originates from neural crest cells during the fourth week after conception.
Meckel's cartilage, the initial nonossifying template for early mandibular growth, forms. A single ossification center for each half of the mandible forms lateral to Meckel's cartilage at the bifurcation of the inferior alveolar nerve and artery into its mental and incisive branches.
From this center, ossification proceeds ventrally to the body and dorsally to contribute to the mandibular ramus. Furthermore, bone deposition begins to proceed superiorly around the neurovascular bundles to provide a bony framework for the developing teeth.
primitive temporomandibular joint begins to organize with condensation of a presumptive condyle and articular discs.
Between the tenth and fourteenth weeks of development, secondary cartilages form that will eventually give rise to the coronoid process, mental protuberance, and condylar head
Secondary cartilages of the mental protuberance form ossicles in the fibrous tissue of the symphysis that will later aid in the conversion of its syndesmosis to a synostosis through endochondral ossification during the first year of life
The secondary cartilages of the coronoid, condyle, mental ossicles, and angular cartilage will all contribute to musculoskeletal structures later in development.
Embryology of the TMJ
Blastemic stage (week 7-8)
Mesenchymal condensation of a presumptive condyle and articular discs.
In humans, buccal movements begin during weeks 7 and 8 of development (lateral pterygoid)
following the initiation of muscle movements by the masticatory apparatus, cavitation of the inferior joint occurs at week 9
inferior joint cavity begins with the appearance of small spaces or clefts between the articular disc and condyle.
start of condylar chondrogenesis
organization of the superior joint cavity develops week 11
movement results in the formation of a recognizable joint capsule.
immobilization has been shown to produce an absence of joint cavities and skeletal anomalies
Maturation (>week 12)
secondary cartilages form that will eventually give rise to the coronoid process, mental protuberance, and condylar head. The secondary cartilage of the coronoid process gives rise to additional intermembranous bone and contributes to formation of the temporalis muscle.
The condylar secondary cartilage is the primitive form of the future condyle, providing the cartilaginous material that will provide the stimulus for endochondral ossification of the condylar neck later in development.
During this time and subsequently throughout development, the condylar cartilage takes on a stratified organization with five principal layers: (1) articular cartilage, (2) chondroprogenitor cells, (3) chondroblasts, (4) nonmineralized hypertrophic chondrocytes, and (5) mineralized hypertrophic chondrocytes.
It is this particular cellular organization that allows the joint to function as both an articular surface and a site of bone deposition, with the first endochondral bone being deposited during the fourteenth week after conception.
With increasing age, the articular portion of the condylar cartilage increases in thickness, while the sizes of the chondroprogenitor cells and chondroblasts remain relatively stable.
With the occlusion of the primary dentition at age three, pressure is placed on the joint which results in the change to nonvascularity of the articular surfaces. Before age three, a fall or other trauma to the joint will often result in a hemarthrosis and possible fibrous ankylosis.
Part of the “functional matrix” of surrounding soft tissues.
While the presence of a particular mandibular subunit and its surrounding tissues is genetically determined, the development of that subunit, and its subsequent maintenance, is a function of the local mechanical strains to which it is subjected.
Remodelling – deposition and resorption
Epiphyseal proliferation is largely responsible for increases in bone length and projection, a process that is dominant during the first 18 years of life.
The epiphysis adapts the intercondylar distance to the widening cartilaginous synchondrosis of the cranial base
From birth to roughly 3 years of age, the postnatal mandible begins to undergo the depository and resorptive changes that make room for the developing dentition and provide the structure of its dental arch.
Downward and forward growth - deposition of bone at the posterior margin of the ramus and corresponding resorption at the anterior margin, a process that provides length to accommodate the deciduous dentition .
mandibular contour and width occur as a function of buccal bone deposition and concomitant lingual resorption
At the third year of life, mandibular growth begins along the vectors that will predominate during much of subsequent development. While it has traditionally been thought that major centers of growth reside within the mandible's lingual tuberosity and condyles, nearly all of its surfaces undergo some form of bone remodeling.
Bone deposition on the medial portion of the developing ramus, combined with posterior growth via the lingual tuberosity, serves to further increase the size of the dental arches. Growth of the condyles in the superior and posterior directions results in an increased vertical length of the ramus.
Black arrows indicate areas of bone resorption. White arrows indicate bone deposition. Note the medial displacement of the coronoid process and the superior-posterior growth of the condyles
At 5 or 6 years of age, the two principal parts of the mandible (the ramus and the body) are distinct anatomical entities whose growth occurs in a largely independent fashion in parallel to changes in the midface.
Increase in anteroposterior length and anterior projection parallels the growth of the middle cranial fossa and pharynx.
The vertical growth of the ramus mirrors the growth of the maxilla and eruption of the maxillary dentition.