Enamel is the external layer of the tooth and is composed of 96% mineral deposits, with water and organic substances completing the remaining matrix [1]. It surpasses bone as the hardest and most mineralized substance in the body. A crystallized form of calcium phosphate known as hydroxylapatite is the main mineral component of enamel [2]. This compound accounts for the strength of enamel but also for its brittleness. Unlike the underlying dentin, enamel is without collagen. Enamel can exceed a thickness of 2 millimeters on the cusps of posterior teeth, but it exists as a thin veneer at the dentinoenamel junction.

Although enamel is the hardest substance in the body, its lifelong exposure to the oral environment presents numerous challenges to its integrity and maintenance. When enamel is lost, there is no capacity for the production of new enamel. The loss of enamel through tooth-to-tooth contact is known as attrition. This is a gradual process that occurs throughout life but can be accelerated when parafunctional habits such as bruxing or clenching occur. The functional aspects of the teeth, such as the incisal edges of anterior teeth and the occlusal surfaces of posterior teeth, are most frequently involved, although the interproximal surfaces of teeth may also be affected. This loss of enamel can dull the incisal edges and flatten the occlusal surfaces of teeth, causing a decrease in the vertical dimension of occlusion and decreasing the penetrating ability of the involved surfaces during mastication. Sensitivity usually does not occur until the enamel has worn away to the point of exposing the underlying dentin. The loss of enamel supporting dental restorations can undermine their support and cause the loss of the restoration.

Dentin is the substance that underlies the enamel of the clinical crown of the tooth and the cementum of the root. Its composition differs from enamel, consisting of 70% inorganic components, 20% organic components, and 10% water [1]. The significantly lower mineral content makes dentin softer than enamel and permits a more rapid progression of decay. It is formed by cells called odontoblasts and comprises the bulk of the tooth structure supporting the enamel.

Most of the organic matrix of dentin is type I collagen, which confers a property of resiliency such that dentin can deform under the forces of occlusion and support the more brittle layer of enamel [12]. Dentin is avascular, like enamel, and the substructure of dentin is a series of minute channels called dentinal tubules that contain cytoplasmic extensions of the progenitor odontoblasts and traverse much of the thickness of dentin. These odontoblastic processes are the main component within the dentinal tubules, and some may extend to the dentinoenamel junction. Cellular organelles such as mitochondria and secretory granules are associated with the cell bodies of the odontoblastic processes. Enamel is completely devoid of these live cellular elements.

The structures that support, protect, and nourish the teeth externally are collectively referred to as the periodontium. This includes the free and attached gingiva, the cementum, the alveolar bone (process) of the maxilla and mandible, and the periodontal ligament. Each structure plays an important role in maintaining the health and stability of the teeth.

The free gingiva is composed of stratified squamous epithelial tissue supported by a matrix of dense connective tissue. This tissue surrounds the tooth in a collar-like fashion and is not attached to the underlying alveolar bone. In some patients, there is a minute linear demarcation, called the free gingival groove, between the free and attached gingiva. The space between the tooth and inner aspect of the free gingiva is the gingival sulcus. Bacteria and plaque that are deposited on and not removed from the gingival sulcus initiate an inflammatory response known as gingivitis. There is no loss of alveolar bone at this point, but the continued assault of bacterial toxins and the host inflammatory response may cause a progressive deepening of the sulcus beyond the 3 mm or less depth associated with ideal gingival health. The development of a periodontal pocket indicates progressive inflammation that will continue to deepen without treatment and will eventually cause resorption of the alveolar bone supporting the tooth and tooth loss. The epithelium of the free gingiva facing the gingival sulcus is called the sulcular epithelium. These cells are generally not keratinized (although some keratinization may extend into the sulcus) and are thinner than the external epithelial surface of the free gingiva (which is keratinized). At the depth of the gingival sulcus, the attachment between the tooth and the gingiva is known as the junctional epithelium. Important in this attachment is the presence of the basal lamina, a noncellular adhesive supporting sheet comprised of glycoproteins secreted by epithelial cells [14]. The epithelial attachment is the part of the junctional epithelium where the epithelial cells adhere to the tooth surface. The gingival sulcus contains gingival (crevicular) fluid that originates from the adjacent gingival connective tissue. This fluid contains substances with antimicrobial properties, antibodies, and plasma proteins that may improve the adhesion of the epithelial attachment [15]. The amount of this filtration product from the adjacent and highly vascular gingival connective tissue increases during gingival inflammation.

