The dorsal surface of the tongue contains numerous mucosal projections called lingual papillae (Figure 15-1). Each lingual papilla is formed by a highly vascular connective tissue core and a covering layer of stratified squamous epithelium. According to their shape, lingual papillae can be divided into four types: (1) filiform papillae (narrow conical), the most abundant; (2) fungiform papillae (mushroom-shaped); (3) circumvallate papillae; and (4) foliate papillae (leaf-shaped), rudimentary in humans but well developed in rabbits and monkeys.

Figure 15-1 Tongue

Taste buds are found in all lingual papillae except the filiform papillae. Taste buds are barrel-shaped epithelial structures containing chemosensory cells called gustatory receptor cells. Gustatory receptor cells are in synaptic contact with the terminals of the gustatory nerves.

Circumvallate (wall-like) papillae are located in the posterior part of the tongue, aligned in front of the sulcus terminalis. The circumvallate papilla occupies a recess in the mucosa and, therefore, it is surrounded by a circular furrow or trench.

Serous glands, or Ebner’s glands, in the connective tissue, in contact with the underlying muscle, are associated with the circumvallate papilla. The ducts of Ebner’s glands open into the floor of the circular furrow.

The sides of the circumvallate papilla and the facing wall of furrow contain several taste buds. Each taste bud, depending on the species, consists of 50 to 150 cells, with its narrow apical ends extending into a taste pore. A taste bud has three cell components (Figure 15-2): (1) taste receptor cells, (2) supporting cells (or immature taste cells), and (3) precursor cells (or basal cells).

Figure 15-2 Taste bud

Taste receptor cells have a life span of 10 to 14 days. Precursor cells give rise to supporting cells (or immature taste cells) which, in turn, become mature taste receptor cells. The basal portion of a taste receptor cell makes contact with an afferent nerve terminal derived from neurons in the sensory ganglia of the facial, glossopharyngeal, and vagus nerves.

Sweet, sour, bitter, and salty are the four classic taste sensations. A fifth taste is umami (the taste of monosodium glutamate). A specific taste sensation is generated by specific taste receptor cells. The facial nerve carries the five taste sensations; the glossopharyngeal nerve carries sweet and bitter sensations.

Taste is initiated when soluble chemicals, called tastants, diffuse through the taste pore and interact with the G-protein α, β, and γ subunits (called gustducin) linked to the taste receptors (designated TR1 and TR2), present in the apical microvilli of the taste receptor cells. As we discussed in Chapter 3, Cell Signaling, guanosine triphosphate (GTP) binding to the α subunit of the G-protein complex activates target molecules (ion channels in the taste receptor cells). Ionic changes within taste cells cause either depolarization (see Figure 15-2) or hyperpolarization of the receptor cells. An increase in intracellular Ca²⁺ triggers the release of neurotransmitters at the afferent synapse with the afferent nerve terminal. Some taste receptor cells respond to only one of the basic taste substances. Others are sensitive to more than one taste substance.

TOOTH

In the adult human, dentition consists of 32 permanent teeth. The 16 upper teeth are embedded in alveolar processes of the maxilla. The lower 16 teeth are embedded in similar alveolar processes of the mandible. The permanent dentition is preceded by a set of 20 deciduous teeth, also called milk or baby teeth. Deciduous teeth appear at about 6 months of age and the entire set is present by age 6 to 8 years. The deciduous teeth are replaced between ages 10 and 12 by the 32 permanent teeth. This replacement process ends at about age 18.

Each of the several types of teeth has a distinctive shape and function: incisors are specialized for cutting; canines, for puncturing and holding; and molars, for crushing.

Each tooth consists of a crown and either single or multiple roots (Figure 15-3). The crown is covered by highly calcified layers of enamel and dentin. The outer surface of the root is covered by another calcified tissue called cementum. The dentin forms the bulk of the tooth and contains a central chamber filled with soft tissue, the pulp. The pulp chamber opens at the apical foramen into the bony alveolar process by the root canal. Blood vessels, nerves, and lymphatics enter and leave the pulp chamber through the apical foramen. Myelinated nerve fibers run along with the blood vessels.

Figure 15-3 Longitudinal section of the tooth

TOOTH DEVELOPMENT

The ectoderm, cranial neural crest, and mesenchyme contribute to the development of the tooth (Figure 15-4). Ameloblasts derive from the ectoderm. Odontoblasts derive from the cranial neural crest. Cementocytes derive from the mesenchyme.

Figure 15-4 Stages of tooth development

Secreted signaling molecules—activin βA, fibroblast growth factor, and bone morphogenetic proteins—mediate the interaction between the dental epithelium and the mesenchyme during tooth morphogenesis. Figure 15-4 illustrates the relevant steps of tooth development.

ODONTOBLASTS

A layer of odontoblasts is present at the periphery of the pulp. Odontoblasts are active secretory cells that synthesize and secrete collagen and noncollagenous material, the organic components of the dentin.

The odontoblast is a columnar epithelial-like cell located at the inner side of the dentin, in the pulp cavity (Figure 15-5). The apical cell domain is embedded in predentin, a nonmineralized layer of dentin-like material. The apical domain projects a main apical cell process that becomes enclosed within a canalicular system just above the junctional complexes linking adjacent odontoblasts.

Figure 15-5 Odontoblasts

A well-developed rough endoplasmic reticulum and Golgi apparatus as well as secretory granules are found in the apical region of the odontoblast. The secretory granules contain procollagen. When procollagen is released from the odontoblast, it is enzymatically processed to tropocollagen, which aggregates into type I collagen fibrils.

Predentin is the layer of den tin adjacent to the odontoblast cell body and processes. Predentin is nonmineralized and consists mainly of collagen fibrils that will become covered (mineralized) by hydroxyapatite crystals in the dentin region. A demarcation mineralization front separates predentin from dentin. Dentin consists of 20% organic material, mainly type I collagen; 70% inorganic material, mainly crystals of hydroxyapatite and fluoroapatite; and 10% water.

Coronal dentin dysplasia (also known as dentin dysplasia, type II) is a rare inherited autosomal defect characterized by abnormal development of dentin, extremely short roots (rootless teeth), and obliterated pulp chambers.

The pulp consists of blood vessels, nerves, and lymphatics surrounded by fibroblasts and mesenchyme-like extracellular elements. Blood vessels (arterioles) branch into a capillary network among the cell bodies of the odontoblasts. An inflammation in the pulp causes swelling and pain. Because there is no space for swelling in the pulp cavity, the blood supply is suppressed by compression, leading rapidly to the death of the pulpal cells.

Cementum

The cementum is a bonelike mineralized tissue covering the outer surface of the root. Like bone, the cementum consists of calcified collagenous fibrils and trapped osteocyte-like cells called cementocytes.

The cementum meets the enamel at the cementoenamel junction and separates the crown from the root at the neck region of the tooth. The outermost layer of the cementum is uncalcified and is produced by cementoblasts in contact with the periodontal ligament, a collagen- and fibroblast-rich and vascularized suspensory ligament holding the tooth in the sockets of the alveolar bone (see Figure 15-3). The strength of the periodontal ligament fibers gives teeth mobility and strong bone attachment, both useful in orthodontic treatment.

AMELOBLASTS

Ameloblasts are enamel-producing cells present only during tooth development. The ameloblast (Figure 15-6) is a polarized columnar cell with mitochondria and a nucleus present in the basal region of the cell. The supranuclear region contains numerous cisternae of rough endoplasmic reticulum and Golgi apparatus.