Paracrine - cell messengers also called cytokines which are locally acting, 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 salivary 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 molecule 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 descending 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 inverse 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 molecule 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.     Plasma - the fluid part of blood, containing proteins and salts, from which the blood cells have been removed. see alsoserum. Plasmid- a circular piece of DNA found in the cell cy lasm of bacteria which is able to reproduce itself independently of it host. Plasmids may transmit a resistance to antibiotics from one bacteria to another. They are of great importance in techniques using for recombinant DNA.     Platelet- derived growth factor - a cytokine found especially in platelets. It stimulates cell proliferation and encourages wound healing.     Platelets - are small colourless discs of cy lasm found in blood. When platelets come into contact with a damaged vessels surface they change in several important ways. They begin to swell, their shape becomes irregular with protruding processes, they become sticky and they release an enzyme which causes the formation of thromboxane, one of the precursors of thrombin. Thromboxane also activates nearby platelets, thus starting a positive feedback which rapidly increases the mass of sticky platelets which form a platelet plug. This process accounts for daily damage to capillary walls. Damage on a larger scale requires other mechanism for haemostasis. Platelets also release serotonin which acts as a powerful vasoconstrictor      Polymers-large molecules made up of many joined units of a more simple molecule. Examples are polysaccharides and polypeptides.     Polypeptides; -chains of amino acids joined by peptide bonds. They are not the size of proteins, but may be biologically very active. Some hormones are peptides, such as insulin which has 51 amino-acid residues. Peptides may also be powerful neurotransmitter substances.     Polyphyodont - continuous replacement of teeth with many generations (from poly =many; phylo = generation and dont =teeth). See also diphyodont     Polysaccharides - long molecules made of chains of sugars linked together. Examples are starch, glycogen and dextrans.     Predentine - the extracellular matrix produced by odontoblasts, which becomes mineralised to form dentine. Similar in structure to osteoid.     Procaryote - a cell in which the genetic material is dispersed throughout the cell in distinction to a eucaryote which has a nucleus and other organelles. Bacteria and blue-green algae are procaryotes     Prognathic- a prominent lower jaw which may bring the lower teeth ahead of the upper teeth     Proline- one of 20 amino acids common in proteins. It is a common amino acid of collagen and like lycine must be hydroxylated by ascorbic acid in order to allow the formation of hydrogen bonds which will hold the triple helix together.      Proline-rich proteins - a group of proteins in saliva which have the ability to bind to calcium. They provide the protective layer of pellicle on the tooth surfaces by binding to the calcium in enamel. They also bind onto microorganism, providing a link between organism and the tooth surface. Proline rich proteins help to de-toxify tanins, which are potentially poisonous plant substances found in tea, and unripe fruit.     Prostaglandins - are members of a class of hormones known as the eicosanoids. They are released by cells which have been damaged and have a powerful ability to sensitise nerve endings causing tenderness to the damaged area, and to cause vasoconstriction by contracting the smooth muscle of arterioles. They belong to a group of compounds which have a similar effect on nervous tissue, known asneuropeptides.