UPPER DIGESTIVE SEGMENT

Mouth (oral cavity)

The oral cavity is the entrance to the digestive tube. Ingestion, partial digestion and lubrication of the food, or bolus, are the main functions of the mouth and its associated salivary glands. We study the salivary glands in Chapter 17 , Digestive Glands.

The mouth, or oral cavity, includes the lips, cheeks, teeth, gums (gingivae), tongue, uvula and palate. The various regions of the oral cavity are lined by three types of mucosae with structural variations:

• 1.

  Lining mucosa (lips, cheeks, ventral surface of the tongue, soft palate, mouth floor and alveolar mucosa).

• 2.

  Masticatory mucosa (gingiva and hard palate).

• 3.

  Specialized mucosa (dorsal surface of the tongue).

There are three transition sites of the oral mucosa:

• 1.

  The mucocutaneous junction (between the skin and the mucosa of the lips).

• 2.

  The mucogingival junction , between the gingiva and alveolar mucosa. The junction, called mucogingival groove , displays changes in the epithelium and the lamina propria. The gingiva mucosa is lined by a keratinized stratified squamous epithelium supported by collagen bundles anchored to the periosteum. The alveolar mucosa consists of a thick non-keratinized epithelium supported by a loose lamina propria containing elastic fibers.

• 3.

  The dentogingival junction (between the mucosa of the gingiva and the enamel of the tooth). It is a sealing point that prevents periodontal diseases.

Except for the teeth, the mouth is lined by a stratified squamous epithelium, with a submucosa, consisting of loose connective tissue, blood vessels and nerves, present only in certain regions (cheeks, lips and part of the hard palate). The oral epithelium includes aggregates of lymphoid tissue. They represent the primary barrier to pathogens.

In some regions, such as the gingiva and parts of the hard palate, the oral mucosa is firmly attached to the periosteum of the subjacent bone. This arrangement is called mucoperiosteum . The oral mucosa lacks a muscularis mucosa.

Lips ( 15-1 )

The lips consist of three regions:

• 1.

  The cutaneous region .

• 2.

  The red or vermilion region .

• 3.

  The oral mucosa region .

The cutaneous region is covered by thin skin with tall dermal papillae (keratinized stratified squamous epithelium with hair follicles and sebaceous and sweat glands). The red or vermilion region is lined by a stratified squamous epithelium supported by connective tissue containing blood vessels responsible for the red color of this region.

Salivary glands are not present in the mucosa of the vermilion region. This region dries out and becomes cracked in cold weather. A sharp vermilion border separates the skin from the vermilion region.

The oral mucosa region, which is continuous with the mucosa of the cheeks and gums, displays minor salivary glands.

The stratified squamous epithelium covering the inner surface of the lips and cheeks is non-keratinized and supported by a dense lamina propria (lining mucosa) and a submucosa, closely bound by connective tissue fibers to the underlying skeletal muscles.

Gingiva, hard and soft palate

Masticatory mucosa covers the hard palate and gingivae and sustains abrasion during food mastication. The gums , or gingivae , are similar to the red region of the lips, except on the free margin, where significant keratinization is seen. The lamina propria of the gums binds tightly to the periosteum of the alveolar processes of the maxillae and mandible and to the periodontal membrane. The gums lack submucosa or glands.

The hard palate is lined by a keratinizing stratified squamous epithelium similar to that of the free margins of the gums. A submucosa is present in the midline but absent in the area adjacent to the gums. Collagenous fibers in the submucosa bind the mucosa to the periosteum of the hard palate, enabling the mucosa to resists shear forces and compression. Areas of fat and glandular tissue cushion the mucosa to protect nerves and blood vessels of the hard palate.

The soft palate and uvula are lined by a non-keratinized stratified squamous epithelium, extending into the oropharynx where it becomes continuous with the pseudostratified ciliated columnar epithelium of the upper respiratory tract. The submucosa is loose and contains abundant mucous and serous glands. Skeletal muscle fibers are present in the soft palate and uvula.

Tongue ( 15-2 )

The anterior two-thirds of the tongue consists of a core mass of skeletal muscle oriented in three directions: longitudinal, transverse and oblique. The posterior one-third displays aggregations of lymphatic tissue, the lingual tonsils.

The dorsal surface of the tongue is covered by a specialized mucosa consisting of a non-keratinized stratified squamous epithelium supported by a lamina propria associated with the muscle core of the tongue.

Serous and mucous glands extend across the lamina propria and the muscle. Their ducts open into the crypts and furrows of the lingual tonsils and circumvallate papillae , respectively.

The dorsal surface of the tongue contains numerous mucosal projections called lingual papillae (see 15-2 ). 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 (wall-like).

• 4.

  Foliate papillae (leaf-shaped), rudimentary in humans but well developed in rabbits and monkeys.

Taste buds are found in all lingual papillae except the filiform papillae .

