The development of the human body is an extremely complex process that begins when an egg is fertilized by a sperm. It continues with a series of cell divisions, multiplications, and differentiation into various tissues and structures. A failure or disturbance that occurs during these processes may result in a lack, excess, or deformity of a body part. These disorders are called developmental disorders, or developmental anomalies.
Inherited disorders are different from developmental disorders in that they are caused by an abnormality in the genetic makeup (genes and chromosomes) of an individual and transmitted from parent to offspring through the egg or sperm. Inherited disorders are discussed in Chapter 6.
The complex process of proliferation and differentiation that takes place in the human body provides numerous possibilities for errors or defects in development. The head and neck region is a common location for such errors because of its intricate sequence and pattern of development. This chapter includes descriptions of developmental disorders of the face, oral cavity, and teeth with which the dental hygienist should be familiar.
Some of the developmental disturbances discussed in this chapter can be identified clinically, whereas others are identified by radiographic examination; still others require biopsy and microscopic examinations. A thorough clinical examination, including examination of extraoral and intraoral structures, is an essential component. Dental radiographs are an important part of the examination. Any developmental anomalies observed either clinically or radiographically are documented in the patient’s record even if no treatment is indicated. The patient is informed of all dental anomalies, their possible implications, and the treatment necessary, if any. In some instances referral to a specialist is indicated. To better understand these developmental disorders, a brief review of the embryonic development of the face, oral cavity, and teeth is included in this chapter.
Development of the face is a process of selective growth or proliferation and differentiation (Figures 5-1 and 5-2). During the third week of embryonic life, an invagination, or infolding, of the ectoderm forms the primitive oral cavity, which is called the stomodeum. Just above the stomodeum is a process called the frontal process, and just below it is a structure called the first branchial arch. Additional branchial arches form below the first branchial arch. All of the face and most of the structures of the oral cavity develop from either the frontal process or the first branchial arch. The first branchial arch divides into two maxillary processes and the mandibular process. The maxillary processes give rise to the upper part of the cheeks, the lateral portions of the upper lip, and part of the palate. The mandibular arch forms the lower part of the cheeks, the mandible, and part of the tongue.
As development continues, two pits (called olfactory pits) mark the future openings of the nose that develop on the surface of the frontal process. They divide the frontal process into three parts: (1) the median nasal process, (2) the right lateral nasal process, and (3) the left lateral nasal process. The lateral nasal processes form the sides of the nose, whereas the median nasal process forms the center and tip of the nose. Later, the median nasal process grows downward between the maxillary processes to form a pair of bulges called the globular process. This continues to grow downward, forming the portion of the upper lip called the philtrum. Most of these developments are completed by the end of the eighth week of embryonic life.
The area of the palate called the premaxilla develops from the globular process. The lateral palatine processes (left and right) are formed from the maxillary processes. These lateral palatine processes then fuse with the premaxilla. The fusion creates a Y-shaped pattern (see Figure 5-2). The nasal septum arises from the median nasal process. The right and left maxillary processes fuse together with the nasal septum at the center of the palate.
The tongue develops from the first three branchial arches. The second and third branchial arches are located just below the first branchial arch (Figure 5-1, B). The body of the tongue forms from the first branchial arch, and the base of the tongue forms from the second and third branchial arches.
Tooth development, or odontogenesis, in the human embryo begins at about the fifth week of embryonic life and involves both ectoderm and ectomesenchyme. The ectomesenchyme is derived from neural crest cells.
Odontogenesis begins with the formation of a band of ectoderm in each jaw called the primary dental lamina. Ten small knoblike proliferations of epithelial cells develop on the primary dental lamina in each jaw (Figure 5-3, A). Each of these proliferations extends into the underlying mesenchyme, becoming the early enamel organ for each of the primary teeth (Figure 5-3, B).
