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Digestive System: Oral Cavity
The digestive system—composed of the oral cavity, alimentary tract, and associated glands—functions in the ingestion, mastication, deglutition (swallowing), digestion, and absorption of food, as well as in the elimination of its indigestible remnants. Regions of the digestive system are modified and have specialized structures in order to be able to perform these varied tasks.
This and the following two chapters detail the histology and functions of the component parts of the digestive system. This chapter details the oral cavity and its contents; Chapter 17 describes the alimentary canal, including its intramural glands (esophagus, stomach, small and large intestines, rectum, and anus); and Chapter 18 discusses the glands of the digestive system that are external to the alimentary canal (major salivary glands, pancreas, and liver and gallbladder).
Oral Mucosa: Overview
The oral mucosa is composed of a wet, stratified squamous epithelium and an underlying dense, irregular, collagenous connective tissue. There are three categories of the oral mucosa: the lining mucosa, masticatory mucosa, and specialized mucosa.
The oral mucosa lines the oral cavity and is composed of a wet, stratified squamous epithelium ( keratinized , nonkeratinized , or parakeratinized ) and an underlying connective tissue. Those regions of the oral cavity that are exposed to considerable frictional and shearing forces (gingiva, dorsal surface of the tongue, and hard palate) are lined or covered by a masticatory mucosa composed of parakeratinized to completely keratinized stratified squamous epithelium with an underlying dense, irregular, collagenous connective tissue. The remainder of the oral cavity is lined or covered by a lining mucosa , composed of a nonkeratinized stratified squamous epithelium overlying a looser type of dense, irregular, collagenous connective tissue ( Table 16.1 ). The regions of the oral mucosa that house taste buds in their epithelium (dorsal surface of the tongue and patches of the soft palate and pharynx) are said to be covered by specialized mucosa , that is, they are specialized to perceive taste.
TABLE 16.1
Summary of the Histological Features of the Oral Mucosa
Modified with permission from Gartner LP. Oral Histology and Embryology , 3rd ed. Baltimore: Jen House Publishing; 2014 (Table 8.9).
| Mucosal Region | Type of Epithelium/Mucosa | Height of CT Papilla | Special Comments |
|---|---|---|---|
| Lip | |||
| Skin aspect | Stratified squamous keratinized | Medium | Hair, sebaceous and sweat glands |
| Vermilion zone | Stratified squamous keratinized | High | Few sebaceous glands(?) |
| Mucosal aspect | Stratified squamous nonkeratinized | Medium | Mucous (mixed) minor salivary glands and Fordyce granules |
| Cheek | |||
| Skin aspect | Stratified squamous keratinized | Medium | Hair, sebaceous and sweat glands |
| Mucosal aspect | Stratified squamous nonkeratinized | Medium | Mucous (mixed?) minor salivary glands and Fordyce granules |
| Gingiva | |||
| Free and attached | Masticatory mucosa | High | Tightly bound to the periosteum |
| Sulcular | Lining mucosa | Low | Lines the gingival sulcus |
| Junctional epithelium | Stratified squamous nonkeratinized | None | Attached to tooth surface and to gingival CT by hemidesmosomes |
| Alveolar mucosa | Lining mucosa | Low | Some minor mucous salivary glands |
| Hard Palate (oral surface) | Masticatory mucosa | High | Adipose tissue in the CT |
| Anterior lateral | Masticatory mucosa | High | Minor mucous salivary glands in the CT |
| Posterior lateral | Masticatory mucosa | High | Tightly bound to periosteum |
| Raphe | Masticatory mucosa | High | Tightly bound to periosteum |
| Hard Palate (nasal surface) | Respiratory epithelium | NA | Mucous glands in the CT |
| Soft Palate (oral surface) | Lining mucosa | Low | Elastic lamina; minor mucous salivary glands |
| Uvula | Lining mucosa | Low | Minor mucous salivary glands in the CT |
| Soft Palate (nasal surface) | Respiratory epithelium | NA | Mucous glands in the CT |
| Uvula | Lining mucosa | Low | Mucous glands in the CT |
| Floor of the Mouth | Lining mucosa | Low | Minor mucous salivary glands in the CT |
| Tongue | |||
| Dorsal surface
Ventral surface |
Specialized mucosa embedded in masticatory mucosa Lining mucosa |
High
Low |
Taste buds; lingual papillae; serous, mucous, and mixed minor salivary glands |
CT, Connective tissue; NA, not applicable.
