Formation and Role of Dental Pellicle




What is the Dental Pellicle?
The pellicle is defined as the acellular layer of adsorbed salivary proteins and other macromolecules on the dental enamel surface approximately 10 micrometers thick. (Ole Fejerskov & Edwina Kidd, 2003)

Composition of the Dental Pellicle

Proteins found in the dental pellicle are not well-defined, but most glycoproteins found in saliva are also found in the pellicle. Components of the dental pellicle include macromolecules such as lysozyme, alpha-amylase, perioxidase, immunoglobulin A, immunoglobinG, glycosltransferase, proline-rich proteins, mucins, albumin and breakdown products from macromolecules from both saliva and bacteria, which participate in the pellicle matrix formation.

The dental pellicle is formed by the selective adsorption of negatively charged salivary glycoproteins preventing continuous deposition of calcium phosphate. The pellicle allows for the subsequent adhesion of micro-organisms, which form dental plaque. (Ole Fejerskov & Edwina Kidd, 2003)


Formation of the Pellicle
The surface of enamel is charged negatively in the normal oral pH range, and is due to the structure of hydroxyapatite, in which phosphate groups are arranged close to the surface of the enamel.Thus postively-charged ions e.g. calcium are attracted to the enamel surface forming a hydration layer with unevenly distributed charges. As calcium is the predominant ion in the hydration layer, the resulting net charge of the hydration layer is positive. Thus, the hydration layer will attract negatively charged macromolecules with acidic side chains and end groups of phosphate or sulfate. The amino acids aspartate and glutamate, which are negatively charged at physiological pH, also have a high affinity for the postively charged hydration layer of the tooth surface.

The exact composition of the hydration layer is dependent on several factors including pH, ionic strength and the types of ions present in the saliva; usually the hydration layer contains mainly calcium and phosphate ions in the ratio 10:1. Other ions present include sodium, potassium and chloride.

The adsorption of the first molecule layer on a clean tooth surface is immediate. The initial rate of formation of the dental pellicle is fast during the first hour, after which it decreases. Rate of formation of the pellicle varies in individuals due to differences in salivary flow and composition. (Ole Fejerskov & Edwina Kidd, 2003)


Role of the Pellicle
The main role of the pellicle is protective in nature. The pellicle protects the enamel against abrasion and attrition, and serves as a diffusion barrier. As the liquid layer in the pellicle is relatively undisturbed, molecule movement due to non-diffusion forces are lower than in most parts of the salivary film, influencing the solubility behavior of the enamel surface. (Ole Fejerskov & Edwina Kidd, 2003)



Plaque Formation: Binding of Bacteria to the Pellicle

Proline-rich proteins which are negatively charged bind to the tooth surface, helping form the dental pellicle. Other proteins are contained in the pellicle, such as alpha-amylase and statherin, which allow for the adherence of more bacteria species to the pellicle. Specific receptors on the surface of fimbria of bacteria such as S. gordonii and mutans streptococci are hence able to interact with the PRP proteins (proline-rich proteins) of the pellicle via a “lock and key” mechanism, as can be seen in the diagram below. Other gram-positive oral microbes such as Streptococcus sanguis are able to attach to the negatively charged salivary glycoproteins contained in the dental pellicle.

Gram-negative oral microbes such as Fusobacterium nucleatum then attach to the gram-positive microbes, forming dental plaque. The dental plaque is cariogenic in nature, and is the first step in the formation of caries. (Ole Fejerskov & Edwina Kidd, 2003; University of Newcastle Dental School, retrieved October 11, 2009)

Pellicle and associated plaque-forming bacteria
(Referenced from "Introduction to Dental Plaque". Accessed 20th Oct 2009.)



References
  1. Kinder, S.A. Microbiology of Dental Plaque. Retrieved September 21, 2009 from http://www.dent.ucla.edu/pic/members/microbio/mdphome.html
  2. University of Newcastle Dental School. Bite-Sized Tutorials: Electrostatic Charge and Bacterial Adhesion. Retrieved October 11, 2009 from http://ncl.ac.uk/dental/oralbiol/oralenv/tutorials/electrostatic.htm
  3. University of Newcastle Dental School. Bite-Sized Tutorials: Why Plaque Forms at Specific Sites. Retrieved October 11, 2009 from http://ncl.ac.uk/dental/oralbiol/oralenv/tutorials/plaquesites.htm
  4. Fejerskov, O., & Kidd, E. (2003). Dental Caries: The Disease and Its Clinical Management (1st ed.). Oxford: Blackwell Munksgaard Ltd.
  5. Introduction to Dental Plaque. Retrieved October 20, 2009 from http://www.dentistry.leeds.ac.uk/OROFACE/PAGES/micro/micro2.html.

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