Tooth Development and Anatomy of the Maxilla and Mandible

Posted on Sep 8, 2024 in Biology

Tooth Development

Stages of Tooth Development

1. Bud Stage

This stage occurs at approximately six weeks of intrauterine life (IUL). There is a thickening of the lining of the jaws due to the proliferation of basal layer cells. This proliferation is induced by the mesenchyme.

2. Cap Stage

This stage begins around the 10th week of IUL. Epithelial cells proliferate, and the deep surface of the tooth buds invaginates. This invagination forms the tooth germ. The proliferating epithelial cells form a cap-like structure, hence the name of the stage. The mesoderm that is incorporated into the tooth germ is called the dental papilla, and the mesoderm surrounding the entire complex is called the dental sac.

The tooth germ can be divided into different areas:

  • Enamel Organ:
    • Outer enamel epithelium: Forms the outer layer of the enamel organ.
    • External dental epithelium: Contacts the dental sac.
    • Stellate reticulum: Central portion of the enamel organ.
    • Inner dental epithelium: Surrounds the dental papilla cells and gives rise to ameloblasts, which produce enamel.
  • Dental Papilla: Forms the dentin and pulp of the tooth.
  • Dental Sac: Forms the cementum and periodontal ligament.

3. Bell Stage

This stage begins in the 3rd month of IUL. The cap from the previous stage grows and takes on the shape of a bell. There are two periods in this stage:

  • Initial Period: Begins at week 14. The enamel organ and oral epithelium are linked by an accumulation of cells called the enamel cord. The enamel organ is composed of:
    • External dental epithelium: Provides protection.
    • Stellate reticulum: Cushions injuries and assists in nutrition.
    • Inner dental epithelium: Forms ameloblasts.
    • Stratum intermedium: Responsible for nutrition.
  • Advanced Period: Tooth germ cells organize and determine the size of the tooth crown. The four layers of the enamel organ differentiate. The external epithelium attaches to the inner lining at the neck, forming the cervical loop, from which the root derives. Cells of the inner dental epithelium closest to the stellate reticulum become preameloblasts, which then differentiate into ameloblasts and begin producing enamel. This enamel deposition starts at the cusp tips and incisal edges and progresses towards the neck. Simultaneously, cells in the dental papilla closest to the inner dental epithelium differentiate into odontoblasts, which form dentin. The pulp forms in the central part of the dental papilla. During this stage, the dental lamina disappears, and the development of the permanent tooth begins.

4. Crown Stage: Calcification

This stage occurs around the 4th month of IUL. Hard tissues of the tooth, starting with the crown and then the root, begin to form. Calcification occurs through the apposition of mineral salts, primarily hydroxyapatite crystals. As dentin thickness increases, the dental papilla narrows, leaving a small space that will become the pulp cavity. Once enamel calcification is complete, the ameloblasts atrophy, which happens before tooth eruption. At birth, most tooth crowns are already calcified. Eruption begins around six months of age, and calcification continues after eruption. Calcification of permanent teeth begins after birth.

5. Root Formation

Enamel formation is followed by root formation. Cells in the dental papilla begin depositing dentin at the neck of the tooth. As more root dentin is deposited, the pulp cavity narrows, forming the root canal, which houses the blood vessels and nerves.

Anatomy of the Maxilla and Mandible

Maxilla

The maxilla is a paired bone that forms the upper jaw. It is located between the oral cavity and the orbital cavity and constitutes a significant portion of the middle third of the face. The two maxillary bones are symmetrical and join at the midline. They articulate with the palatine bone to form the hard palate. The maxilla is light and contains a cavity called the maxillary sinus. Key structures of the maxilla include:

  • Alveolar Process: Contains the sockets for the upper teeth. The canine eminence is the most prominent.
  • Palatine Process: A horizontal projection that forms part of the floor of the nose and the hard palate. The two palatine processes join to form the anterior nasal spine.
  • Frontal Process: An ascending process that articulates with the frontal bone.
  • Zygomatic Process: Forms part of the orbital floor and a small area of the cheek.

Other bones associated with the maxilla include:

  • Palatine Bone: Located behind the maxilla, forming the posterior part of the nasal cavity floor and the hard palate roof. It has a vertical portion that contributes to the nasal cavity and a horizontal portion that contains the greater palatine foramen and lesser palatine foramina, which allow passage for nerves and blood vessels supplying the soft palate.
  • Zygomatic Bone: Forms the cheekbone and the lower outer rim of the orbit. It serves as an attachment point for muscles.
  • Nasal Bone: A paired bone located at the upper part of the nose. It is a flat, rectangular bone that articulates with the maxilla and the frontal bone.
  • Lacrimal Bone: A paired, small, rectangular bone located on the inner wall of the orbit. It contains the posterior lacrimal crest and the nasolacrimal duct opening.
  • Inferior Nasal Conchae: Paired bones located inside the nasal cavity. They increase the surface area for humidifying and warming inhaled air.
  • Vomer: A single, flat bone that forms part of the nasal septum.

Mandible

The mandible is the lower jaw bone. It is a single, U-shaped bone that houses the lower teeth. It is the only movable bone in the skull and articulates with the temporal bone at the temporomandibular joint (TMJ). The mandible has two main parts:

  • Body: The horizontal, horseshoe-shaped portion of the mandible. It has upper and lower surfaces and anterior and posterior aspects.
    • Upper Surface: Contains the alveolar sockets for the lower teeth.
    • Anterior Aspect: Features the mandibular symphysis (midline ridge), the mental protuberance (chin), and mental foramina (openings for nerves and blood vessels).
    • Posterior Aspect: Includes the genial tubercles (muscle attachment points) and the mylohyoid line, which divides the inner surface into the sublingual fossa (above) and the submandibular fossa (below). These fossae house the sublingual and submandibular salivary glands, respectively.
  • Ramus: The vertical, plate-like portion of the mandible. Each side of the mandible has a ramus that projects upwards from the posterior end of the body. The ramus has several important landmarks:
    • Coronoid Process: The anterior projection of the ramus, serving as an attachment point for muscles.
    • Condyle: The posterior projection of the ramus that articulates with the temporal bone to form the TMJ.
    • Mandibular Notch: The depression between the coronoid process and the condyle.
    • Mandibular Foramen: An opening on the inner surface of the ramus that allows passage for the inferior alveolar nerve, which supplies sensation to the lower teeth. The lingula is a bony projection located anterior to the mandibular foramen.
    • Angle of the Mandible (Gonion): The junction point between the body and the ramus. It is an important landmark in orthodontics.

Hyoid Bone

The hyoid bone is a small, U-shaped bone located in the neck, above the larynx. It does not articulate directly with any other bone but is suspended by ligaments and muscles. It serves as an attachment point for muscles involved in swallowing and speech.