Ducts of the three pairs of major salivary glands (parotid, submandibular, and sublingual) open into the oral cavity, delivering saliva to moisten the mouth. These major salivary glands (see Chapter 18 ) also manufacture and release the enzyme salivary amylase to break down carbohydrates; lactoferrin and lysozymes , antibacterial agents; and secretory immunoglobulin ( IgA ). In addition, minor salivary glands , located in the connective tissue elements of the oral mucosa, add to the flow of saliva into the oral cavity. It is in the oral cavity that food is moistened with saliva, chewed, and formed, by the tongue, into spherical masses about 2 cm in diameter. These spherical masses, each known as a bolus , are forced by the tongue into the oral pharynx to be swallowed.
The lips form the anterior boundary, and the palatoglossal folds form the posterior boundary of the oral cavity. The structures of interest in and about the oral cavity are the lips, teeth and their associated structures, palate, and the tongue.
Clinical Correlations
Occasionally, bad breath ( halitosis ) occurs in almost everyone, especially when the individual’s mouth is dry. However, in approximately a quarter of all adults, halitosis is a constant phenomenon. This chronic condition is caused mostly by anaerobic bacteria that reside in the crevices of the posterior aspect of the tongue and in the gingival sulcus (the shallow groove between the teeth and gums). These bacteria emit noxious gases—such as mercaptans, hydrogen sulfide, and methyl and dimethyl sulfides—and as they digest decomposing food remnants, they form malodorous compounds, such as indoles and skatoles. In most cases, gentle scraping of the tongue surface, meticulous dental hygiene, and the use of mouthwashes containing zinc and certain ions can alleviate the bad breath for a few hours. Unfortunately, as yet, there is no permanent solution to the problem of chronic bad breath. In fewer than 10% of individuals with chronic halitosis, other factors may be involved, such as deep cavities in the teeth, tonsillar crypts, sinuses, nasal cavity, and the stomach.
Lip
The lip has three regions: the skin aspect, vermilion zone, and mucous (internal) aspect.
Entry into the oral cavity is guarded by the upper and lower lips . The core of the lips is composed of skeletal muscle fibers that are responsible for lip mobility. Each lip may be subdivided into three regions: the skin (external) aspect, vermilion zone, and mucous (internal, wet) aspect ( Fig. 16.1 ).
The skin aspect ( external aspect ) of the lip is covered with thin skin and is associated with sweat glands, hair follicles, and sebaceous glands. This region is continuous with the vermilion zone , which is also covered by thin skin. However, the vermilion zone has neither sweat glands nor hair follicles, although occasional, nonfunctional sebaceous glands are present in its dermis. The interdigitation between the epidermis and dermis ( the rete apparatus ) is highly developed so that the capillary loops of the dermal papillae are close to the surface of the skin, imparting a pink color to the vermilion zone. The absence of functional glands in this region necessitates the occasional moistening of the vermilion zone by the tongue.
The mucous aspect ( internal aspect ) of the lip is always wet and is lined by stratified squamous nonkeratinized epithelium. The subepithelial connective tissue is of the dense, irregular collagenous type and houses numerous, mostly mucous, minor salivary glands.
The core of the lip is composed of dense, irregular, collagenous connective tissue that surrounds bundles of skeletal muscle, the orbicularis oris and other muscles of facial expression that control the movements of the upper and lower lips.
Clinical Correlations
Dormant herpes simplex virus type 1 (HSV type 1) inhabits the ganglia of the fifth cranial nerve (CN V, trigeminal ganglion). Occasionally, the virus migrates from the ganglion along the nerve fibers, infecting the oral cavity and the lips, forming small blisters that rupture. These blisters release a clear virus-rich fluid that makes this condition, known as herpetic stomatitis , especially contagious. After the fluid is released, the blisters usually become ulcerated and exceptionally painful. Although herpetic stomatitis affects mostly children, when it affects adults, it becomes a more serious condition.