The keratinized attached gingiva is continuous with the free gingiva and extends to the facial alveolar mucosa of both the mandibular and maxillary arches. Lingually, it is continuous with the loose alveolar mucosa of the mandibular arch and blends imperceptibly with the firmly bound palatal mucosa of the maxillary arch. Its width can vary from 1–9 mm [17]. Unlike the free gingiva, the attached gingiva is tightly bound to the underlying alveolar bone by periosteal gingival fibers [18]. The exterior layer of the attached gingiva is composed of stratified squamous epithelium supported by a connective tissue matrix. There are few elastic fibers in its connective tissue matrix, as the attached gingiva is designed for minimal movement. This histologic consideration allows the attached gingiva to afford the periodontium protection against external injury, stabilize the gingival margin, and absorb the forces of the alveolar mucosa and the frenal attachments [1]. The surface of the attached gingiva has a stippled, "orange peel" texture that is absent from the gingival margin. The microscopic basis for stippling is alternating protuberances and depressions in the underlying tissue. Inflammation and edema from periodontal disease or odontogenic infections that extend into the attached gingiva will reduce or eliminate the stippling; the texture will return after the restoration of tissue health.

The calcified tissue covering the roots of the teeth is known as cementum, and it is formed by cells called cementoblasts. By weight, its composition is about 50% inorganic material and 50% organic material and water. Cementum is thinnest at the cementoenamel junction (CEJ), the point of union of the coronal enamel and the cementum of the root [1]. At the CEJ and for the coronal two-thirds of the root, the cementum lacks cellular components and is known as acellular cementum. The more permeable cellular cementum is present in the apical one-third of the tooth [1].

The primary function of the cementum is to provide a surface into which the periodontal ligaments can insert and anchor the tooth to the alveolar bone. There are no blood vessels or nerves within the cementum, and it is not seen intra-orally unless gingival recession and periodontal disease occurs. In periodontal disease, the apical migration of the gingival tissue due to alveolar bone loss exposes the previously protected cementum, which is then susceptible to abrasion and caries. Root planing and scaling procedures upon cementum will not illicit a painful response, as this substance is without innervation. However, sensitivity can occur if exposed cementum is abraded and the previously protected dentinal tubules are subjected to thermal stimuli, acidic substances, or sweets. Likewise, if the cementum is removed during planing, exposure of the underlying dentin may result in pain. Unlike enamel, cementum is formed throughout life, which allows for the continuous reattachment of the fibers of the periodontal ligament.

Ligaments are usually associated with connecting bones together at the joints. The periodontal ligament (PDL) connects the cementum of the tooth to the alveolar bone and has width ranging from 0.15–0.38 mm [4]. Radiographically, it is seen as a narrow black line surrounding the tooth. When widening of the PDL is seen on conventional periapical films, pulpal pathology or systemic medical conditions are usually the etiologic factor. Beyond its support and attachment of the tooth to the bone, it protects the underlying tissue from bacterial invasion. The PDL also has neural input that responds to the sensations of pain, touch, and pressure.

The PDL is composed of an interconnecting meshwork of fibers between the alveolar bone and cementum. Type I collagen fibrils are the principal constituent of the periodontal ligament [23]. Collagen-producing fibroblasts are bountiful in the periodontal ligament, with other cells such as macrophages, osteoblasts, and cementoblasts also present [24].

