“Structural biology” is a general term referring to the classical macromorphology and histology of tissues, as well as their function, including the biochemistry of the cells and the intercellular substances.
Basic knowledge of the normal structural biology of periodontal tissues and their dynamics (mediator-guided homeostasis, “turnover”) is a prerequisite for full understanding of pathobiological changes in the periodontium, which can involve adaptations of the normal structures or an imbalance of otherwise normal functions (Schroeder 1992).
The term “periodontium” encompasses four different soft and hard tissues: gingiva, root cementum, alveolar bone, and the periodontal ligament, which attaches root cementum to bone. Each of these four tissues can be further differentiated in terms of structure, function and localization.
The gingiva is one portion of the oral mucosa. It is also the most peripheral component of the periodontium. Gingiva begins at the mucogingival line, and covers the coronal aspect of the alveolar process. On the palatal aspect, the mucogingival line is absent; here, the gingiva is a part of the keratinized, non-mobile palatal mucosa.
The gingiva ends at the cervix of each tooth, surrounds it, and forms there the epithelial attachment by means of a ring of specialized epithelial tissue (junctional epithelium; p. 10). Thus the gingiva provides for the continuity of the epithelial lining of the oral cavity.
The gingiva is demarcated clinically into the free marginal gingiva, ca. 1.5 mm wide; the attached gingiva, which may be of varying width; and the interdental gingiva.
Healthy gingiva is described as “salmon” pink in color; in Blacks (seldom also in Caucasians) the gingiva may exhibit varying degrees of brownish pigmentation. Gingiva exhibits varying consistency and is not mobile upon the underlying bone. The gingival surface is keratinized and may be firm, thick and deeply stippled (“thick phenotype”), or thin and scarcely stippled (“thin phenotype”; Müller & Eger 1996, Müller et al. 2000).
The attached gingiva becomes wider as a patient ages (Ainamo et al. 1981). The width varies between individuals and among various groups of teeth in the same person. Although it was once believed that a minimum width of attached gingiva (ca. 2 mm) is necessary to maintain the health of the periodontium (Lang & Löe 1972), this concept is not accepted today. However, a wide band of attached gingiva does offer certain advantages in the case of periodontal surgery, both therapeutically and esthetically.
Apical to the contact area between two teeth, the interdental gingiva assumes a concave form when viewed in labiolingual section. The concavity, the “col,” is thus located between the lingual and facial interdental papillae and is not visible clinically. Depending on the expanse of the contacting tooth surfaces, the col will be of varying depth and breadth. The epithelium covering the col consists of the marginal epithelia of the adjacent teeth (Cohen 1959, 1962; Schroeder 1992). The col is not keratinized. In the absence of contact between adjacent teeth, the keratinized gingiva courses uninterrupted from the facial to the oral aspect.
The marginal gingiva attaches to the tooth surface by means of the junctional epithelium, an attachment that is continuously being renewed throughout life (Schroeder 1992).
The junctional epithelium (JE) is approximately 1–2 mm in coronoapical dimension, and surrounds the neck of each tooth. At its apical extent, it consists of only a few cell layers; more coronally, it consists of 15–30 cell layers. Subjacent to the sulcus bottom, the JE is about 0.15 mm wide.
The junctional epithelium consists of two layers, the basal (mytotically active) and the suprabasal layer (daughter cells). It remains undifferentiated and does not keratinize. The basal cell layer interfaces with the connective tissue via hemidesmosomes and an external basal lamina. Healthy JE exhibits no rete ridges where it contacts the connective tissue. JE turnover rate is very high (4–6 days) compared to oral epithelium (6–12 days, Skougaard 1965; or up to 40 days, Williams et al. 1997).
The epithelial attachment to the tooth is formed by the JE, and consists of an internal basal lamina (IBL) and hemidesmosomes. It provides the epithelial attachment between gingiva and tooth surface. This can be upon enamel, cementum or dentin in the same manner. The basal lamina and the hemidesmosomes of the epithelial attachment are structural analogs of their counterparts comprising the interface between epithelium and connective tissues.
All cells of the JE are in continual coronal migration, even those cells in immediate contact with the tooth surface. Such cells must continually dissolve and reestablish their hemidesmosomal attachments. Between the basal lamina and the tooth surface, a 0.5–1μm thick “dental cuticle” is observed; this is possibly a serum precipitate or a secretion product of the junctional epithelial cells.
The sulcus is a narrow groove surrounding the tooth, about 0.5 mm deep. The bottom of the sulcus is made up of the most coronal cells of the junctional epithelium, which are sloughed (exfoliated) in rapid succession. One lateral wall of the sulcus is made up of the tooth structure, the other wall is the oral sulcular epithelium (OSE; Schroeder 1992).