Growth and Development of the Mandible: Prenatal and Postnatal Stages
The growth and development of the mandible is a complex process involving prenatal and postnatal stages, mechanisms of bone growth, and the role of various embryological features. It highlights key concepts such as cell division, types of ossification, and the importance of Meckel's cartilage in mandible development. The complexity of these processes spans from embryonic stages to adulthood, where growth generally ceases around age 20.
Fundamental Concepts and Terminology
To understand mandibular development, it is essential to define several biological terms:
- Embryology: It is the study of formation and development of embryo or fetus from the moment of its inception up to the time when it is born as an infant.
- Growth: Defined as "The self multiplication of living substance" (J.S. Huxley) or simply "An increase in size" (Todd).
- Development: "Development is progress towards maturity" (Todd).
- Cell Division: The process by which a parent cell divides into two or more daughter cells, essential for growth, repair, and maintenance.
The role of stem cells is also crucial. Multipotent stem cells, such as Mesenchymal stem cells (MSCs), are capable of differentiating into a range of cell types, including osteoblasts and chondrocytes, which are vital for bone and cartilage formation.
Prenatal Embryology of the Mandible
About the 4th week of IUL (Intrauterine Life), the developing brain and the pericardium form two prominent bulges on the ventral aspect of the embryo, separated by the primitive oral cavity or the stomaodeum. The pharyngeal arches are laid down on the lateral and ventral aspect of the cranial most part of the foregut. The first arch, known as the mandibular arch, forms the lateral wall of the stomodeum.
The mandibular process of both sides grows ventro-medially and fuses in the midline. They now form the lower border of the stomodeum, which includes the lower lip and the lower jaw. This process is derived from the 1st branchial arch around the 41st-45th day of IUL.
The Role of Meckel’s Cartilage
Meckel's cartilage provides a template for guiding the growth of the mandible. While a major portion of the Meckel’s cartilage disappears during growth, the remaining part develops into several specific anatomical structures.
The following table summarizes the derivatives of Meckel's cartilage based on the provided material:
| Structure Category | Anatomical Derivative |
|---|---|
| Ossicles | Mental ossicles, Incus, and Malleus |
| Ligaments | Anterior ligament of the malleus, Spheno-mandibular ligament |
| Bone Features | Spine of the sphenoid bone |
Mechanisms of Bone Growth and 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 spreads below and around the inferior alveolar nerve. Spread of the intramembraneous ossification dorsally and ventrally forms the body and ramus of the mandible.
Secondary Accessory Cartilages
Several skeletal sub-units develop via secondary cartilages during the prenatal period:
- Condylar Process: This develops into a cone-shaped cartilage by about the 10th week and starts ossification by the 14th week. It then migrates inferiorly and fuses with the mandibular ramus by about the 4th month.
- Coronoid Process: Secondary accessory cartilages appear in the region of the coronoid process by about 10-14th week of IUL.
- Mental Region: One or two small cartilages appear and ossify in the 7th month of IUL to form a variable number of mental ossicles in the fibrous tissue of the symphysis.
Postnatal Growth of the Mandible
While the mandible appears in the adult as a single bone, it is developmentally and functionally divisible into several skeletal sub-units. Basal bone forms one unit, to which is attached the alveolar process, coronoid process, condylar process, angular process, the ramus, the lingual tuberosity, and the chin.
After birth, various regions continue growing through bone deposition and resorption to accommodate the erupting teeth and enlarging muscles. For example, the ramus moves progressively posterior by a combination of deposition and resorption. Resorption occurs on the anterior part of the ramus while bone deposition occurs on the posterior region.