The portion of the maxillary or mandibular arch that supports and protects the teeth is known as the alveolar bone. A boundary at the level of the root apices separates the alveolar process from the basal bone of the mandible or the maxilla. The terms alveolar bone and alveolar process are often used interchangeably; for the purpose of this course, the bone that surrounds and supports the teeth in either the mandibular or maxillary arch is considered the alveolar process. Anatomically, the alveolar process can be divided into separate areas, but in support and retention of the teeth, it is one functional unit. It is composed of a thin inner socket wall of compact (cortical) bone called the alveolar bone proper and the supporting alveolar bone, which consists of facial (outer) and lingual (inner) plates of compact bone, interior to which is the spongy cancellous bone. Radiographically, the alveolar bone proper is known as the lamina dura and appears as a radiopaque band that surrounds the tooth root. It is the alveolar bone proper to which the fibers of the periodontal ligament attach. When radiographs reveal loss of density in the lamina dura, it can indicate resorption of this bone as a symptom of inflammation or infection of the periodontal tissues.

The portion of the maxillary or mandibular arch that supports and protects the teeth is known as the alveolar bone. A boundary at the level of the root apices separates the alveolar process from the basal bone of the mandible or the maxilla. The terms alveolar bone and alveolar process are often used interchangeably; for the purpose of this course, the bone that surrounds and supports the teeth in either the mandibular or maxillary arch is considered the alveolar process. Anatomically, the alveolar process can be divided into separate areas, but in support and retention of the teeth, it is one functional unit. It is composed of a thin inner socket wall of compact (cortical) bone called the alveolar bone proper and the supporting alveolar bone, which consists of facial (outer) and lingual (inner) plates of compact bone, interior to which is the spongy cancellous bone. Radiographically, the alveolar bone proper is known as the lamina dura and appears as a radiopaque band that surrounds the tooth root. It is the alveolar bone proper to which the fibers of the periodontal ligament attach. When radiographs reveal loss of density in the lamina dura, it can indicate resorption of this bone as a symptom of inflammation or infection of the periodontal tissues.

Masticatory mucosa is designed to withstand compressive and shearing forces during mastication and the abrasive nature of food against soft tissue. In order for the masticatory mucosa to perform these functions and retain its original shape, the tissue must be firmly secured. The junction between the epithelial cells of the mucosa proper and the underlying lamina propria is a convoluted border that extends the available surface area for attachment and allows for a stronger union between the layers. In the free and attached gingiva, fingerlike extensions called rete pegs extend from the epithelial layer into the lamina propria and provide a more secure mechanical attachment [29]. The epithelial attachment of the junctional epithelium secures the attached gingiva to the tooth. Because there are few elastic fibers in the connective tissue, movement and flexibility in the gingiva is ideal in order to withstand masticatory forces and protect the underlying tissue [17].

The mucosa that overlies the hard palate is also considered masticatory mucosa, as the tongue thrusting a bolus of food against the hard palate aids in the breakdown of food. In addition, complete or partial dentures use the unyielding hard palate as a sturdy foundation to which masticatory stresses can be transferred. The palatal mucosa is bound tightly to the bone of the hard palate by dense connective tissue. Pronounced ridges of epithelial tissue called rugae extend horizontally across the anterior region of the hard palate and also assist with food breakdown.

Most of the mucosa present in the oral cavity is categorized as lining mucosa, which is flexible and distensible. This includes the mucosa of the inside of the lips (labial) and cheeks (buccal), the ventral surface of the tongue and the floor of the mouth beneath it, and the alveolar mucosa beginning at the attached gingiva and blending with the labial or buccal mucosa. Lining mucosa consists of stratified squamous epithelium that, unlike the masticatory mucosa, is not keratinized. This tissue is designed for movement during function, and the histology reflects this function.

The tongue is divided into the anterior two-thirds (or body) and the posterior one-third (or base) and each has different embryologic origins. The dorsum of the tongue is masticatory mucosa, the epithelium of which contains numerous projections called papillae, most of which are filiform papillae. These flame-like projections have a surface layer of keratinized stratified squamous epithelium suited for the masticatory function of compressing a bolus of food against the hard palate. The mucosa between the filiform papilla is not keratinized. There are no taste buds present in the filiform papillae.

