bone – It also ossifies both endochondrally and intra-
membranously from 11 centres.
• Ethmoid Bone – It shows only endochondrial ossification. It ossifies
from 3 centres.
• Sphenoid Bone – It ossifies by both types from 15 ossification centres.
The cranial base or chondrocranium is important as a junction between the
cranial vault and the facial skeleton, being shared by both. The cranial
base is relatively stable during growth as compared to the cranial vault
and the face. Therefore the cranial base can be taken as a basis against
which the cranial vault and facial skeleton can be compared.

8. Flexure Of The Cranial Base -
During the embryonic and early fetal period, the cranial base becomes
flexed in the region between the pituitary fossa and the spheno-occipital
junction. The face is tucked under the cranium. This flexure of the cranial
base is accompanied by a corresponding flexure of the developing brain
stem.
The spinal chord and foramen magnum, which during the early stages of
development were directed backwards, now become directed
downwards.

9. Uneven growth of Cranial Base
The uneven growth of the cranial base is due to the uneven nature of
growth seen in different regions of brain.
The flexure of cranial base aids in increasing the neurocranial capacity
and is the main reason for the downward displacement of face during
growth.
At 10th week of intra uterine life, the flexion of the base is about 65
degrees. This flattens out a bit at the time of birth.

10. Pre-Natal Embryology Of Maxilla
Around the 4th week of intra uterine life, a prominent bulge appears on
the ventral aspect of the embryo corresponding to the developing brain.
Below the bulge, a shallow depression which corresponds to the primitive
mouth appears called stomadeum. The floor of the stomadeum is formed
by the buccopharyngeal membrane which separates the stomadeum from
the foregut.
By around 4th week of intrauterine life, 5 brachial arches form in the
region of the future head and neck. Each of these arches gives rise to
muscles, connective tissue, vasculature, skeletal components and neural
components of the future face. The 1st brachial arch is the mandibular
arch and plays an important role in the development of the naso-
maxillary region.
The mesoderm covering the developing forebrain proliferates and forms a
downward projection that overlaps the upper part of the stomadeum.
This downward projection is called Fronto-nasal process.

11. The stomadeum is thus overlapped superiorly by the frontonasal process,
while the mandibular arches of both the sides form the lateral walls of
the stomadeum. The mandibular arch gives a bud from it’s dorsal end
called the Maxillary Process. The maxillary process grows ventro-
medio-cranial to the main part of the mandibular arch, which is now
called Mandibular Process. Thus, at this stage the primitive mouth or
stomadeum is overlapped from above by the frontal process, below by
the mandibular process and on either sides by the maxillary processes.
The ectoderm overlying the fronto-nasal process shows bilateral localized
thickening above the stomadeum. These are called as the Nasal
Placodes. These placodes soon sink and form the nasal pits.
The formation of these nasal pits divides the fronto nasal process into 2
parts – medial and lateral nasal processes.
The two mandibular processes grow medially and fuse to form the lower
lip and lower jaw. As the maxillary process undergoes growth, the fronto
nasal process becomes narrow so that the two nasal pits come closer.

13. Development Of Palate
The palate is formed by contributions of the maxillary process, palatal
shelves given off by maxillary process and Fronto – nasal process.
The fronto-nasal process gives rise to the premaxillary region while the
palatal shelves form the rest of the palate. As the palatal shelves grow
medially, their union is prevented by the presence of the tongue. Thus,
initially the developing palatal shelves grow vertically downwards towards
the floor of the mouth.

14. Around 7th week of intra-uterine life, a quick transformation in the
position of the palatal shelves occurs and they change from a vertical to
a horizontal position.
By 8.5 weeks of intra-uterine life, the two palatal shelves are in close
approximation with each other. Initially they are covered by an epithelial
lining but as they join the epithelial cells degenerate. The connective
tissue from the palatal shelves intermingle with each other and fuse.
The entire palate doesn’t contact and fuse at the same time. Initially
contact occurs in the central region of the secondary palate , then
closure occurs anteriorly and posteriorly.
The mesial edges of the palatal processes fuse with the free lower end of
nasal septum and thus separates the two nasal cavities from each other
and the oral cavity.

