antigenic that the response is not effective. This may explain why Streptococcus sanguis and Streptococcus mitior are able to remain constant members of the oral cavity throughout life. If a patient is treated with immunosuppressive drugs (e.g., used in bone marrow transplants), the secretion of sIgA is reduced, and the oral flora may become invaded with organisms from the gut. The presence of large numbers of gut organisms in the oral cavity causes a mucositis which is difficult to control. This mucositis suggests a wider protective role of sIgA. sIgA may be most effective in controlling the population of exogenous organisms in the mouth (see Chapter 4.7.3 Antigen Tolerance).
Fig. 4.6 Diagrammatic representation of the action of sIgA on salivary organisms. (1) some organisms are unaffected by sIgA and adhere selectively to salivary pellicle, while others (2) adhere to oral mucosal surfaces. IgA causes clumping (3) of susceptible bacteria and blocks sites (4) of adhesion to other organisms. Some bacteria (5) are able to split the IgA dimer.
4.2.3 Rate of Flow of Saliva
The mouth is bathed by a resting flow of saliva which during the day is a total of about 1 L. This ensures that the mouth and throat are kept moist at all times. During sleep the rate of flow is almost unmeasurable and accounts for the dryness of the mouth on waking. In other circumstance, particularly feeding, the rate of secretion of saliva increases. The following factors are important in the stimulation of saliva which is driven by the activity of both sympathetic and parasympathetic nerve pathways.
• Taste: Taste is the most potent salivary stimulant. Different modalities vary in their power to stimulate saliva flow. Sour food ranks highest through salt and sweet to bitter tasting foods. One of the functions of the water in saliva is to dissolve and spread food chemicals (see Chapter 10.6 Taste).
• Mastication: When food is held between the teeth, a little saliva flows from the ipsilateral (same side) glands. As further bite force is exerted, receptors in the periodontal ligament send impulses to the brain and the rate of flow is increased. These events have been described as the masticatory–salivary reflex.
• Touch: Stimulation of nerve endings in the oral mucosa, particularly by spicy or irritating substances, produces an increase in flow rate of saliva. Hot or cold water also has a stimulating effect. In some areas of the oral cavity, such as the soft palate, even light touch causes an increased flow of saliva. This may be a response to enable swallowing and a protective response to the onset of retching.
• Sight, smell, and thought: The presence of food, or even the thought of it, increases salivary secretions. Some foods like lemons or onions have a particularly strong effect even at a distance from the mouth.
• Nausea and vomiting: Copious amounts of saliva are released during vomiting. This may be a protective reflex, protecting the oral cavity from stomach acids which are sufficiently concentrated to damage the oral mucosa and etch tooth enamel.
• Nutrition: Malnutrition causes irreversible damage to the salivary glands of rats if it occurs during infancy. During later years, malnutrition in human children has been shown to reduce the rate of saliva secretion and the protein content.
• Drugs: Several drugs which are mood altering (antidepressants and tranquilizers) reduce salivary secretions.
• Ageing: During ageing all mucosal secretions are reduced, and this may lead, in the oral cavity, to the onset of root surface caries.
• Radiation: One of the undesirable side effects of radiation treatment for oral tumors is damage to the acinar cells of the salivary glands. The xerostomia (dry mouth) which develops is usually permanent and requires special preventive measures to be put in place in order to prevent caries in patients who have had radiation treatment.
Saliva is an active secretion from both major and minor salivary glands. The three major salivary glands are the parotid, submandibular, and sublingual glands. The composition of saliva varies according to the types of secretory gland (see Appendix D.2 Salivary Gland Secretion).
4.2.4 Gingival Crevicular Fluid
Filter paper inserted into the gingival sulcus soon becomes saturated with crevicular fluid. This fluid derives from the connective tissue of the gingiva, passes through spaces in the junctional epithelium, and enters the gingival sulcus. The rate of flow and composition of the gingival crevicular fluid (GCF) are influenced by the following factors:
• The fluid flow is minimal in the morning and reaches a maximum at the end of the day.
• GCF flow is increased by tooth brushing and mastication.
• Pregnancy causes the rate of GCF to increase. There is an increase in gingival inflammation represented by a tendency for gingival bleeding during pregnancy.
• Fluid flow is minimal during the middle of the menstrual period.
• GCF flow is increased in diabetics.
• One of the earliest signs of gingival inflammation is an increase in GCF flow. There is some correlation between the rate of flow and the severity of the gingival inflammation.
The spaces between the cells of the junctional epithelium comprise 18% of its volume. These spaces are large enough to allow the passage of large molecules and cells (see Chapter 3.7 Junctional Epithelium). Thus, bacteria and their toxic products and other antigens may enter the gingival connective tissue through the junctional epithelium. This constant minor invasion causes a mild inflammatory state in the lamina propria of the junctional epithelium. Hence, neutrophils which have migrated into the junctional epithelium in response to the presence of bacterial products are flushed out into the GCF fluid. From this route, 25,000 neutrophils enter the mouth every 15 minutes. Other cells found in the GCF are desquamated epithelial cells.
In health, the GCF is neither exactly like serum or an inflammatory exudate. There are slightly higher concentrations of inorganic ions than in serum and higher levels of carbohydrates. The serum immunoglobulins, IgG, IgM, and IgA, are all found in GCF (not the secretory version of IgA which is found in saliva). Their presence may regulate the entrance of bacteria into the epithelium. One of the functions of antibodies is in triggering the complement cascade of reactions which produce enzymes capable of inflicting damage to the cell walls of bacteria. These enzymes are also capable of damaging host cells, and this may be another reason for the inflammatory state of the lamina propria under the junctional epithelium. The enzymes found in GCF are evidence of the battle between invading bacteria and the defending neutrophils. For example, there is acid phosphatase (from cell breakdown), glucuronidase, lysozyme, hyaluronidase, and collagenases.
Review Questions
1. What do you understand by the term keystone organism?
2. What features of the oral cavity make it possible to describe it as an ecosystem?
3. What makes saliva supersaturated?
4.
