Prenatal and Postnatal Growth and Development of the Mandible
Growth is defined as “The self multiplication of living substance” (J.S. Huxley), “An increase in size” (Todd), or the “Quantitative aspect of biological development per unit of time” (Moyers). Development is defined as “progress towards maturity” (Todd).
Prenatal Embryology and the Mandibular Arch
About the 4th week of IUL, the developing brain and the pericardium form 2 prominent bulges on the ventral aspect of embryo. These bulges are separated by the primitive oral cavity or the stomaodeum. The floor of the stomatodeum is formed by the buccopharyngeal membrane, which separates it from the foregut. The pharyngeal arches are laid down on the lateral and ventral aspect of the cranial most part of the foregut that lies in close approximation with the stomadeum.
Initially there are 6 pharangeal arches, but the 5th one usually disappears as soon as it is formed leaving only 5. They are separated by 4 branchial grooves. The 5th arch is called the mandibular arch and the 2nd arch, hyoid arch. Each of these 5 arches contain a central cartilage rod that forms the skeleton of the arch, muscular components termed as branchiomere, a vascular component, and a neural element.
The mandibular arch form the lateral wall of the stomodeum. It gives off a bud from its dorsal end, bud is called maxillary process. The mandibular process of both sides grow towards each other and fuse in the midline. They now form the lower border of the stomodeum i.e. the lower lip and the lower jaw.
The Role of Meckel’s Cartilage
Meckel's cartilage is derived from the 1st branchial arch around the 41st-45th day of IUL. It provides a template for guiding the growth of the mandible. A major portion of the meckel’s cartilage disappears during growth, but the remaining part develops into several specific structures.
| Structure Category | Derivatives of Meckel’s Cartilage |
|---|---|
| Ossicles | The mental ossicles, Incus and malleus |
| Ligaments | Anterior ligament of the malleus, Spheno-mandibular ligament |
| Bone Features | Spine of the spenoid bone |
Types of Ossification
Ossification begins in the mandible through intramembranous and endochondral bone formation. The ossifying membrane is located lateral to the meckel’s cartilage and its accompanying neurovascular bundle. From this primary centre, ossification spread below and around the inferior alveolar nerve and its incisive brand and upward to form a trough for a accommodating the developing tooth bud. Spread of the intramembraneous ossification dorsally and ventrally forms the body and ramus of the mandible.
Secondary Accessory Cartilages
- Condylar Process: About the 5th week of IUL, an area of the mesenchymal condensation can be seen above the ventral of developing mandible. This develops into a cone-shaped cartilage by about 10th week and starts ossification by 14th week.
- Coronoid Process: Secondary accessory cartilages appear in the region of the coronoid process by about 10-14th week of IUL.
- Mental Region: In this region, on the either side of the symphysis, one or two small cartilages appear and ossify in the 7th month of IUL to form a variable numbers of mental ossicles in the fibrous tissue of the symphysis.
Skeletal Sub-units and Functional Components
While the mandible appears in the adult as single bone, it is developmentally and functionally divisible into a several skeletal sub-unit. Basal bone forms one unit, to which various processes are attached:
- Alveolar process
- Coronoid process
- Condylar process
- Angular process
- The ramus
- The lingual tuberosity
- The chin
Post-natal Growth of Mandible
After birth, various regions such as the ramus, body, angle, and condyle continue growing through bone deposition and resorption to accommodate the erupting teeth and enlarging muscles. The ramus moves progressively posterior by a combination of these processes. Growth generally ceases around age 20.