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Anatomy and Physiology
Edgard El Chaar1 and Thierry Abitbol2
1Department of Periodontics, University of Pennsylvania, Dental Medicine, Philadelphia, PA, USA
2Arthur Ashman Department of Periodontics and Implant Dentistry, New York University, NY, USA
Overview of Gingival Tissue and Periodontium
Macroscopically, the dental organ appears to be limited to the dental crown surrounded by soft tissue, which is scalloped around a well delineated circular line called the cementoenamel junction (CEJ).
The morphology of the dental crowns change shape from the midline to the posterior dentition and with it, the function and dimension change. The soft tissue changes as well to accommodate these differences. That soft tissue will comprise two distinct parts, the mucosa and the keratinized tissue separated by the mucogingival line (MGL). In the keratinized zone, we have the attached gingiva and the marginal gingiva. The latter is a non‐attached tissue that extends few millimeters from the margin to the junctional epithelium (JE), delineating the sulcus that is a gap on the inner side of the non‐attached gingiva around the CEJ. JE is non‐keratinized epithelium as it is on the border of the attached and non‐attached gingiva.
In a microscopic cross‐sectional view, we can appreciate the different parts of hard and soft tissue interacting together making the periodontium of the dental organ from its incisal tip apically (Figure 1.1). This figure shows the intricate components of the dental organ and the harmony needed to make this interaction work reminding us of a well‐tuned opera. All of this hard and soft tissue requires irrigation from perfectly lined up blood circulation, which originates from an alveolar artery dividing itself in a periodontal ligament (PDL) branch, dental branch, and an intra‐marrow branch. Inside the mucosa, an intricate circular system leads to a supra‐periosteal artery crossing into the keratinized gingiva in a linear manner up to the gingival collar in which the three arteries, PDL, intra‐marrow, and supra‐periosteal form the gingival crevicular plexus (Figure 1.2).
Radiographically, the PDL space can be observed and its width ranges between 0.1 and 0.25 mm (Figure 1.3). Any widening beyond these margins is considered a “widening of the periodontal ligament,” which can be a sign of inflammation either from an early periodontal disease at the coronal level or excessive occlusal trauma.
This harmonious intricate system called the dental periodontium will be reviewed in this manual, wishing you great reading and enjoyment.
Embryonic Development
At approximately four to five weeks into embryonic development, there is downgrowth of the ectoderm of the primitive oral stomatodeum into the underlying ectomesenchyme. At the terminal end of this downgrowth, the cells form a knoblike structure or bud. Cells in the surrounding ectomesenchyme begin to concentrate around this bud.
Several weeks later, this ectodermal bud has developed into a cuplike structure with four distinct layers: an outer enamel epithelium (OEE), an inner enamel epithelium (), a stellate reticulum (SR), and a stratum intermedium (SI). Directly beneath the IEE, cells of the underlying ectomesenchyme have condensed into a dental papilla (DP). Surrounding these two structures is a third condensation, the dental follicle (DF), which will give rise to most of the cementum, periodontal ligament, and alveolar bone.
At the apical extent of the root, the IEE and OEE have fused to form Hertwig’s epithelial root sheath (HES). More coronally, this root sheath breaks down to form islands of epithelial cells in the developing PDL space, the epithelial rests of Malassez (ERM). The breakdown of the root sheath and subsequent exposure of the dentin (D) to the DF allows cells in the DF nearest the developing root surface to differentiate into cementoblasts (CB) and lay down the first cementum matrix (CM). Further away from the tooth follicle, cells differentiate into fibroblasts and lay down the first bundles of collagen in the PDL (Figures 1.3–1.6).
Figure 1.1 Different components of the periodontium in a cross section.
Figure 1.2 Vascularity in the periodontal ligament.
Figure 1.3 Dimension of periodontal ligament.
Figure 1.4 Bud stage in embryologic formation of the dental organ.
Formation of the Epithelial Attachment
After formed, the enamel is covered by an epithelium called the reduced dental epithelium (RED) extending to CEJ. During eruption, the tip of the tooth approaches the oral mucosa leading to a fusion of the RED with oral epithelium (OE). Once the tip emerges, the RED is termed Epithelial Attachment. As the tooth erupts, the attached epithelium gradually separates from its surface creating a groove called the Gingival Sulcus.