15. Ossification of Palate – Occurs from 8th week of intra-uterine life. Its is
intra-membranous type of ossification. The palate ossifies from a single
center derived from the maxilla. The most posterior part of the palate
does not ossify and forms the soft palate. The mid-palatal suture ossifies
by 12-14 years.
Development of Maxillary Sinus – It forms around 3rd month of intra-
uterine life. It develops by expansion of the nasal mucous membrane into
the maxillary bone. Later the sinus enlarges by resorption of the internal
wall of maxilla.

16. Pre-Natal Embryology Of Mandible
Around 4th week of intra-uterine life, the developing brain and
pericardium form two prominent bulges on the ventral aspect of the
embryo. These bulges are separated by the primitive oral cavity or
stomadeum. The floor of the stomadeum is by bucco-pharyngeal
membrane which separates it from the foregut.
The pharyngeal arches are laid down on the lateral and ventral aspects of
the cranialmost part of the foregut which lies in close approximation with
the stomadeum. Initially there are six pharyngeal arches, but the 5th one
disappears as soon as it is formed leaving only five. They are separated
by four branchial grooves. The first arch is called as the mandibular arch
and the second arch, hyoid arch. The other arches do not have any
specific names.
Each of these five arches contain a central cartilage rod that forms the
skeleton of the arch, a muscular component, a vascular component and a
neural component.

17. The mandibular arch forms the lateral wall of the stomadeum. It gives a
bud at its dorsal end which is called as maxillary process. It grows
ventro-medially, cranial to the main part of the arch, which is now called
as the mandibular process. The mandibular processes grow towards each
other and fuse in midline. They now form the lower border of the
stomadeum i.e. the lower lip and the lower jaw.
Meckel’s Cartilage
It is derived from the 1st branchial arch around the 41st – 45th day of
intra-uterine life. It extends from the cartilagenous otic capsule to the
midline or symphysis and provides a template for guiding the growth of
the mandible. A major part of Meckel’s cartilage disappears during
growth and the remaining part develops into – mental ossicles, incus and
malleus, spine of sphenoid bone, anterior ligament of malleus and
spheno-mandibular ligament.
The first structure to develop in the primordium of the lower jaw is the
mandibular division of the trigeminal nerve. This is followed by
mesenchymal condensation forming the first branchial arch. Neurotrophic
factors produced by the nerve induce osteogenesis in the ossification
centers.

18. A single ossification center for each half of mandible arises in 6th week of
intra-uterine life in the region of the bifurcation of the inferior alveolar
nerve into mental and incisive branches.
The ossifying membrane is located lateral to the Meckel’s cartilage. From
this primary center, ossification spreads below and around the inferior
alveolar nerve and its incisive and branch and upwards to form a trough
for accommodating the developing tooth buds. Spread of the
intramembranous ossification dorsally and ventrally forms the body and
ramus of the mandible.
As ossification continues, the Meckel’s cartilage becomes surrounded and
invaded by bone. Ossification stops at the site that later becomes
mandibular lingula from where the Meckel;s cartilage continues into the
middle ear and develops into the auditory ossicles [malleus and incus].

20. Endochondral Bone Formation
It is seen only in 3 areas of mandible – condylar process, coronoid
process and mental region.
• Condylar Process – At about 5th week of intra-uterine life, an area of
mesenchymal condensation can be seen above the ventral part of the
developing mandible. This develops into a cone shaped cartilage by
about 10th week and starts ossification by 14th week. It then migrates
inferiorly and fuses with the mandibular ramus by about 4 months. Much
of the cone shaped cartilage is replaced by bone by the middlw of fetal
life, but its upper end persists into adulthood acting both as a growth
cartilage and an articular cartilage.
• Coronoid Process – Secondary accessory cartilages appear in the
region of the coronoid process by about the 10-14th week of intra-uterine
life. This cartilage is believed to grow as a response to the developing
temporalis muscle. The coronoid accessory cartilage becomes
incorporated into the expanding intra-membranous bone of the ramus
and disappears before birth.

21. • Mental Region – In the mental region, on either side of the symphysis,
one or two small cartilages appear and ossify in the 7th month of intra-
uterine life to form variable numbers of mental ossicles in the fibrous
tissues of the symphysis. These ossicles become incorporated into the
intra-membranous bone when the symphysis ossifies completely during
the first year of post-natal life.

22. THANK YOU