The tooth germ is composed of three parts: (1) the enamel organ, (2) the dental papilla, and (3) the dental sac, or follicle (Figure 5-3, C). The enamel organ develops from ectoderm, and the dental papilla and dental sac or follicle develop from mesoderm. Cell differentiation in the enamel organ progresses to produce ameloblasts, which form enamel. In the dental papilla odontoblasts are produced to form dentin. The permanent, or succedaneous, enamel organs form at the same time.
Formation of dental hard tissues occurs during the fifth month of gestation (Figure 5-4). Dentinogenesis is the formation of dentin. Dentin is the first mineralized tooth tissue to appear. When it begins to form, the mesenchymal tissue within the tooth germ is called the dental papilla. After dentin is produced, the dental papilla is called the dental pulp. Enamel is the product of the enamel organ. Enamel matrix begins to form shortly after dentin, and mineralization and maturation of enamel follow the formation of the matrix. Amelogenesis refers to the formation of enamel. The enamel is highly mineralized epithelial tissue and 90% of its volume is occupied by hydroxyapatite crystals.
The dental sac, or follicle, that surrounds the developing tooth germ provides cells that form cementum, the periodontal ligament, and alveolar bone. Cementogenesis (the formation of cementum) occurs after crown formation is complete. An epithelial structure called the Hertwig epithelial root sheath proliferates to shape the root of the tooth and induces the formation of the root dentin. Cells of the Hertwig epithelial root sheath must break up and pull away from the root surface before cementum can be produced. Very little cementum is produced until the tooth has erupted and is in occlusion and functioning. Root length is not completed until 1 to 4 years after the tooth erupts into the oral cavity.
Ankyloglossia is a developmental anomaly of the tongue. It is an extensive adhesion of the tongue to the floor of the mouth, caused by a short lingual frenum. The condition is often referred to as “tongue-tied.” Ankyloglossia is derived from the Greek words ankylos, meaning adhesion, and glossa, meaning tongue. This adhesion results from the complete or partial fusion of the lingual frenum to the floor of the mouth. It is four times more common in boys than girls. Total ankyloglossia is rare. Partial ankyloglossia appears clinically as a very short lingual frenum connecting the anteroventral portion of the tongue to the floor of the mouth (Figure 5-5). Patients with a short lingual frenum may exhibit no adverse effects, but some may have problems with speech. Gingival recession and bone loss can occur if the frenum is attached high on the lingual alveolar ridge. Surgical removal of a portion of the lingual frenum, known as frenectomy, is the usual treatment for ankyloglossia.
Commissural lip pits are epithelium-lined blind tracts located at the corners of the mouth (commissures) (Figure 5-6). These tracts may be shallow, or they may be several millimeters deep. They are a relatively common developmental anomaly. The cause of commissural lip pits is not clear; both the incomplete fusion of the maxillary and mandibular processes and the defective development of the horizontal facial cleft have been suggested. They are more often seen in adult males. The commissural lip pit may be observed during examination.
Another lip pit that occasionally may be seen is the congenital lip pit (paramedian lip pit). It occurs near the midline of the vermilion border of the lower lip and may appear as either a unilateral or bilateral depression. They are inherited as an autosomal dominant trait. No treatment is indicated for lip pits.
A Lingual thyroid, or ectopic lingual thyroid nodule, is a mass of thyroid tissue located on the tongue away from the normal anatomic location of the thyroid gland. It is an uncommon developmental anomaly that results from the failure of the primitive thyroid tissue to migrate from its developmental location in the area of the foramen cecum on the posterior portion of the tongue to its normal position in the neck.
Clinically, the lingual thyroid nodule appears as a smooth nodular mass at the base of the tongue posterior to the circumvallate papillae on or near the midline. It can be asymptomatic or can cause a feeling of fullness in the throat or difficulty in swallowing. Microscopically, the lingual thyroid is composed of immature or mature thyroid tissue.
