Chapter 8. Investing organic layers on enamel surfaces
Overview
Throughout its life, the crown of a tooth is covered by an organic layer or integument. Before the tooth erupts into the oral cavity the crown is covered by the overlying oral mucosa, the coronal part of the dental follicle, and the vestiges of the enamel organ (plus its associated primary enamel cuticle). After emerging into the mouth, parts of the integument of enamel organ origin are lost by degeneration of its epithelial component and by attrition or abrasion of the underlying cuticular component. In the region of the gingival crevice or sulcus, the primary (or pre-eruptive) enamel cuticle acquires additional matter from the lining epithelium and, coronal to the gingival margin, from saliva. The salivary layer is known as the acquired pellicle. Oral bacteria adhere initially to the enamel cuticle and later to the acquired pellicle, to form the dental plaque. Dental plaque may mineralize to form dental calculus.
Learning objectives
You should:
• know the origins of the acquired pellicle
• understand the mechanisms of attachment of bacteria and proteins to the acquired pellicle, leading to plaque formation
• appreciate how different dietary carbohydrates influence plaque matrix and how that matrix affects cariogenicity
• know how dental calculus is formed.
Soft tissues covering an erupting tooth
The soft tissues covering an erupting tooth comprise oral mucosa and the subjacent connective tissue of the dental follicle. Between the dental follicle and the enamel is an epithelial layer that is the remains of the enamel organ — the reduced enamel epithelium; a basal lamina (primary enamel cuticle) is interposed between the enamel surface and the reduced enamel epithelium. The reduced enamel epithelium and the primary enamel cuticle comprise Nasmyth’s membrane.
The erupted healthy tooth
A number of organic layers cover the erupted healthy tooth.
Primary enamel cuticle
The original reduced enamel epithelium is lost, leaving the primary enamel cuticle initially covering the exposed enamel. This cuticle immediately acquires an organic element of salivary origin, the acquired pellicle. The primary enamel cuticle is in intimate contact with the underlying organic enamel matrix. Generally approximately 30 nm thick, the cuticle acquires accretions in the region of the gingival crevice, which derive from crevicular epithelium and from plasma and may increase the cuticle to about 5 μm thick.
Acquired pellicle
Where the enamel surface is exposed to wear, either by attrition or abrasion, the vestigial enamel organ is worn away, but the enamel rapidly acquires a layer of acquired pellicle. Indeed, this pellicle always forms a protective coat following any wear. This acellular layer is derived mainly from salivary proteins, but includes elements from crevicular fluid and bacteria.
Dental plaque
Dental plaque is the combination of bacteria embedded in a matrix of salivary proteins and bacterial products superimposed on the acquired pellicle. Dental plaque is an example of a biofilm, a term used to describe communities of microbes attached to surfaces. Early plaque is composed of mainly Gram-positive, facultative, anaerobic cocci and filaments. With time, the deposit will thicken, although in non-pathological, supragingival situations its microfloral composition is unlikely to vary greatly. Plaque can be described as a soft, adherent, predominantly microbial mass which accumulates on the tooth surface in the absence of oral hygiene measures. Although it contains a large variety of micro-organisms, it is also composed of a rich organic matrix, derived from saliva, foodstuffs and microbial metabolism.
Mechanisms of plaque formation
It is clear that there are distinct stages to the formation of plaque:
• Firstly, there is the initial transport of bacteria to tooth surface.
• This is followed by reversible adsorption of the bacteria on to the pellicle surface.
• Less reversible attachment of bacteria to tooth surface then occurs.
• These stages are repeated, forming new layers of bacterial adherence.
• Finally, there is growth of the attached organisms, leading to the formation of a ‘climax’ community.
The transport of micro-organisms to the pellicle-coated tooth is via passive means, and local variations in the chemical composition of the pellicle can influence the pattern of microbial deposition.
Attachment of bacteria
Initial reversible attachment of bacteria to the tooth surface involves the formation of long-range, physicochemical interactions between micro-organisms and pellicle proteins. Such interactions include electrostatic interactions (ionic and hydrogen bonds), Van der Waals forces (induced dipole formation) and hydrophobic bonds. As micro-organisms are generally negatively charged due to nature of the cell surface molecules and acidic proteins are present in acquired pellicle, these interactions usually involve the formation of calcium bridging between the pellicle proteins and bacterial membranes for successful interaction. Less reversible attachment of bacteria to tooth surface involves short-range interactions between adhesins on the microbial surface and receptors in the acquired pellicle. These are usually specific and irreversible.
Bacteria–host interactions
Examples of bacteria–host interactions include:
• lectin-like bacterial proteins interacting with carbohydrates, or with oligosaccharides within salivary glycoproteins
• hydrophobic fimbriae with hydrophobic portions within salivary proteins
• specific protein–protein interactions by antigenically and functionally distinct fimbriae.
Macromolecules and bacterial adherence
A function of salivary macromolecules is in the aggregation and subsequent removal of bacteria. For plaque formation to proceed, it is necessary that not all bacteria aggregate in saliva before they reach the tooth surface. Conformational changes of proteins on adsorbed surfaces allow for the binding of specific proteins. Examples of this include the removal of sialic acid by bacterial neuraminidase facilitating the adherence of certain bacteria.
As plaque formation continues, there is multiplication of attached organisms to produce confluent growth and a biofilm is formed. The production of proteases by bacteria removes inhibitors of growth (e.g. IgA, histatin) and provides nutrients for further microbial growth. Growth of a pioneer population creates conditions suitable for colonization of bacteria with more demanding growth requirements (dependent on nutrient supply, O 2 tension, pH, ionic concentration, toxins within plaque fluids) and organisms within the plaque begin to synthesize extracellular polymers.
Where the plaque is associated with chronic inflammatory periodontal disease and becomes subgingival, a more complex flora develops with anaerobic Gram-negative organisms predominating, to include cocci, rods, filaments and many motile forms (particularly spirochaetes). The microbial composition of dental plaque will vary, not only with the stages of maturity of the deposit, but also from individual to individual, from tooth to tooth, from site to site and from surface to surface. Plaque can be seen on all tooth surfaces that are not subject to constant abrasion, especially in areas that are difficult to clean such as occlusal pits and fissures, interproximal regions and the gingival margin.
Plaque matrix and extracellular polysaccharides
Plaque matrix and extracellular polysaccharides are as important in the functions of plaque as the micro-organisms. The plaque matrix is the intercellular material between bacteria. It can be directly related to the diet consumed and be a key source of carbohydrate for acid production during caries. The matrix consists of carbohydrates from foodstuffs as well as polysaccharides produced by bacterial metabolism. It also contains salivary proteins and glycoproteins that have precipitated out of solution. The plaque matrix is directly related to the diet consumed and its consistency, and composition may vary with the presence or absence of diet. In the absence of diet (e.g. stomach tube feeding), the plaque matrix is thin and porous and composed mainly of precipitated salivary glycoproteins. These are incorpo/>
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