Figure 1.5 Bell stage in embryologic formation. (OEE: outer enamel epithelium; IEE: inner enamel epithelium; SR: stellate reticulum; SI: stratum intermedium; DF: dental follicle; DP: dental papilla).
Figure 1.6 Apical formation of the different periodontal ligament. ERM, epithelial rest of malassez; HES, Hertwig’s epithelial root sheath.
Formation of the Cementum, Periodontal Ligament, and Alveolar Bone
The deposition of cementum on the root surface that gradually thickens toward the PDL space is somewhat similar to the deposition of alveolar bone that thickens the alveolar bone support from the opposite side of the ligament space. As a result, the cementum does have some structural and biochemical similarities (as well as some critical differences) with alveolar bone.
As with the development of the alveolar bone proper, an organic matrix of cementum composed primarily of type I and type III collagen is secreted by a layer of formative cells (the cementoblasts) over the thin hyaline‐like layer secreted by HES covering the root dentin. This fine fibrillar matrix calcifies to form a relatively uniform and well‐organized layer of cementum free of cellular elements called primary acellular cementum. This first thin layer of mineralized cementum contains only the fibrillar matrix from the cementoblasts themselves. These fibers are therefore called intrinsic fibers of cementum.
As the cementum continues to thicken by apposition of cementum by the cementoblast layers, this thickening cementum will encounter and incorporate bundles of the forming periodontal ligament. These ligament bundles incorporated into the cementum surface will calcify along with the surrounding intrinsic fibers to form a significant portion of the more superficial layers of the cementum. These insertions of calcified ligament fibers are termed extrinsic fibers of the cementum. A similar entrapment and calcification process occurs on the forming alveolar bone side. The general term for these calcified insertions of bundles of ligament fibers into the cementum and bone are Sharpey’s fibers. On the cementum side, these Sharpey’s fibers are much thinner in diameter and insert at closer intervals when compared with the alveolar bone side.
These differences in the pattern of insertion have clinical importance in the distribution of forces that are generated within the PDL during occlusion, tooth movement, and traumatic forces. Specifically, these forces are more evenly distributed along the cementum surface and are more concentrated along the more widely spaced insertions on the alveolar bone side. As a result, in response to mechanical forces, there is generally a remodeling of the periodontal housing on the alveolar bone side and not on the cementum side. This prevents the possibility of significant cementum and root resorption. In addition, the root cementum is protected from this relatively extensive remodeling because it is avascular, and therefore not as exposed to osteoclast‐like precursor cells in the circulation. Although small areas of microscopic cementum resorption and repair have been frequently observed in histologic sections, more extensive resorption of cementum is usually not seen unless there is a force on the tooth of a high enough magnitude, or duration, or both, that cannot be accommodated by the remodeling of the alveolar bone.
As the tooth completes actively erupting into the oral cavity and meets its opposing tooth in the other arch, the formation of cementum becomes somewhat less regular and organized. This type of cementum formation that occurs over the more organized primary cementum is called secondary cementum. It occurs mainly along the apical one third of the root. During the formation of secondary cementum, cells in the layer of secreting cementoblasts will often become entrapped within the CM. These entrapped cementoblasts become cementocytes similar in appearance to the entrapped osteoblasts that become osteocytes on the alveolar bone side. These areas of cementum that contain cementocytes are called cellular cementum. Layers of cellular cementum are generally seen in the apical one third of the root surface. In secondary cementum, these layers of cellular cementum often alternate with layers of acellular cementum.
Soft Tissue Physiology
Gingiva
The gingiva consists of free and attached tissue. The attached gingiva is the portion of the gingiva that is firm, dense, stippled, and tightly bound to the underlying periodontium, tooth, and bone. The free gingival margin is defined as the coronal border of the free gingiva that surrounds the tooth and is not directly attached to the tooth surface. The free gingival margin generally corresponds to the base of the gingival sulcus. It is present in 30–40% of adults and most frequently occurs in the mandibular premolar and incisor regions. The mucogingival junction (MGJ) represents the junction between the gingiva (keratinized) and alveolar mucosa (non‐keratinized) (Lindhe 1983).
