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.