Teeth
Each tooth, whether deciduous or permanent, has a crown, a cervix, and a root.
Humans have two sets of teeth: 20 deciduous ( milk ) teeth , which are replaced by 32 permanent (adult) teeth composed of 20 succedaneous teeth (i.e., teeth that succeed their deciduous forerunners) and 12 molars that did not have deciduous counterparts. Both the deciduous and permanent dentitions are evenly distributed between the maxillary and mandibular arches.
The various teeth have different morphological features, numbers of roots, and functions. They assist in seizing prey, cutting smaller pieces from large chunks, and macerating the chunks to form a bolus. Only the general structure of teeth is discussed here.
Each tooth is suspended in its bony socket, the alveolus , by a dense, irregular, collagenous connective tissue, the periodontal ligament (PDL) . The gingiva also supports the tooth, and its epithelium seals the oral cavity from the subepithelial connective tissue spaces ( Fig. 16.2 ).
The portion of the tooth that is visible in the oral cavity is called the clinical crown , and the region housed within the alveolus is known as the root . The portion between the crown and the root is the cervix . The entire tooth is composed of three calcified substances, which enclose a soft, gelatinous connective tissue, the pulp , located in a continuous space subdivided into the pulp chamber and root canal. The root canal communicates with the PDL space via a small opening, the apical foramen, at the tip of each root. It is through this opening that blood and lymph vessels, as well as nerves, enter and leave the pulp ( Figs. 16.3 and 16.4 ).
Enamel, Dentin, and Cementum
Enamel, dentin, and cementum are the mineralized components of the tooth.
Enamel, dentin, and cementum are the mineralized structures of the tooth. Enamel covers dentin of the crown that surrounds the pulp chamber and cementum covers dentin of the root that surrounds the root canal. Thus, dentin is located both in the crown and the root and forms the bulk of the mineralized components of the tooth. Enamel and cementum meet each other at the cervix of the tooth.
Enamel
Enamel overlies dentin of the crown; it is composed of 96% calcium hydroxyapatite crystals and is the hardest substance in the body.
Enamel , the hardest substance in the body, consists of 96% calcium hydroxyapatite and 4% organic material and water. It is translucent; its coloration is due to the color of the underlying dentin. Enamel consists of large crystals. Each crystal is coated with a thin layer of keratin-like high-molecular-weight glycoproteins, tyrosine-rich enamelins , amelogenins , and ameloblastin .
Cells known as ameloblasts manufacture enamel daily in 4- to 8-μm segments known as rod segments that adhere to one another, forming cylindrical-like enamel rods ( enamel prisms ), which extend from the dentinoenamel junction (DEJ) to the enamel surface. The orientation of the crystals within rods varies so that the enamel rod has a cylindrical head to which a tail (interrod enamel), shaped like a rectangular solid, is attached. Because ameloblasts die before the tooth erupts into the oral cavity, enamel cannot be repaired by the body.
Clinical Correlations
Caries ( cavities ) usually result from the accumulation of microorganisms in and on slight defects of the enamel surface. As these bacteria metabolize nutrients in the saliva and on the tooth surface, they produce acids that begin to decalcify the enamel. As the bacteria proliferate in the cavity that they have “excavated,” they and the toxins that they release enlarge the caries.
Fluoride increases the hardness of enamel, especially in young individuals, making the enamel more resistant to caries. The incidence of cavities has been greatly reduced by the addition of fluoride to the public water supply and toothpastes and by its topical application in the dental office. As the individual ages, the enamel crystals enlarge, and there is less space available for the exchange of hydroxyl ions for fluoride ions. Therefore, the use of fluoride treatments in adults is not nearly as effective as it is for young children.
Because enamel is elaborated in daily segments during its formation, the quality of the enamel produced varies with the health of the mother during prenatal stages. However, in enamel formed after birth, the quality of the enamel depends on the health of the individual. The enamel rod thus mirrors the metabolic state of the mother or the individual during the time of enamel formation, resulting in successive rod segment sequences of hypocalcified and normally calcified enamel. These alternating sequences, analogous to growth rings in a tree trunk, are evident histologically and are called striae of Retzius .