On occasion, the size of the lingual thyroid necessitates its removal. However, this nodule may be the patient’s only functioning thyroid tissue. Therefore it is necessary to establish the presence of functioning thyroid tissue elsewhere before any nodular lesion located in the posterior aspect of the tongue is removed. If a normally located thyroid gland is lacking or nonfunctional, the lingual thyroid is not removed.
The cysts discussed in this chapter are all related to the development of the face, jaws, and teeth. Some have a distinctive microscopic appearance, and a definitive diagnosis is based on microscopic examination of the tissue. Other cysts are lined by less distinctive epithelium. The diagnosis of these cysts is based on both the microscopic appearance of the tissue and the location of the cyst.
The most common cyst observed in the oral cavity is caused by pulpal inflammation and is called a radicular cyst (periapical cyst). This cyst is described in Chapter 2. It develops from a preexisting periapical granuloma found at the apex of a nonvital tooth. The radicular cyst is always associated with a nonvital tooth and diagnosis can be made only through microscopic examination because the radiographic appearance can be similar to other lesions. A residual cyst is a radicular cyst that remains after extraction of the offending tooth. Because cysts are commonly observed in the jaws and surrounding soft tissues, the dental hygienist should understand their diagnostic criteria, pathogenesis, and prognosis. The preliminary identification of a cystic lesion is within the scope of dental hygiene practice.
Developmental cysts are classified as odontogenic (related to tooth development) and nonodontogenic (not related to tooth development). Cysts are also classified according to location, cause, origin of the epithelial cells, and microscopic appearance (Table 5-1). Developmental cysts can vary in size from small, asymptomatic lesions to large lesions that can cause expansion of bone. Very large and long-standing lesions can resorb tooth structure or move teeth. Oral cysts that occur within bone are called intraosseous cysts, and cysts that occur in soft tissue are called extraosseous cysts.
Radiographically, cysts within bone generally appear as well-circumscribed radiolucencies. All cysts may appear unilocular, but some are more likely to appear as multilocular radiolucencies. When a cyst is found within soft tissue, there is no radiographic feature.
A dentigerous cyst, also called a follicular cyst, forms around the crown of an unerupted or developing tooth (Figure 5-7). It is the second most common odontogenic cyst after the radicular cyst. The epithelial lining originates from the reduced enamel epithelium after the crown has completely formed and calcified. Fluid accumulates between the crown and the reduced enamel epithelium. The reduced enamel epithelium results from remnants of the enamel organ. The most common location for the dentigerous cyst is around the crown of an unerupted or impacted mandibular third molar. However, a dentigerous cyst may form around the crowns of other unerupted or impacted teeth such as the maxillary cuspid or a supernumerary tooth. The cyst may develop in both males and females. However, there is a higher incidence in males and this cyst is most often seen in young adults in their twenties and thirties. The size of these cysts can range from small and asymptomatic to very large. When large, it is capable of displacing teeth or causing a fracture of the mandible.
Radiographically, the dentigerous cyst appears as a well-defined, unilocular radiolucency around the crown of an unerupted or impacted tooth (Figure 5-8, A and B). Microscopically, the lumen is most characteristically lined with cuboidal epithelium surrounded by a wall of connective tissue (Figure 5-8, C). It may also be lined with stratified squamous epithelium. The lumen may be filled with a watery or serous fluid.
Treatment of a dentigerous cyst involves the complete removal of the cystic lesion and usually the tooth involved. If it is not removed, the cyst wall continues to enlarge. In addition, a risk exists that a neoplasm (i.e., ameloblastoma, mucoepidermoid carcinoma, intraosseous squamous cell carcinoma) may develop (see Chapter 7).
Because the tooth erupts through the cyst, this condition usually does not require treatment. On occasion, the dome of the eruption cyst is removed to expose the crown. The tooth is then allowed to erupt naturally.
A primordial cyst develops in place of a tooth and is most commonly found in place of the third molar or posterior to an erupted third molar (Figure 5-9). It originates from remnants and degeneration of the enamel organ. A history that the tooth was never present is an essential component of the diagnostic process.