The tongue is divided into the anterior two-thirds (or body) and the posterior one-third (or base) and each has different embryologic origins. The dorsum of the tongue is masticatory mucosa, the epithelium of which contains numerous projections called papillae, most of which are filiform papillae. These flame-like projections have a surface layer of keratinized stratified squamous epithelium suited for the masticatory function of compressing a bolus of food against the hard palate. The mucosa between the filiform papilla is not keratinized. There are no taste buds present in the filiform papillae.

Some patients may develop benign migratory glossitis (or geographic tongue), a condition that involves the filiform papillae. With this condition, the temporary loss or desquamation of the filiform papillae leaves irregular and erythematous areas, although the location of these areas can change daily. The papillae will regenerate, but the condition often recurs. The etiology of these lesions is unknown, although genetic, autoimmune, and environmental factors have been implicated. There is no common or universally effective treatment, and no cure is available. The exposed mucosal surfaces can be sensitive to thermal, acidic, or mechanical stimuli, so treatment is usually palliative. Although the lesions resolve spontaneously, they should be monitored and any lesions that do not heal should be biopsied.

The vermilion zone is the margin of the lips between the skin on the exterior surface and the labial mucosa on the interior surface. This thin epithelial layer is keratinized, with capillary loops in its connective tissue core that allow a copious surface perfusion of blood and account for the vermilion (red) color of the lips. The tissue of the vermilion zone is considered a specialized mucosa. It has a dense submucosal layer with a firm attachment to the underlying musculature [34].

There are no mucous glands in the vermilion zone, so saliva is necessary to keep the area moist. Excessive desiccation of the vermilion zone due to extremes of temperature or excessive exposure to the wind leads to the condition known as "chapped lips." Commercial lip balms can palliate the symptoms associated with this problem.

Before the major and minor salivary glands and their unique histology can be discussed, it is essential to review the main components of saliva and their contribution to oral health. Saliva is a unique substance that functions as a tissue lubricant, as a medium for the remineralization of enamel, as an antimicrobial agent, and as a critical component in the self-cleansing mechanism for the teeth. Adequate salivary flow is essential for good oral health, as the decreased flow of saliva increases retention of plaque on the teeth and the mucosa and thus increases the risk of caries and periodontal disease. It is 99.5% water, with an assortment of 0.5% solutes accounting for the remainder [35].

Mucous secretions in saliva provide tissue lubrication, which is especially significant for patients who wear partial or complete dentures. Inadequate lubrication of the tissues supporting these prostheses can cause ulcerations and preclude their use. Immunoglobulin A (IgA), which can inhibit the adherence of some strains of streptococci bacteria to the oral mucosa, is secreted in saliva from the parotid salivary glands and the minor salivary glands [36]. Lysozyme, a hydrolytic enzyme secreted by the major and minor salivary glands, disrupts the structural integrity of the cell walls of certain bacteria [22]. Amylase is an enzyme in saliva that begins the breakdown of carbohydrates contained in food while it is in the oral cavity. Remineralization of small areas of enamel is mediated through an exchange of calcium and phosphate ions present in the saliva. There are trace amounts of several other organic and inorganic solutes in saliva that contribute to the overall health of the oral and maxillofacial complex.

Saliva originates in one of the three paired major salivary glands (parotid, sublingual, and submandibular) and numerous minor salivary glands. The basic secreting units of each gland are the acini. Serous acini produce a watery secretion, and mucous acini produce a thick, viscous secretion. The parotid gland produces serous secretions only, while the sublingual and submandibular glands produce both mucous and serous secretions. The minor salivary glands are widely distributed in the mucosa and submucosa of the oral cavity and produce predominantly mucous secretions. The major salivary glands are considered tubuloacinar, as their secretions are produced by acini and the initial portion of the duct system that transports these secretions into the oral cavity [34]. They are also considered exocrine glands, because their secretions must be brought to the surface by a series of ducts and tubes. This is in contrast to endocrine glands, such as the pituitary and adrenal glands, whose hormones enter the bloodstream directly. The production of saliva is an involuntary activity regulated by the sympathetic and parasympathetic divisions of the autonomic nervous system.

