Paracrine - cell messengers also called cytokines which are locally acting1, produced by neighbouring cells or the extracellular matrix, as distinct from as distinct from endochrine or hormonal messengers.

Parakeratinised - an epithelium in which the superficial cells have not lost their nuclei, but have become filled with keratin. see also keratinised

Parasympathetic - part of the autonomic nervous system concerned with maintaining routine functions. Always acts as a balance to activity of the sympathetic nervous system.

Pathogenic - able to cause disease.

Pellicle - a thin layer of salivary4 proteins which forms on the surface of enamel.

Peptide bonds - a covalent bond made between the carbon atom of the carboxyl group of one amino acid and the nitrogen atom from the amine group of another. In the process a molecule6 of water is removed. Peptide bonds allow chains of amino acids to form polypeptides and proteins When peptide bonds are broken apart they need water to reform the amino acids. This process is known as hydrolysis, and occurs during cooking and in digestion..

Periaquaductal grey - is an integrative centre for inputs form the autonomic nervous system, the limbic system and from sensory and motor pathways. It has an inhibitory affect on pain transmission due to descending10 connections through the raphe nucleus along the corticospinal tract to the cells of the dorsal horn.

Pericytes- small cells lying next to the endothelial cells of capillaries which have the capacity to differentiate into osteoblasts.

Periodontal pocket - loss of epithelial attachment to the tooth, producing an increase in gingival sulcus depth beyond the normal 1-2 mm.

Periosteum - a connective tissue layer containing osteoblasts on the external aspect of all bones. see alsoendosteum.

Peritubular dentine - see intratubular dentine

pH - a measure of how acid or alkali a solution is. As the pH gets lower, the solution is more acid. At a pH of 7 the solution is neither acid nor alkali. pH is the inverse1 of the logarithm of the concentration of hydrogen ions.

Phagocytosis - the ingestion of small particles, bacteria or viruses into the cell, by engulfing it in a vacuole.

Phosphate - a salt in which the negatively charged part is a phosphorus molecule3 joined to four oxygen molecules as PO4.

Phospho-proteins - proteins which contain available phosphate groups.

Phospholipids - are the most common lipids in our cell membranes. The head group contains a phosphate and is readily soluble in water. Phospholipids are also found inmatrix vesicles, and they provide the first step in the synthesis of prostaglandins

Planktonic - a form of life style in which an organisms floats freely in a fluid without significant attachment or association with other living forms.

Plaque is a film of bacteria in a matrix of salivary and bacterial polymers. It can be called a biofilm as it has a complex population of organisms which when mature, reach an ecological balance with one another.