Primordial cysts are most often seen in the young adult; no sex predilection has been reported. Clinically, the cyst usually is asymptomatic and is discovered on radiographic examination. Radiographically, it is a well-defined, radiolucent lesion that can be either unilocular or multilocular. The microscopic appearance and diagnosis of the primordial cyst may vary. Microscopically, the lumen is lined by stratified squamous epithelium surrounded by parallel bundles of collagen fibers. A layer of orthokeratin or parakeratin can cover the epithelium. The term primordial cyst simply refers to its development in place of a tooth. For this reason, biopsy and microscopic examination of primordial cysts are essential. Microscopically, a primordial cyst can be an odontogenic keratocyst or a lateral periodontal cyst.
Treatment of a primordial cyst involves surgical removal of the entire lesion. The prognosis depends on the histology; the risk of recurrence depends on the microscopic diagnosis. For example, if the cyst is microscopically an odontogenic keratocyst, the risk of recurrence is greater than if the cyst is lined by nonkeratinized stratified squamous epithelium.
An odontogenic keratocyst (OKC) is an odontogenic developmental cyst that is characterized by its unique microscopic appearance and frequent recurrence. The lumen is lined by epithelium that is 8 to 10 cell layers thick and surfaced by parakeratin. The basal cell layer is palisaded and prominent; the interface between the epithelium and the connective tissue is flat (Figure 5-10).
In 2005 the World Health Organization reclassified the odontogenic keratocyst as keratocystic odontogenic tumor on the basis of its aggressive behavior, unique histopathology, and specific genes identified in the epithelial cells. The diagnosis of odontogenic keratocyst is still frequently used and often both diagnoses are used together.
These cysts are most often seen in the mandibular third-molar region, and there is a slight predilection for males. Radiographically, the odontogenic keratocyst frequently appears as a well-defined, multilocular, radiolucent lesion (Figure 5-10, C). Unilocular lesions may also occur. The radiographic appearance of an odontogenic keratocyst can be identical to that of an odontogenic tumor. The odontogenic keratocyst can move teeth and resorb tooth structure but does not usually cause expansion of bone. The odontogenic keratocyst is also associated with nevoid basal cell carcinoma syndrome (Gorlin syndrome) (see Chapter 6).
Treatment of an odontogenic keratocyst is rather aggressive because of the high recurrence rate (Figure 5-10, D and E). The cyst generally extends beyond the borders that are seen on the radiograph because it extends between the trabeculae of bone. Therefore thorough surgical excision and osseous curettage are recommended. Careful follow-up and evaluation are essential.
The calcifying odontogenic cyst is a developmental nonaggressive cystic lesion lined by odontogenic epithelium that closely resembles the epithelium of the odontogenic tumor called an ameloblastoma; in addition, microscopically, it has a characteristic feature called ghost cells. In general, the cyst is treated conservatively and does not recur. A solid variant of the calcifying odontogenic cyst is called the odontogenic ghost cell tumor; it has been suggested that this solid variant represents a neoplasm rather than a cyst. Because of its microscopic resemblance to the ameloblastoma, the calcifying odontogenic cyst is described in detail in Chapter 7.
The lateral periodontal cyst is named for its location. It is seen most often in the mandibular cuspid and premolar area. It presents as an asymptomatic, unilocular or multilocular radiolucent lesion located on the lateral aspect of a tooth root (Figure 5-11, A). When multilocular, it is known as a botryoid cyst (Figure 5-11, B). Microscopically, the lateral periodontal cyst and the botryoid cyst show a thin epithelial lining with focal epithelial thickenings. The gingival cyst exhibits the same type of epithelial lining as the lateral periodontal cyst and is located in the soft tissue of the same area. The gingival cyst appears as a small bulge or swelling of the attached gingiva or interdental papillae (Figure 5-11, C).