Width and Thickness of the Gingiva
Bowers (1963) measured the widths of the facial attached gingiva in the primary and permanent dentitions of 240 subjects. The width of attached gingiva ranged from 1 to 9 mm. Values were greatest in the incisor regions (especially the lateral incisor) and the least in the canine and first premolar sites. The maxilla usually exhibited a broader zone of the attached gingiva than the mandible. Clinically healthy gingiva was noted in subjects with less than 1 mm of the attached gingiva, but the tissue was usually inflamed in areas of no attached gingiva. Buccal–lingual tooth position affected the amount of the attached gingiva present, and high frenum and muscle attachments were generally associated with narrow zones of attached gingiva. Facially positioned teeth had narrower zones of attached and keratinized tissue than well‐aligned or lingually positioned teeth. As teeth moved lingually, an increase in the width of attached and keratinized tissue and a slight decrease in clinical crown height were observed. Teeth moving facially had a decrease in the width of the attached and keratinized tissue.
Voigt et al. (1978) measured the width of lingual attached gingiva in the mandible. The keratinized tissue ranged from 1 to 8 mm. Greatest widths were recorded on the first and second molars (4.7 mm), decreasing at premolar and third molar sites. The smallest widths were observed on the incisors and canines (1.9 mm).
Goaslind et al. (1977) measured the thickness of the free and attached facial gingiva in a population consisting of 10 males (ages 25–36). Results demonstrated considerable variation of gingival thickness among subjects and among areas within individual subjects. Free gingival thickness averaged 1.56–0.39 mm, increased from anterior to posterior and was directly proportional to sulcus depth. Thickness of the attached gingiva averaged 1.25–0.42 mm, increased from anterior to posterior in the mandibular arch, remained relatively constant in the maxillary anterior, and was inversely proportional to attached gingival width. The overall mean thickness for all areas was 1.41 mm.
Histological Composition
As discussed in the tooth development section, while tooth emerges and eruption continues, three distinct zones of epithelium form: outer epithelium, crevicular epithelium, and the JE. Each is different in stratification, organization, and function.
Like the epidermis, the OE has multiple layers:
- Stratum basale: one to two layer of cuboidal‐shaped cells that divide and migrate to the superficial layers
- Stratum spinosum or prickle cell layer: spinous‐shaped cells with large intercellular spaces
- Stratum granulosum: flattened granular cells with flattened and condensed nuclei, increased accumulation of keratohyalin granules, and intracellular and extracellular membrane‐coated granules
- Stratum corneum: flattened cells packed with keratin; nuclei may be undiscernible known as orthokeratinized or may have visible dense nuclei called parakeratinized; cells shed and are replaced by cells from the deeper layers migrating upward.
In both the basal and prickle cell layers connect to each other via desmosomes, which appear microscopically as a thickening. Each half is made of a hemi‐desmosome that attach to underlying cell through intermediate filaments. Within the oral gingival epithelium, there are several other cells not derived from keratinocytes. These include melanocytes that transfer melanin pigment granules to the surrounding basal layer of keratinocytes, Langerhans cells that are part of the reticulo‐endothelium system and are responsible for processing and presenting foreign antigens to the immune system, and Merkel cells that may be responsible for perception of sensation in the gingiva.
On the outer layer, in 45% of the patient, stippling is noticed. It used to be thought that its presence is a sign of health but later it was refuted. Based on Karring and Loe (1970), the stippling coincides with the intersection with epithelial ridges. Epithelial (rete) ridges represent areas of epithelial proliferation into the underlying connective tissue (CT). These are believed to promote anchoring of epithelium to the CT by increasing the surface area of attachment. They are more pronounced in the gingiva than in the alveolar mucosa.
The CT of the gingiva consists of cells, fibers, and ground substance (proteoglycans [PGs] and glycoproteins [GPs]). Cells constitute about 5% of the CT and include fibroblasts (65%), mast cells, PMNs, macrophages, lymphocytes, and plasma cells. Fibers account for approximately 60–65% of the CT, with collagen predominating reticulin and elastic fibers. Ground substance comprises 35% of the CT and consists of protein‐polysaccharide macro‐molecules made up of PGs and GPs. The PGs contain glycosaminoglycans (GAGs
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