The free surface of a newly erupted tooth is covered by a basal lamina–like substance, the primary enamel cuticle , manufactured by the same ameloblasts that elaborated enamel. This cuticle wears away shortly after the tooth’s emergence into the oral cavity.
Dentin
Dentin forms the bulk of the tooth; it is composed of 70% calcium hydroxyapatite and is the second hardest substance in the body.
Dentin , the second hardest tissue in the body ( Figs. 16.4 and 16.5 ), is somewhat elastic in nature, which acts to protect the brittle enamel covering it from becoming fractured. Dentin is yellow and composed of 65% to 70% calcium hydroxyapatite, 20% to 25% organic materials (specifically, type I collagen , proteoglycans, and glycoproteins), and 10% bound water.
Cells known as odontoblasts manufacture dentin and maintain their association with it for the life of the tooth. Odontoblasts are located at the periphery of the pulp; their cytoplasmic extensions, odontoblastic processes , occupy tunnel-like spaces within dentin. These extracellular fluid–filled spaces, known as dentinal tubules , extend from the pulp to the DEJ in the crown or dentinocemental junction in the root. Unlike ameloblasts, odontoblasts remain functional for the life of the tooth. Therefore, dentin has the capability of self-repair; reparative dentin is elaborated on the surface of preexisting dentin within the pulp cavity, reducing the pulp cavity’s size with age.
During dentinogenesis, odontoblasts manufacture about 4 to 8 μm of dentin every day. The quality of dentin, as of enamel, varies with the health of the mother prenatally or of the individual postnatally. Thus, along the length of the dentinal tubule, dentin displays alternating regions of normal calcification and hypocalcification. These are recognizable histologically as lines of Owen analogous to the striae of Retzius in enamel.
Clinical Correlations
Dentin sensitivity is mediated by sensory nerve fibers that are closely associated with odontoblasts, their processes, and the dentinal tubules. Disturbance of the tissue fluid within dentinal tubules is believed to depolarize the nerve fibers, sending a signal to the brain, where the signal is interpreted as pain.
Cementum
Cementum overlies dentin of the roots; it is composed of about 50% calcium hydroxyapatite and approximately 50% organic matrix and bound water. Therefore, it is approximately as hard as bone.
The third mineralized tissue of the tooth is cementum, a substance that is restricted to the root ( Figs. 16.3, 16.4, 16.6, and 16.7 ). Cementum is composed of 45% to 50% calcium hydroxyapatite and 50% to 55% organic material and bound water. Most of the organic material is composed of types I, III, and XII collagen, with associated glycosaminoglycans, proteoglycans, and glycoproteins.
The apical region of cementum is similar to bone in that it houses cells called cementocytes within lenticular spaces, known as lacunae . Processes of cementocytes extend from lacunae within narrow canaliculi that extend toward the vascular PDL. Because of the presence of cementocytes, this type of cementum is called cellular cementum . The coronal region of cementum is very thin and has no cementocytes and is called acellular cementum . Both cellular cementum and acellular cementum have cementoblasts , cells that are responsible for the formation of cementum. These cells lie on a thin layer of uncalcified cementum, known as cementum matrix , that covers cementum and abuts the PDL. Cementoblasts continue to elaborate cementum for the life of the tooth.
Type I collagen fibers of the PDL, known as Sharpey fibers , are embedded in cementum and in the alveolus. In this fashion, the PDL suspends the tooth within its bony socket. Odontoclasts ( cementoclasts ) are large, multinucleated cells that resemble osteoclasts and are able to resorb both cementum and dentin. During exfoliation, the replacement of deciduous teeth by their succedaneous counterparts, odontoclasts resorb cementum (and dentin) of the root.
Clinical Correlations
- 1.
Cementum does not resorb as readily as does bone, a property that orthodontists use to their advantage in moving improperly positioned teeth. By placing the correct force on a tooth, the orthodontist reshapes the bony socket, causing the tooth to be moved into its proper position.
- 2.
Cementum is continuously being elaborated, especially in the apical region of the root. This process compensates for the continuous eruption process of the tooth, which occurs in response to the abrasion of the occlusal surface due to the mechanical action of chewing. In order to maintain opposing teeth of the upper and lower arches in occlusion, the teeth must continuously erupt, albeit at a very slow rate. As the teeth move in an occlusal direction, the constant width of the periodontal ligament must also be maintained. All of these requirements are accomplished by the addition of cementum onto the root surface, especially in the region of the apex of the root. Due to the apposition of cementum, the diameter of the apical foramen becomes constricted, and, occasionally, even its location may be altered with age.