The lateral periodontal cyst is found most often in males. No sex predilection has been reported for gingival cysts. Both the lateral periodontal cyst and the gingival cyst are treated by surgical excision.
The botryoid odontogenic cyst is closely related to the lateral periodontal cyst. The microscopic appearance is the same for both. The diagnosis botryoid odontogenic cyst is used for multicystic and multilocular variants. The botryoid odontogenic cyst is also most common in the mandibular cuspid and premolar area of the mandible. The botryoid odontogenic cyst is reported to have a greater recurrence potential than the lateral periodontal cyst. These are also generally treated by conservative enucleation of the cyst. A few cases of recurrence of lateral periodontal cysts have been reported. Some of these have been multilocular variants and were probably better classified as botryoid odontogenic cysts.
The glandular odontogenic cyst is a relatively recently defined odontogenic cyst. It was initially identified as a separate entity in 1988 and was called a sialo-odontogenic cyst. The name glandular odontogenic cyst was adopted by the World Health Organization in 1992 and this has become the preferred name for this cyst.
The glandular odontogenic cyst exhibits a distinctive microscopic appearance. They are usually multicystic lesions. Some of the microscopic features include eosinophilic cuboidal cells on the surface of the epithelium and in areas, columnar cells with cilia are seen elsewhere on the surface. Intraepithelial microcysts, or ductlike spaces, within the epithelium and clear epithelial cells and mucous cells are also noted. Epithelial thickenings showing whorls of cells similar to those in the lateral periodontal cyst and botryoid cyst are often also seen.
The glandular odontogenic cyst often presents as an enlargement of the bone; the posterior mandible and anterior maxilla are the most commonly reported locations. Both men and women may develop this cyst and there is a peak incidence in the fifth decade. The radiographic appearance may be unilocular but is often multilocular, appearing similar to that of an ameloblastoma or odontogenic keratocyst. When not completely surgically removed, these cysts have a high recurrence rate.
A nasopalatine canal cyst (incisive canal cyst) is a developmental cyst located within the nasopalatine canal or the incisive papilla. When found in the papilla, it is referred to as a cyst of the palatine papilla. This cyst arises from epithelial remnants of the embryonal nasopalatine ducts. The lesion is most commonly seen in adults between 40 and 60 years of age, and a strong predilection exists for males. The cyst is usually asymptomatic. There may be a small pink bulge near the apices and between the roots of the maxillary central incisors on the lingual surface. The adjacent teeth are usually vital.
Radiographically, the nasopalatine canal cyst is a well-defined, radiolucent lesion that is often heart shaped (Figure 5-12, A), resulting from the anatomic Y shape of the canal. When it is heart shaped, it is evenly distributed to the right and left of the midline.
Microscopically, the cyst is lined by epithelium that varies from stratified squamous to pseudostratified ciliated columnar epithelium (Figure 5-12, B). The connective tissue wall contains nerves and blood vessels that are normally found in the area and may also contain inflammatory cells.
Treatment of a nasopalatine canal cyst is surgical enucleation. It is especially important that surgery take place in the edentulous patient before the fabrication of a prosthesis. Recurrence is rare.
A median palatine cyst appears as a well-defined unilocular radiolucency and is located in the midline of the hard palate (Figure 5-13). The cyst is thought to be a more posterior form of a nasopalatine canal cyst. Microscopically, the median palatine cyst is lined with stratified squamous epithelium that is surrounded by dense fibrous connective tissue. The median palatine cyst is treated by surgical enucleation. The prognosis is good, and recurrence is rare.
Radiographically, a globulomaxillary cyst is a well-defined, pear-shaped radiolucency found between the roots of the maxillary lateral incisor and cuspid (Figure 5-14). Although it was once thought to be a developmental fissural cyst, it is now believed to be of odontogenic epithelial origin. The size of the lesion can vary; however, when it is large enough, a divergence of the roots can result. The adjacent teeth are usually vital. Pulp testing can rule out a periapical cyst or periapical granuloma. Surgical enucleation of the globulomaxillary cyst is recommended. The diagnosis is determined by the microscopic evaluation of the lesion. The prognosis and recurrence depend on the final diagnosis.