Taste buds are barrel-shaped structures embedded in the stratified epithelium of the tongue, palate and epiglottis. Each taste bud, depending on the type and location, consists of 50 to 150 elongated chemosensory cells, called taste receptor cells , extending from the base of the taste bud to a taste pore. The basal portion of the taste receptor cells makes contact with an afferent nerve terminal derived from neurons in the sensory ganglia of the facial, glossopharyngeal and vagus nerves.

Taste receptor cells have a life span of 10 to 14 days. Sweet, sour, bitter and salty are the four classic taste sensations. A fifth taste is umami (the taste enhanced by monosodium glutamate).

Circumvallate 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; 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 papillae. The ducts of Ebner's glands open into the floor of the circular furrow.

The sides of the circumvallate papillae and the facing wall of furrow contain several taste buds.

Types and function of taste receptor cells ( 15-3 )

A taste bud consists of three cell types:

• 1.

  Type I taste receptor cells represent about 50% of the total cells of a taste bud. They have a glial-like supporting function.

• 2.

  Type II taste receptor cells (about one-third of the taste bud population) have chemosensory receptors for sugars, amino acids and/or bitter tastants. They express taste G protein-coupled receptors type 1 (T1R) or T2R. T1R1, T1R2 and T1R3 subtype receptors are coexpressed in taste bud cells.

  Type II cells lack synaptic vesicles and communicate with type III cells through receptors for adenosine triphosphate (ATP), serotonin and GABA (γ-aminobutyric acid).

• 3.

  Type III taste receptor cells , the less numerous cell type, do not express taste G protein-coupled receptors but can detect sour taste by a T1R/T2R-independent mechanism. Type III cells have synaptic regions containing synaptic vesicles.

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.

When a sweet tastant diffuses through the taste pore of a taste bud, it interacts with a T1R present in the apical microvilli of the taste receptor cells. Taste receptors can form heterodimers (T1R2+T1R3) or homodimers (T1R3+T1R3).

T1Rs are linked to the G-proteinα, βand γsubunit complex, called gustducin . Binding of the αsubunit of the G-protein complex to phospholipase

C (PLC) triggers the production of the second messengers inositol triphosphate (IP3) and diacylglycerol (DAG) which activate ion channels in the taste receptor cells.

An influx of Na ⁺ within taste cells causes depolarization of the taste receptor cells. An increase in intracellular Ca ²⁺ , released from intracellular storage sites, triggers the release into the extracellular space of ATP and neurotransmitters at the synapses with gustatory afferent nerve terminals.

In summary , taste receptor cells can detect and discriminate sweet, bitter or umami tastants by cell depolarization, gustducin–dependent Ca ²⁺ and Na ⁺ signaling and release of ATP and neurotransmitters.

The salty taste of Na ⁺ is detected by direct influx of Na ⁺ through membrane ion channels to depolarize the plasma membrane.

Some taste receptor cells respond to only one of the basic taste substances. Others are sensitive to more than one taste substance.

Tooth ( 15-4 )

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 (see 15-4 ). 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.

Tooth development ( 15-5 )

The ectoderm, cranial neural crest and mesenchyme contribute to the development of the tooth. Ameloblasts derive from the ectoderm. Odontoblasts derive from the cranial neural crest. Cementocytes derive from the mesenchyme.

Secreted signaling molecules, activin βA, fibro-blast growth factor and bone morphogenetic proteins, mediate the interaction between the dental epithelium and the mesenchyme during tooth morphogenesis.

A step-by-step description of the relevant steps of tooth development can be found in 15-5 . The sequential steps include:

• 1.

  The bud stage , in which neurodermal cells induce the overlying ectodermal placode cells to proliferate and form the epithelial tooth bud.

• 2.

  The early cap stage , defined by the proliferation of cells of the epithelial tooth bud that invaginate into the underlying mesoderm to form the early cap.

• 3.

  The late cap stage , characterized by cells at the growing end of the dental bud, forming a cap-like structure. The epithelial tooth bud is now lined by the outer and inner dental epithelium . Note that the bud of the future permanent tooth develops from the dental lamina which links the developing cup with the overlying ectodermal epithelium. An important event is the emergence of the enamel knot , an indication that tooth development has started.

• 4.

  During the bell stag e, the outermost cells of the primitive dental papilla, close to the enamel knot site, differentiate into dentin-producing odontoblasts . A single layer of enamel-secreting ameloblasts develops in the inner dental epithelium portion of the enamel knot.

  Enamel, produced by ameloblasts, moves downward and dentin moves outward . Odontoblasts produce non- mineralized predentin , which later calcifies to form dentin . The primitive dental papilla becomes the dental pulp .

• 5.

  Tooth eruption marks the completion of tooth development. It is important to emphasize that ameloblasts of the erupted tooth disappear and enamel can no longer be replaced.