Formation and functions of pellicle
Saliva is seldom in direct contact with the tooth surface but is separated from it by the acquired pellicle, defined as an acellular layer of salivary proteins and other macromolecules, approximately 10 um thick, adsorbed onto the enamel surface. It forms a base for subsequent adhesion of microorganisms, which under certain conditions may develop into dental plaque. The pellicle layer, although thin, has an important role in protecting the enamel from abrasion and attrition, but it also serves as a diffusion barrier.
Figure 106 shows that the undisturbed pellicle is formed in different layers. There are many nonattaching bacteria close to the outer surface of the pellicle. Because of abrasion, for example from toothbrushing, the thickness will vary between 2 and 10 um, depending on the toothbrushing intervals. Saxton (1976) showed that complete removal of the pellicle requires about 5 minutes’ cleaning with pumice in a rotating rubber cup.
Figure 107 shows a groove made in the pellicle with a knife. Such grooves were earlier thought to be abrasive defects in the enamel surface resulting from the use of abrasive toothpaste. The pellicle shown in Figure 108 was removed by intensive cleaning for about 5 minutes. The pellicle-free enamel surface was partly covered with nail varnish, while the outer part was exposed to saliva in vivo for several hours.
Figure 108 shows the thickness of the new pellicle compared to the naked enamel surface after removal of the nail varnish.
Movement of molecules by forces other than diffusion is less frequent in the pellicle than in most other parts of the salivary film. The relatively undisturbed layer of liquid in the pellicle influences the solubility behavior of the enamel surface. Adsorption to the enamel of macromo- lecules, usually originating from the saliva, is selective; certain macromolecules show a higher affinity for the mineral surface than do others. In the normal oral pH range, the enamel surface has a negative net charge, because of the structure of hydroxyapatite, in which phosphate groups are arranged close to the surface. Count -erions (of opposite charge), for example, calcium, are attracted to the surface, forming a hydration layer of unevenly distributed charges. The exact composition of this layer will be determined by several factors (eg, pH, ionic strength, and the type of ions present in the saliva). Normally the hydration layer close to the enamel surface contains mainly calcium and phosphate ions in the proportion of 10:1, but other ions, such as sodium, potassium, fluoride, and chloride must also be present (see the formation of phosphate- and protein- coated CaF2 crystals in the pellicle; see Figs 101, 102, and 103). Because of the domination of calcium, the resulting net charge of the enamel surface with its hydration layer is positive, implying that the hydration layer will attract negatively charged macromolecules, as illustrated by
Waerhaug more than 25 years ago (Figs 109 and 110).
Negative charges on macromolecules are found in acidic side chains with end groups of phosphate or sulfate. These side chains have a high affinity to the tooth surface.
Recent research has shown that the bulk of the pellicle consists of salivary micellelike
structures of great importance for reducing diffusion through the pellicle and
reducing friction between the teeth and other oral tissues.
Not all the proteins contributing to the pellicle are well defined. However, salivary
proteins, such as amylase, lysozyme, peroxidase, IgA, IgG, and glycosyltransferase,
participate in formation of the pellicle matrix, together with mucins and breakdown
products from macromolecules of both salivary and bacterial origin. Of special
interest are the acidic PRPs, mentioned earlier, which bind via their amino-terminal
segments to the tooth surface, leaving their carboxy-terminal regions directed toward
the oral cavity, where they may interact with oral microorganisms.
During the first hour, pellicle formation is rapid, and then decreases. It seems likely
that adsorption of the first layer of molecules onto a clean surface is instantaneous.
The formation rate varies among individuals, probably as a result of differences in
salivary composition, the frequency of oral hygiene, and diet composition.