Pulp
Pulp, a richly vascularized and innervated loose connective tissue, is surrounded by dentin and communicates with the periodontal ligament via the apical foramen.
The pulp of the tooth, located in the pulp cavity, is a proteoglycans and glycosaminoglycans-rich loose, gelatinous connective tissue with some lymphatic elements and a very rich vascular and nerve supply. The pulp has two anatomical regions that are continuous with each other: the coronal pulp , located in the pulp chamber of the crown, and radicular pulp , located in the root canals of the roots. Blood vessels, lymph vessels, and nerve fibers exit and enter the pulp from the periodontal ligament via a small opening, the apical foramen , at the tip of each root. Therefore, teeth with multiple roots have an apical foramen at the tip of each root.
It is customary to subdivide the pulp histologically into three concentric zones around a central core of the pulp : the odontoblastic zone is composed of a single layer of odontoblasts whose processes, referred to as the odontoblastic process , extend into the adjacent dentinal tubules of dentin; the cell-free zone forms the layer deep to the odontoblastic zone, and as its name implies, is mostly devoid of cells; and the cell-rich zone , consisting of fibroblasts and mesenchymal cells, is the deepest zone of the pulp, adjacent to the pulp core ( Figs. 16.8 and 16.9 ).
The core of the pulp resembles most other loose connective tissues but lacks adipose cells. Another notable difference is that the pulp core is highly vascularized and, occasionally, coronal pulp houses calcified elements called pulp stones ( denticles ).
The nerve fibers of the pulp are of two types: (1) sympathetic (vasomotor) fibers control the luminal diameters of blood vessels, and (2) sensory fibers are responsible for the transmission of pain sensation. Most of these pain fibers are thin myelinated fibers that form Raschkow plexus , just deep to the cell-rich zone and are believed to be Aδ fibers that conduct sharp pain. Additionally, some nonmyelinated C fibers also enter the Raschkow plexus and are responsible for the conveyance of dull pain. As nerve fibers continue through this plexus, the Aδ fibers lose their myelin sheath, pass through the cell-free zone, and penetrate the space between odontoblasts to enter the dentinal tubule. Some nerve fibers synapse on the odontoblasts or their processes instead of entering the dentinal tubules. Others enter the dentinal tubule where they may or may not synapse with the odontoblastic process.
Clinical Correlations
Hemorrhage of the pulp is evident clinically as dark discoloration of the tooth. Because the pulp may recover, hemorrhage should not be the sole indicator for extirpation of the pulp. If the pulp is determined to be diseased, then endodontic procedures should be performed. Otherwise, it may become necrotic and the infection may spread to the adjacent periapical tissues of the periodontal ligament via the apical foramen (or accessory foramina).
Odontogenesis
Odontogenesis begins with the appearance of the dental lamina.
The first sign of odontogenesis (tooth development) occurs between the sixth and seventh weeks of gestation, when the ectodermally derived oral epithelium of the maxillary and mandibular arches undergoes mitotic activity ( Fig. 16.10 ), forming a horseshoe-shaped band of epithelial cells known as the dental lamina . The connective tissue surrounding the dental lamina is known as an ectomesenchyme because it is derived from neural crest material. The dental lamina and the ectomesenchyme are separated from each other by a well-defined basement membrane .
The process of tooth development is dependent on epithelial-mesenchymal interaction , accomplished by the synchronized release of a number of signaling molecules and gene products by the oral epithelium and the ectomesenchyme ( Table 16.2 ).
TABLE 16.2
Signaling Molecules and Gene Products During Odontogenesis
| Tissue | Signaling Molecules | Gene Products |
|---|---|---|
| Oral epithelium | Fibroblast growth factor 8 Transforming growth factor-β |
Wnt sonic hedgehog |
| Ectomesenchyme | Activin ßA Bone morphogenic protein 4 |
MSX-1 and MSX-2 or DLX-1 and DLX-2 |
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