**2.3 Periodontal ligament**

The periodontal ligament is the soft and specialized connective tissue situated between the cementum covering the root of the tooth and the bone forming the socket wall (alveolodental ligament) [4]. The periodontal ligament consists of cells and an extracellular compartment comprising collagenous and noncollagenous matrix constituents. The cells include osteoblasts and osteoclasts, fibroblasts, epithelial cell rests of Malassez, monocytes and macrophages, undifferentiated mesenchymal cells, and cementoblasts and odontoclasts. The extracellular compartment consists mainly of well-defined collagen fiber bundles embedded in an amorphous background material, known as ground substance [4, 8]. These bundles of collagen fibers can be classified into the following groups, according to their disposition (e.g., **Figure 1c**): crestal alveolar fibers (CAF), horizontal fibers (HF), oblique fibers (OF), and apical fibers (AF) [6].

The main function of the periodontal ligament is to support the teeth in their sockets and at the same time allow them to withstand the considerable forces of

mastication. In addition, the periodontal ligament has the capacity to act as a sensory receptor necessary for the proper positioning of the jaws during mastication, and, very importantly, it is a cell reservoir for tissue homeostasis, regeneration, and repair [4].

### **2.4 Gingiva**

Gingiva is a portion of the oral mucosa covering the tooth-carrying part of the alveolar bone and the cervical neck of the tooth. Three parts of the gingiva can be distinguished (e.g., **Figure 1d**): (1) free gingiva (FG), (2) interdental gingiva (IG), and (3) attached or inserted gingiva (AG) [6]. Histologically, the epithelial component of the gingiva shows regional morphological variations that reflect the adaptation of the tissue to the tooth and alveolar bone. These include the epithelium that covers a connective tissue, chorion, or lamina propria. A keratinized stratified squamous epithelium protects the lamina propria of the gingiva on its masticatory surfaces and a nonkeratinized epithelium protects the lamina propria on its crevicular and junctional surfaces [6, 9]. The junctional epithelium plays a crucial role since it essentially seals off periodontal tissues from the oral environment. Its integrity is thus essential for maintaining a healthy periodontium. Periodontal disease sets in when the structure of the junctional epithelium starts to fail, an excellent example of how structure determines function [4].

During pathological conditions, such as inflammation, the periodontal connective tissues, including the gingiva, undergo many changes. Clinically detected gingival overgrowth is one of the alterations that occurs in chronic periodontitis. It is caused by a variety of etiological factors and is exacerbated by local bacterial biofilm accumulation, because the periodontopathogen products act on the gingival tissues activating cellular events that induce the alteration of connective tissue homeostasis and the destruction of the alveolar bone [9]. Likewise, junctional epithelial cells differ considerably from those of the gingival epithelium. There are polymorphonuclear leukocytes and monocytes that pass from the subepithelial connective tissue through the junctional epithelium and into the gingival sulcus. The mononuclear cells, together with molecules they secrete and others originating from junctional epithelial cells, blood and tissue fluid, represent the first line of defense in the control of the perpetual microbial challenge [4].
