“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.
10Periodontal StructuresLeft Side:Transmission electron photomicrograph (TEM) of root formation in humans (ca. 6-year-old)This TEM depicts the growing demarcation between dentin, cementum and periodontal ligament during root formation. Initial mineralization of the “cementoid” directly apposed to the dentin, with penetrating collagen fibers and fibroblast-like cementoblasts, which are involved in the formation of acellular exogenous fiber cementum.A DentinB CementoidC Radiating collagen fibersD Cementoblast (fibroblast-like) building acellular exogenous fiber cementCourtesy D. Bosshardt, H. Schroeder
Gingiva
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).
11Healthy GingivaThe free gingival margin courses parallel to the cementoenamal junction. The facial interdental papillae extend to the contact area of adjacent teeth. A gingival groove can be observed in some areas, demarcating the free gingival margin from the attached gingiva.Right: The radiograph depicts normal interdental septa. In the original radiograph, the crest of the alveolar bone was observed ca. 1.5 mm apical to the CEJ.12Variations in Consistency of Healthy GingivaLeft: Firm, fibrous gingiva = “thick” phenotype.Right: Delicate, scarcely stippled gingiva = “thin” phenotype. The subjacent bone covering the roots is clearly visible.Thicker gingiva provides better conditions for treatment and wound healing (blood flow; stable position of the gingival margin).13Healthy, Pigmented GingivaNote the symmetrical pigmentation of the attached gingiva in this 16-year-old African female.Right: This pigmentation results from the synthesis of melanin by melanocytes located in the basal layer of the epithelium. The melanocytes in this histologic section appear as brown spots.
Gingival Width
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.
Col—Interpapillary Saddle
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.
14Mean Width of Attached Gingiva• In the maxilla, the facial gingiva in the area of the incisors is wide, but narrow around the canines and first premolars. On the palatal aspect, the marginal gingiva blends without demarcation into the palatal mucosa.• In the mandible, the lingual gingiva in the area of the incisors is narrow, but wide on the molars. On the facial, the gingiva around the canines and first premolars is narrow (arrow), but wide around the lateral incisors.15Variability of Gingival WidthWidth of attached gingiva can vary dramatically. The three patients depicted here, all about the same age, exhibit gingival width varying from 1 to 10 mm in the mandibular anterior area.Right: After staining the mucosa with iodine (Schiller or Lugol solution) the mucogingival line is easily visible because the non-keratinized alveolar mucosa is iodine-positive while the keratinized gingiva is not (cf. p. 161).16Col—Interpapillary SaddleThe col consists essentially of a connection between the junctional epithelia (JE; p. 10) of any two adjacent teeth. Tooth morphology, width of the tooth crowns and relative tooth position will determine the extent of the contact surfaces (hatched), their breadth (2–7 mm, red bar), as well as the depth (1–2 mm) of the interpapillary saddle.I IncisorP PremolarM Molar
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).
Junctional Epithelium
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).
17Junctional Epithelium and Gingiva in Orofacial SectionThe gingiva consists of three tissues:• Junctional epithelium• Oral epithelium• Lamina propria (connective tissue)The junctional epithelium (JE) assumes a key role in maintenance of periodontal health: It produces the epithelial attachment and therefore creates the firm connection of soft tissue to the tooth surface. It is quite permeable, and thus serves as a pathway for diffusion of the metabolic products of plaque bacteria (toxins, chemotactic agents, antigens, etc.). There is also diffusion in the opposite direction, of host defense substances (serum exudates, antibodies, etc.). Even when the gingivae do not appear inflamed clinically, the JE is constantly transmigrated by polymorphoneuclear leukocytes (PMNs) moving towards the sulcus (p. 55, Fig. 109). The red arrows depict the migration of daughter cells from the basal layer toward the gingival sulcus. The circled areas A-C are depicted in detail on page 11.
Epithelial Attachment
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.
Gingival Sulcus
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).
18Gingival Sulcus and Junctional EpitheliumThe junctional epithelial cells (1) are oriented parallel to the tooth surface and are sharply demarcated (broken line) from the more deeply staining cells of the oral sulcular epithelium (2). All of the daughter cells that emanate from the entire 1–2 mm length of the basal layer of the junctional epithelium must transmigrate the exceptionally narrow (100–150 μm) sulcus bottom (red arrow). Note the polymorphonuclear leukocytes (circled), which emigrate from the venule plexus in the subepithelial connective tissue (3) without altering it in any way.Left: In the enlargement, a portion of the most coronal JE cells (cf. empty black arrow in lower power view) is shown still manifesting hemidesmosomes and an internal basal lamina attached to the enamel surface.Courtesy H. Schroeder19Internal Basal Lamina and HemidesmosomesEach JE cell adjacent to the tooth forms hemidesmosomes (HD) that enable these cells to attach to the internal basal lamina (IBL) and ultimately to the surface of the tooth. Remnants of enamel crystals are visible at the left. The long arrows indicate intercellular spaces between three JE cells (•).Left: The basal lamina is comprised of two layers: the Lamina lucida (LL) and the Lamina densa (LD).20Apicalmost Portion of the Junctional EpitheliumIn a young, healthy patient, the JE ends apically at the cementoenamel junction. Daughter cells of the cuboidal basal cells (B) migrate toward the sulcus (red arrows). If a JE cell comes into contact with the tooth surface, it establishes the attachment mechanism described above. The internal basal lamina (IBL) is continuous with the external basal lamina (EBL) around the apical extent of the JE (black arrowhead).
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