A median mandibular cyst is a rare lesion. As its name indicates, it is located in the midline of the mandible. The origin of the median mandibular cyst is also unclear. Some believe it to be of odontogenic origin, ranging from odontogenic cysts to tumors. Because no mandibular midline fusion occurs, there is no evidence of epithelial entrapment. The surrounding teeth are vital. Radiographically, a well-defined radiolucency is seen below the apices of the mandibular incisors. A median mandibular cyst is treated by surgical removal; the prognosis is good, and recurrence is rare.
A nasolabial cyst is a soft tissue cyst with no alveolar bone involvement. The origin of this cyst is uncertain. At present it is thought to originate from the lower anterior portion of the nasolacrimal duct. The cyst is observed in adults 40 to 50 years of age; a strong predilection (4 : 1) exists for females.
Clinically, there may be an expansion or swelling in the mucolabial fold in the area of the maxillary canine and the floor of the nose. Usually no radiographic change is associated with this cyst. However, when the lesion is large enough, expansive pressure can cause the resorption of bone (Figure 5-15). Microscopically, the cyst is lined with pseudostratified, ciliated columnar epithelium and multiple goblet cells. Treatment of a nasolabial cyst is surgical excision; the prognosis is good, and recurrence is rare.
A cervical lymphoepithelial cyst (branchial cleft cyst) is located on the lateral neck at the anterior border of the sternocleidomastoid muscle. It is composed of a stratified squamous epithelial lining surrounded by a well-circumscribed component of lymphoid tissue and connective tissue (Figure 5-16, B). It appears to arise from epithelium entrapped in a lymph node during development rather than associated with a branchial cleft.
The intraoral lymphoepithelial cyst is most commonly seen on the floor of the mouth and the lateral borders of the posterior tongue (Figure 5-16, A). This lymphoepithelial cyst appears as a pinkish-yellow, raised nodule when seen intraorally.
An epidermal cyst presents as a raised nodule in the skin of the face or neck. Microscopically, the cyst is lined by keratinizing epithelium that resembles the epithelium of skin (epidermis). The cyst lumen is usually filled with keratin scales. Most epidermal cysts are thought to originate from the epithelium of the hair follicle. On occasion, when located in the skin of the cheek, the nodule may be noted intraorally from the buccal mucosal aspect and the skin. An epidermoid cyst is treated by surgical excision, and the prognosis is good.
A dermoid cyst is a developmental cyst that is often present at birth or noted in young children. It is more common in other parts of the body than in the head and neck. When the dermoid cyst occurs in the oral cavity, it is usually found in the anterior floor of the mouth. The cyst may cause displacement of the tongue and may have a doughlike consistency when palpated.
Microscopically, the dermoid cyst is lined by orthokeratinized, stratified squamous epithelium surrounded by a connective tissue wall. The lumen is usually filled with keratin. Hair follicles, sebaceous glands, and sweat glands may be seen in the cyst wall. A benign cystic teratoma has a cystic component that resembles the dermoid cyst. In addition, teeth, bone, muscles, and nerve tissue may be found in the wall of this lesion. Teeth are usually not found in the malignant form of the teratoma. Treatment of the dermoid cyst is surgical excision; the prognosis is good, and malignant transformation has been reported but is extremely rare.
A thyroglossal tract (duct) cyst forms along the same tract that the thyroid gland follows in development, from the area of the foramen cecum to its permanent location in the neck (Figure 5-17). Most of these cysts occur below the hyoid bone. The epithelial lining varies with location. Cysts above the hyoid bone are usually lined with stratified squamous epithelium, and those below the hyoid bone with ciliated columnar epithelium. Thyroid tissue may also be found within the connective tissue wall.