Growth of Skull & Jaws

Dr.Noor AlTamimi

• Endochondral and Intramembranous ossification• Types of cartilaginous growth• Synchondroses • Growth of condylar cartilage

Contents

Skull Growth• There are several interrelated factors determine

the growth of skull & jaws.

• General Effects: which influence the overall shape & growth rate of bones include:

Hormonal influencesNutritional influencesGenetic influencesSocio-economic influences

• Local effects: The growth pattern followed by a particular part. The effect of enclosed tissues (Capsular Matrices) on the adjacent

bones. Muscle attachments, joints, adjacent soft tissues influence the

detailed architecture of bones (Periosteal Matrices)

Bone Formation

• There are two types of bone formation and both start with coalescence of mesodermal or ectomesenchymal cells into dense groups in a process known as Condensation.

• Endochondral ossification (ECO):

When the condensed mesenchymal cells differentiate into chondroblasts which form a cartilaginous template of the bone and the subsequent replacement of cartilage tissue by bone.

• Cartilage replacing bones those that develop by ECO include post cranial skeleton - except the clavicles – plus the cranial base.

• Intramembranous ossification (IMO):

The direct conversion of mesenchymal tissue into bone with no catrilaginous precursors.

• Dermal bones which mineralize by IMO inlcudes the cranial vault, lower jaw, facial skeleton & the clavicles

Cranial vault are formed by IMO and follow Neural growth pattern.

Facial skeleton undergo IMO and follow Somatic growth curve.

Cranial base develop by ECO and conform Neural growth then adjust to somatic growth pattern.

Cartilaginous Growth

Appositional growth results in the increase of the diameter or

thickness of the cartilage. The new cells derive from the perichondrium and occur on the surface of the cartilage model

Interstitial growth

results in an increase of cartilage mass and occurs from within. Chondrocytes undergo mitosis within their lacuna, but remain imprisoned in the matrix, which results in clusters of cells called "isogenous groups."

Synchondroses

• are the cartilaginous joints of the skull which form when neighboring centers of ossification within a continuous mass of hyaline cartilage enlarge and encroach upon each other.

• Cranial base synchondroses are important growth centers of the craniofacial skeleton and the last sites in the cranium to terminate growth.

HISTOLOGICAL STRUCTURE

Progenitor zoneProliferative zonesHypertrophic zone

• Spheno-ethmoidal synhondrosis:

An active growth site prenatally, but the cartilage is replaced by fibrous tissue shortly after birth where growth continues after its conversion to a suture.

Ossification at this synchondrosis contributes to the enlargement of the cranial base anterior to the pituitary fossa.

• Spheno-occipital synchondrosis

Principal site of elongation of the basicranial axis & growth continues until early teens.

The joint is grown over, starting on the intracranial surface & is completely obliterated at maturity, no joint is visible between the sphenoid & occipital bones after this age.

Growth of the Mandible

• By 6 weeks of development, Meckel’s cartilage forms a rod of cartilage stretching from the otic capsule to the midline in each first pharyngeal arch. The anterior ends of the two cartialges are united by a bridge of ectomesnchyme.

The Mandible is a Dermal bone which develop in close association with, but not from Meckel’s cartilage.

• By 7 weeks, ossification center appear in the V of the mental & incisive nerves of each half of the mandible, then it spreads anteriorly & posteriorlly on the lateral aspect of the nerves & the mandible. Then it spreads beneath these structures & spreads upward to form a U shape enclosing the teeth & nerves.

• The U is later roofed front to back to form the Mandibular canal.

• The Body of the mandible is produced by single center of ossification & runs from the anterior midline symphysis to the area which will become the mandibular foramen.

As the body of the mandible grows it will contact Meckel’s cartilage & causes its resorption.

• The ramus is formed by 10th week by rapid backward spread of ossification, rudementary backward pointing coronoid & condyalr processes are visible.The part of Meckel’s cartilage lying medial to the ramus is resorbed, but its perichondrium remains as the Sphenomandibular ligament.

• Secondary cartilages are cartilages that appear in the mandible - which shouldn’t happen since it’s a dermal bone –

• are derived from cell lines which have already differentiated to produce dermal bone.

• Growth sites producing bone by ECO

3 Secondary Cartilages

2 Cartilages in the Ramus-Condylar Cartilage-Coronoid Cartilage Symphyseal cartilages

Condylar Cartilage The largest & most important one, appears at 12

weeks on the superior & lateral aspects of the condylar process.

Active throughout the growth period until maturity.

Undergoes enlargement & forms a conical mass in the condyalr process, as far as the mandibular foramen. As the cartilage grows it’s replaced inferiorly by bone.

By 5 months, much of the original cartilage has ossified, leaving a narrow zone of growth cartilage persisting beneath the articular surface of the condylar head of the mandible.

Coronoid CartilageAppear at 12 weeks as a cartilaginous strip on

the anterior bordeer of the summit of the coronoid process, but ceases activity & disappears within weeks

Symphyseal cartilagesSmall nodules of secondary cartilage which

develop in the midline symohysis .

The mandible at birth is still separated into right & left halves by the mental symphysis which fuses at abut the end of the 1st post natal year as the cartilages ossify.