The parotid salivary glands are probably the best known among the major salivary glands. They are the largest among the major salivary glands, but they produce only about 20% of the total saliva [27]. Each parotid gland is wrapped in a subcutaneous connective tissue capsule located anterior and inferior to the ear and lies to the exterior of both the masseter and buccinators muscles. The parotid (Stensen) duct pierces the buccinator muscle and enters the oral cavity in a papilla on the buccal mucosa opposite the maxillary first or second molar area. As noted, the secretions of the parotid gland are purely serous in nature. The acinar cells that produce the secretions appear pyramidal and have an abundance of secretory vesicles called zymogen granules within the cytoplasm. Alpha amylase, a substance that begins the digestion of starches, is produced by the serous acini [1]. The ducts in the parotid gland are more than a conduit for saliva to the oral cavity; they are also involved in the secretion and absorption of varied substances.

The ducts originating in the secreting acini are narrow and intercalated and absorb chloride from and secrete bicarbonate ion into the acinar secretion. These ducts are lined with cuboidal or columnar cells. Striated (secretory) ducts continue from the intercalated ducts and resorb sodium and secrete potassium. A series of smaller excretory ducts merge to form a larger excretory duct that will discharge the saliva into the oral cavity. Myoepithelial cells with actin-containing contractile filaments lie adjacent to the acinar secretory cells and the intercalated ducts. The contraction of these myoepithelial cells helps to propel the contents of the secreting acinar cells into the lumen of the ducts and beyond. Bicarbonate secretion from the parotid gland is an important component of the buffering capacity of the oral cavity. A decrease in the bicarbonate production causes an increase in the acidity (lower pH) of the oral environment, which favors the proliferation of cariogenic bacteria and the resident fungal organism Candida albicans.

The submandibular gland is considered a mixed gland because its secretions are both serous and mucous, though serous secretions predominate. These bilateral glands are located inferior and medial to the posterior aspect of the mandible and superior to the digastric muscles. They enter the oral cavity via Wharton ducts. Small mucosal papilla (called sublingual caruncles) near the anterior midline on either side of the lingual frenum in the floor of the mouth mark the entrance of Wharton ducts into the oral cavity. Submandibular glands are smaller in size than the parotid glands, but they account for approximately 70% of salivary production [1]. The duct system and myoepithelial cells associated with the serous acini of the parotid gland remain unchanged in the submandibular gland [3]. However, mucous acini are also present and are responsible for the production of mucus, a glycoprotein-laden, viscous secretion whose primary functions are lubrication and protection of the tissue [31]. Each mucous cell contains numerous membrane-bound secretory granules, and the collective amount of mucus within these granules compresses the nucleus within each cell toward the cell membrane. After these granules have discharged their contents into the lumen of each acinar unit, the compression of the nucleus against the cell membrane is relaxed. Microscopically, the mucous acini have a pale appearance compared to the darker-staining granules of the serous acini.

The sublingual gland is also a mixed gland, with the mucous secretions predominating over the serous secretions. It is the smallest of the major salivary glands and produces about 5% of the salivary volume [27]. The sublingual gland is anterior to the submandibular gland and lies just beneath the mucosa of the floor of the mouth, between the mandible and the genioglossus muscle. Its inferior border is the mylohyoid muscle [30]. Unlike the parotid and submandibular glands, the sublingual gland does not have a connective tissue capsule surrounding it nor is it a single unit. It is comprised of a posterior portion (the greater sublingual gland) and an anterior portion (the lesser sublingual gland) with a duct system for each portion. The greater sublingual gland drains into a main duct (Bartholin duct), while the lesser sublingual gland drains into smaller multiple ducts (the ducts of Rivinus) [23]. Bartholin duct may unite with Wharton duct or may open in the floor of the mouth with individual sublingual papilla. The basic histology of the mucous and serous units is consistent with that of the parotid and submandibular glands.

Sialolithiasis is a benign condition in which calcium-laden crystalized minerals known as sialoliths, salivary calculi, or salivary stones become deposited in a salivary gland or a salivary duct. It is the most common disease affecting the salivary glands. More than 80% of cases occur in the submandibular gland or its duct, 6% involve the parotid gland, and only about 2% involve the sublingual and minor salivary glands [11]. The exact etiology of sialoliths is not known.