PRE NATAL DEVELOPMENT PHASE of Mandible

PRE NATAL DEVELOPMENT PHASE of Mandible

About the fourth week of intrauterine life, the pharyngeal arches are laid down
The first arch is called the mandibular arch and the second arch the hyoid arch.

Component of each arch?

DEVELOPMENT OF MANDIBLEINTRAMEMBRANOUS BONE FORMATION
The first structure to develop in the primodium of the lower jaw is the mandibular division of trigeminal nerve that precedes the mesenchymal condensation forming the first [mandibular] arch.

At around 36 -38 days of intrauterine life there is ectomesenchymal condensation

Some mesenchymal cells enlarges , and form osteoblasts
These osteoblasts secrete a gelatinous matrix called osteoid and result in ossification of an osteogenic membrane.
The resulting intramembranous bone lies lateral to meckel’s cartilage of first [mandibular ] arch.
In the sixth week of the intrauterine life a single ossification centre for each half of the mandible arises in the bifurcation of inferior alveolar nerve into mental and incisive branches

Intramembranous bone formationThe Mandible

Mandible forms by intramembranous bone formation just lateral to Meckel’s cartilage





During seventh week of intrauterine life bone begin to develop lateral to meckel’s cartilage & continues until the posterior aspect is covered with bone
Between eigth & tewelth week of intrauterine life mandibular growth accelerate , as a result mandibular length increases.

Ossification stops at a piont , which later become mandibular lingula, the remaining part of meckels cartilage continues to form sphenomandibular ligament & spinous process of sphenoid.
Secondary accseeory cartilage appear between tenth & fourteenth week of intrauterine life to form head of condyle , part of coronoid process & mental protuberance

THE CONDYLAR PROCESS-

At fifth week of intruterine life , an area of mesenchymal condensation is seen above the ventral part of developing mandible.

At about tenth week it develops in cone shaped cartilage.

It migrate inferior & fuses with mandibular ramus at about 4 month.
This cone shaped cartilage is replaced by bone but its upper end persists acting as growth cartilage & articular cartilage.
ENDROCHONDRAL BONE FORMATION
• Endrocondral bone formation is seen in 3 areas of mandible
• The condylar process
• The coronoid process
• The mental process

THE CORONOID PROCESS-

Secondary accessory cartilage appear in region of coronoid process at about 10- 14 week of intrauterine life.

This cartilage become incorporated into expanding intramembranous bone of ramus & disappear before birth.
THE MENTAL REGION-
In mental region , on either side of symphysis , one or two small cartilage appear and ossify in seventh week of intrauterine life to become mental ossicles.

These ossicles become incorporated into intramembranous bone when symphysis ossify completely.

POST NATAL GROWTH PHASE

At birth the two rami of the mandible are short , condylar development is minimum and there is no articular eminence in glenoid fossa. A thin layer of fibrocartilage & connective tissue exists at the midline of symphysis to separate right & left mandibular bodies.
At fourth month of age and end of first year symphysial cartilage is replaced by bone


During first year of life appositional growth is active at alveolar border, at distal & superior surfaces of the ramus, at the condyle, along the lower border of mandible and on its lateral surface.

After first year of life these changes occurs

Mandibular growth become more selective , condyle shows considerable activities, mandible moves and grows downward & forward.
Appositional growth occurs on posterior border of the ramus and on the alveolar process.
Resorption occurs along the anterior border of ramus lenthening the alveolar border & maintaining the anterior- posterior dimension of ramus.












Gonial angle changes after little muscle activity.

Transverse dimension is mainly due to growth at posterior border in an expanding V pattern.

• Scott divides the mandible into three basic types of bone –

• Basal
• Muscular
• Alveolar
• Basal portion is tube like central foundation running from condyle to the symphysis.
• Muscular portion [gonial angle &coronoid process] is under influence of masseter, internal pterygoid & temporal muscle. They determine the ultimate form of the mandible in these areas.
• Alveolar portion exists to hold the teeth & gradually resorbed in the event of tooth loss.

Pre natal development of maxilla

Centers of ossification develop in the mesenchyme of the maxillary processes of the first branchial arch (intra membranous ossification)

Spreads posteriorly below the orbit towards the developing zygoma and anteriorly toward the future incisor region and superiorly to form the frontal process


Ossification also spreads into the palatine process to form the hard palate

At the union between the palatal process and the main body of the developing maxilla is the medial alveolar plate – together with the lateral plates – development of the maxillary teeth

A zygomatic or malar cartilage appears in the developing zygomatic processes and contributes to the development of the maxilla

DEVELOPMENT OF THE SKULL

The skull can be divided
Neurocranium: Bones which surround and encase the base of the brain & form support capsules for sense organs
Viscerocranium: Bones which surround the structures of the face and palate. Each has separate origins, as follows:
Neurocranium
1. Formed by endochondral ossification (sphenoid, occipital, ethmoid, petrous part of temporal bone).
2. Formed by intramembranous ossification. These bones form the flat bones of the vault of the skull around the brain (calveria). They include the frontal and parietal bones.

Viscerocranium (face skeleton)

bones are all formed from derivatives of pharyngeal arches I-III
1. Cartilagenous bones Malleus, incus, stapes (Arches I and II) Hyoid (Arches II and III)
2. Membranous bones: Maxilla, zygomatic, squamous, temporal (maxillary prominence of Arch I) Mandible (mandibular prominence of arch I)

DEVELOPMENT OF THE SKULL

Endochondral bone formation
Ethmoid, sphenoid, petrous part of temporal bone and basioccipital bones





Intra membranous bone formation
Cranial vault (brain case), maxilla and mandible