بسم الله الرحمن الرحيم
((ولقد
خلقنا الانسان في احسن تقويم))
Prenatal growth of mandible
Introduction :
Growth can be defined an
increase in size Development is increase in complexity This terms very impotant
in orthodontist May give rise to malocclusion,congenital malformation,facial deformities,etc.
Can be divided into two periods: Prenatal&postnatal .
prenatalneonatal)periods
Can be divided into three
periods :
1-period of ovum:from time of
fertilization to end of second week.
2-period of embryo:from the
second week to the 8th week.
3-peroid of fetus;from8th week
to 40week .
PRENATAL GROWTH PROPER :
The growth of the cranial,
facial and oral structures begins around the 21st day (period of embryo) after
conception. At this stage the embryo is about 3 mm in size and the head
begins to take shape.
Although there is progressive
increase in the size of the embryonic disc, yet, the head and tail ends of the
disc (x, y) remain relatively close together. This results in the disc to bulge
upwards into the amniotic cavity. As the disc enlarges further, the embryonic
disc becomes folded on itself at the head and tail ends. These are called the
head and tail folds.
After the formation of head fold, the developing
brain and the pericardium forms two prominent bulgings in the ventral aspect of
the embryo .
In between them, there is a depression called
the stomodaeum, the floor of which is formed by the buccopharyngeal membrane.
This membrane separates the stomodaeum from the foregut.
Soon the mesoderm covering the developing brain proliferates and forms a
downward projection that overlaps the upper part of the stomodaeum. This
downward projection is called the frontonasal
process.
As is evident till now, the neck is not yet
present. The neck is formed by the elongation of the region between stomodaeum
and the pericardium. This is achieved partly by a descent of the developing
heart and mainly due to the appearance of a series of mesodermal thickenings in
the wall of the cranial most part of the foregut. These are called the
pharyngeal or the branchial arch .
These are 4 in number. Although there is a 5th arch, it
disappears soon after formation . Only the first two arches are named; the
mandibular arch and the hyoid arch respectively. Each of these arches is
separated by a groove and is supplied by a nerve that innervates the striated
muscle of the arch along with the overlying ectoderm and endoderm.
At this stage, the mid-sagittal section of the
embryo looks like .
At this stage each mandibular
arch forms the lateral wall of the stomodaeum .
This arch gives off a bud
from its dorsal end and is called the maxillary process . It grows
ventromedially cranial to the main part of the arch which is now called the
mandibular process.
the facial
skeleton :
1. The upper
face (corresponding to frontonasal process.
2. The middle face (corresponding to maxillary process .
3 The lower face (corresponding to mandibular process .
the upper face--frontal bone .
the frontonasan process--nasal bone&lacrimal
bone.
maxillary processes--sphenoid bone(medial
pterygoid plates).
vomer-greater wing of sphenoid and lateral
pterygoid plate-palatine bones.
maxilla-zygomatic bones-temporal bone (squamous
portion).
the mandibular processes-- mandible-temporal
bone(tympanic ring) .
THE MAXILLA :
A primary intramembranous
ossification center appears for each maxilla in the 8th week IU at the
termination of the infraorbital nerve just above the canine tooth dental
lamina. Secondary cartilages appear at the end of the 8th week iU in the
regions of the zygomatic and alveolar processes that rapidly ossify and fuse
with the primary intramembranous center. Two further intramembranous /
premaxilary centers’ appear anteriorly on each side in the 8th week iu and
rapidly fuse with the primary maxifiary center.
Single ossification centers appear for each of the zygomatic bones and the
squamous portions of the temporal bones in the 8th week iu.
Development of the Palate :
• involves the formation of a primary palate, a
secondary palate and fusion of their processes
primary palate :
forms from an internal
swelling of the intermaxillary/premaxillary process (fusion of medial nasal
processes)
secondary palate:
forms from the two
lateral palatine shelves or processes
develop as internal
projections of the maxillary prominences .
Primary palate :
• fusion of the median nasal processes gives rise to the
median palatine process – fuses to form the primary palate
Secondary Palate :
the common oronasal cavity is
bounded anteriorly by the primary palate and occupied by the developing tongue only
after the development of the secondary palate can oral and nasal cavities by
distinguished .
nasal septum :
three outgrowth appear in the
oral cavity grows downward through the oral cavity it encounters the primary
and secondary palates.
two palatine shelves :
closure of the secondary
palate is likely to involve the hardening of the palatine shelves – mechanism
remains unknown + the withdrawl of the tongue .
Mandible :
• the cartilage of the first branchial arch associated
with the formation of the mandible = Meckel’s cartilage.
• 6 weeks: Meckel’s cartilage forms a rod surrounded by
a fibrocellular capsule .
• the two cartilages do not meet at the midline but are
separated by a thin line of cartilage = symphysis.
• at 7 weeks intramembranous ossification begins in this
mesenchyme and spreads anteriorly and posteriorly to form the bone of the
mandible.
the bone spreads anteriorly
to the midline of the developing lower jaw – the bones do not fuse at the
midline – mandibular symphysis forms (from meckel’s cartilage) which fuses
shortly after birth.
• the ramus develops from rapid ossification posteriorly
into the mesenchyme of the first arch
growth of the mandible until
birth is influences by the appearance of three secondary (growth) cartilages.
1. condylar – 12th week,
developing ramus by endochondral ossification, a thick layer persists at birth
at the condylar head.
2. coronoid – 4 months,
disappears before birth.
3. symphyseal – appears in
the connective tissue at the ends of the Meckel’s cartilage, gone after 1 year
after birth.
Development of the Tongue :
begins to develop about 4
weeks localized proliferation of the mesenchyme results in formation of several
swellings in the floor of the oral cavity .
• the oral part (anterior two-thirds) develops from the
fusion of two distal tongue buds or lateral lingual swellings and a median
tongue bud (tuberculum impar)
• the pharyngeal part or root of the tongue (posterior
one-third) develops from the copula and the hypobranchial eminence (forms from
the 2nd, 3rd and 4th branchial arches)
these parts fuse (adult =
terminal sulcus) muscles of the tongue arise from occipital somites which
migrate into the tongue area .