- ◈ EXTERNAL ANATOMIC FEATURES
- ◈ DEVELOPMENT OF PERIODONTIUM
- • Early Development of Cementum
- • Later Development of Cementum
- • Development of Junctional Epithelium
INTRODUCTION
The term periodontium arises from the Greek word peri meaning around and odont meaning tooth, thus it can be simply defined as the “tissues investing and supporting the teeth”. The periodontium is composed of the following tissues namely alveolar bone, root cementum, periodontal ligament (supporting tissues) and gingiva (investing tissue).
The various diseases of the periodontium are collectively termed as periodontal diseases. Their treatment is referred to as periodontal therapy. The clinical science that deals with the periodontium in health and disease is called periodontology. The branch of dentistry concerned with prevention and treatment of periodontal disease is termed periodontics or periodontia.
EXTERNAL ANATOMIC FEATURES
The oral mucosa consists of three zones:
- Masticatory mucosa: It includes the gingiva and the covering of the hard palate.
- Specialized mucosa: It covers the dorsum of the tongue.
- Lining mucosa: It is the oral mucous membrane that lines remainder of the oral cavity. Among all the structures of the periodontium, only the gingiva is visible clinically. The gingiva is divided anatomically into free or marginal, attached and interdental gingiva. The border or groove between marginal and attached gingiva is called as a free gingival groove, a shallow depression on the faciogingival surface that roughly corresponds to the base of the gingival sulcus. The junction between the attached gingiva and alveolar mucosa is called as mucogingival line or junction (Fig. 1.1).
The normal gingiva is pink in color (salmon coral pink) and accumulation of melanin pigmentation is normal. The surface of the gingiva exhibits an orange peel-like appearance referred to as stippling. In health, the gingiva completely fills the embrasure spaces between the teeth and is known as the interdental gingiva. In the posterior teeth, where the contact areas between the teeth are usually broad, the interdental gingiva consists of two papillae, facial and lingual which are connected by the col.
The significance of col is that, it is made up of nonkeratinized epithelium and hence represents the most frequent site for initiation of disease process.
DEVELOPMENT OF PERIODONTIUM
To understand the development of periodontal tissues one has to have a clear understanding of the root formation. Development of cementum and roots of the teeth starts once the formation of enamel is completed. The outer and inner epithelia together form the epithelial root sheath of Hertwig, which is responsible for determining the shape of the root.
Early Development of Cementum
The outer and inner epithelial layers become continuous (without stratum intermedium or stellate reticulum) in the area of the future cementoenamel junction and form a two-layered sheath, which grows into the underlying mesenchyme. The apical portion of the root sheath remains constant whereas the coronal portion, which is associated with dentin and cementum formation moves in the direction of the oral cavity. The root sheath bends horizontally at the level of future cementoenamel junction forming the epithelial diaphragm, following which the cervical opening becomes smaller (Figs 1.2 and 1.3).
Once the crown formation is complete the cells of the inner enamel epithelium loose their ability to form enamel and is called reduced enamel epithelium. They retain the ability to induce perimesenchymal cells to differentiate into odontoblasts and to proceed with the formation of predentin and dentin.5
After the dentin formation is completed, certain changes occur in the root sheath. Recent studies have shown that, the epithelial cells of the root sheath produce a layer on the root dentin, which is 10 μm thick, has a hyaline appearance and contains fine granules and fibrils. This layer is called the hyaline layer of Hopewell Smith or intermediate cementum. The epithelial cells of the root sheath secrete enamel proteins such as amelogenin or enameloids. The root sheath at this stage becomes discontinuous and enables the surrounding follicular mesenchyme to come in contact with the amelogenin. These follicular cells then differentiate into cementoblasts and deposit the organic matrix of cementum on the root surface (Figs 1.4 and 1.5).
Later Development of Cementum
Cementoblasts are cuboidal cells that are arranged on the outer surface of the hyaline layer. These cells are responsible for the deposition of the organic matrix of cementum, which consists of proteoglycan ground substance, intrinsic collagen fibers and is followed by subsequent mineralization of the organic matrix (Fig. 1.6).
Mineralization starts with the formation of a thin layer called cementoid. Mineral salts are derived from the tissue fluid containing calcium and phosphate ions and are deposited as hydroxyapatite crystals.
The disintegrated Hertwig's root sheath slowly moves away from the root surface and remain in the periodontal ligament as epithelial cell rests of Malassez. The periodontal ligament forms from the dental follicle soon after root formation begins. Before a tooth erupts, fibers from the follicle are incorporated in the cementum and they lie parallel to the root surface. Once the tooth erupts, the fibers are arranged in an oblique manner and are regarded as the precursor of the periodontal ligament fibers. As the cementum continues to increase in thickness, more fibers become incorporated into the cementum and eventually called as Sharpey's fibers, when periodontal ligament becomes established.
Alveolar bone forms around the periodontal ligament. With continuous bone deposition the periodontal ligament space gradually becomes narrower.
The alveolar process develops during the eruption of the teeth and cells responsible for bone formation are osteoblasts (Fig. 1.7).
Development of Junctional Epithelium (Fig. 1.8)
When the enamel formation is complete the ameloblasts become shorter, and they leave a thin membrane on the surface of the enamel called primary enamel cuticle. The inner enamel epithelium after laying down enamel reduces to a few layers of flat cuboidal cells, which is then called as reduced enamel epithelium. It covers the entire enamel surface extending till the cementoenamel junction. During eruption, the tip of the tooth approaches the oral mucosa leading to fusion of the reduced enamel epithelium with the oral epithelium. As the crown emerges into the oral cavity the former ameloblasts that are in contact with the enamel get transformed into junctional epithelium. Coronally, the junctional epithelium is continuous with the oral epithelium. As the tooth erupts, the reduced enamel epithelium grows shorter gradually. A shallow groove, the gingival sulcus may develop between the gingiva and the tooth surface.
Hence, the ameloblasts pass through two phases, in one they form enamel and in the other phase they help in formation of primary epithelial attachment or junctional epithelium. When the junctional epithelium forms from the ameloblasts it is called primary epithelial attachment. Junctional epithelium that forms after surgical therapy takes its origin from the basal cells of oral epithelium instead of ameloblasts. This is called secondary epithelial attachment.
REVIEW QUESTIONS
- Define periodontology and periodontics.
- What are the parts of periodontium?
- Describe the development of structures of periodontium.
BIBLIOGRAPHY
- Jansen BG, Van Rensburg. Oral Biology, Chapter 8. Quintessence Publishing Co Ltd, 1995; 301–7.
- Varma BRR, Nayak RP. Current Concepts in Periodontics, Chapter 2. Arya Publishing, 2002; 4–8.
- Bhasker SN. Orbans, Oral Histology and Embryology, 10th edition. CBS Publishers and Distributers, New Delhi, 41–4.
- Tencate. Oral Histology, Development, Structure and Function, 3rd edition. Jaypee Brothers Medical Publishers, 228–43.