Quadrate - one of the bones which together with the articular bones and the dentary, made/make up a reptile's jaw. In mammals the quadrate bone is incorporated into the middle ear as the incus.

    Ramus - the vertical part of the mandible which supports the coronoid and the condylar processes.

    Recombinant DNA - is DNA from a plasmid into which has been inserted a foreign gene. The plasmid is then introduced into a host cell, often the bacterium E.coli. The host cell may then express the foreign gene and secrete the desired protein. This process, commonly known as genetic engineering, has been used to great effect in synthesising proteins such as insulin and interferon.

    Reduced enamel epithelium - (REE) the epithelium produced by the combination of the external and internal enamel epithelium. The REE remains covering the enamel crown until the tooth erupts when it fuses with the oral epithelium. The REE remaining on the enamel surface becomes the junctional epithelium.

    Refined carbohydrates - natural carbohydrates from which other bulk such as fibres have been removed. Granulated sugar is a refined form of sugar cane.

    Remineralisation - the replacement of mineral salts lost by de-mineralisation of a solid salt.

    Remodelling - of bone refers to the constant removal by osteoclasts and rebuilding by osteoblasts The mass of bone can be controlled constantly by altering the balance between removal and rebuilding. The shape of a bone can also be altered by removing in one place and building somewhere else, without necessarily changing the total mass of a bone mass.

    Repositioning - of a tooth refers to its movement within the entire dentition which involves the remodelling of the tooth socket. Repositioning of teeth occurs naturally due to continued eruption and mesial (or distal) drift.

    Rest Position - a position the jaw adopts when at rest with the lips lightly together.

    Reticular fibres - are fine type III collagen fibres forming a net-like supporting framework or reticulum. They are found around small blood vessels, nerve cells, muscle fibres and in particular beneath epithelial membranes as part of thebasal lamina.

    Reticular formation - in the central core of the medulla, it consists of several structures, including the periaquaductal grey. The reticular formation integrates information from many sources and influences sensory motor and autonomic activity. It is involved in aversive drive (behaviour which is an instinctive turning away from the unpleasant).

    Retinoic acid - a product of retinol(Vitamin A) which binds onto cell membranes and controls cell division and differentiation through gene expression.

    Ribosomes - - structures in the cy lasm of cells which attach onto messenger RNA. At the ribosome, the code of nucleotides on the mRNA is translated into a series of aminoacids.

    RNA - Ribosenucleic acid - seenucleic acids.

    Root resorption - resorption of cementum and underlying root dentine by osteoclasts. Temporary zones of root resorption may occur during orthodontic tooth repositioning. More extensive and irreversible root resorption may occur if the root becomes ankylosed.

    Rugae - raised ridges of epithelium, each with its core of lamina propria, found on the anterior wall of the hard palate.