darwinian medicine - an approach to the treatment of infective diseases which takes account of the co-evolution between the host and its parasite.

    deciduous - from the latin "falling" it applies both to trees which lose their leaves in winter and teeth which are lost to make way for the permanent set.

    demineralisation - reduction in amount of mineral in tissue. this reduction occurs when the crystals of apatite are dissolved, usually in an acid environment.

    dental abscess- an abscess around the apex of a tooth due to spread of infection from the pulp .

    dental papilla - the condensation of dental mesenchyme which provides the stem cells from which ondontoblasts, cementoblasts and osteoblasts will form the pulp-dentine, cementum and alveolar bone of the tooth socket.

    dentary - one of several bone which together made up the lower jaw in early reptiles. during evolution the other bones, the articular and quadrate bones, became part of the inner ear, and the dentary became the single the mandible of mammals.

    dentine - a hard material like bone which forms the root and inner core of the crown of teeth. unlike bone, dentine has fine tubules which contain the elongated process of odontoblasts, the dentine forming cells.

    dentine-pulp - a term used to describe the unity between dentine and pulp, and to view it as one integrated tissue.

    depolarisation - all cells have a slight difference in electrical potential between the inside and outside of the cell membrane. this difference is called a membrane potential and is due to a greater number of sodium ions (positively charged) outside the cell than inside. this imbalance is maintained by a membrane pump which pushes sodium ions out of the cell. another pump also pushes potassium ions into the cell so there should be no difference in the balance of positive ions. but the potassium ions leak back out again, so there is always a potential difference across the membrane. nerve cells have the ability to depolarise or reverse the membrane potential so that the inside is positive and outside negative. this reversal is short lived and is soon corrected, but it is long enough to influence the adjacent parts of the membrane and to be carried, like a wave, all the way along a nerve axon to the next nerve where it reaches a synapse the reversal is caused by a sudden opening of cell membrane gates which allow a flood of sodium ions into the cell. this flood causes the inside to become positive, but the gates are soon shut and potassium gates opened, which allows potassium ions to flood out and restore the membrane potential. this can all happen several times in one second, but after a while there is no flood, and the sodium pump has to get to work to build up enough pressure for the depolarisation to work again.

    dermatan sulphate - a glycosaminoglycan found in skin, tendon, blood vessel and heart valves.

    desmosomes -one of the types of cell junctions by which cells join or communicate with each other. desmosomes consist of a round plaque of protein, desmoplakin on the cell membrane. into the plaque are attached fine filamanents which are part of the cell's cytoskeleton. so the plaque is attached to the skeleton of the cell. where the filaments enter the plaque the are so dense as to be visible with a light microscope. they are then called tonofilaments. the plaque of one cell adheres to the plaque of another. this system of joining cells is designed to resist mechanical separation, so we see desmosomes joining epithelial cells which hold tightly to each other. if epithelium is processed for histology, some shrinkage occurs and the epithelial cells separate from each other, except where the desmosomes hold them together. the pulled out tags of cell membrane give these cells a star-like shape, and so they are called the stellate cells.

    desquamation - the detachment of cells from the surface of an epithelium.

    dextrans - polysaccharides made by bacteria. they have a slimy consistency and contribute to the sticky nature of plaque.

    differentiate - change in the pattern of genes expressed by a cell resulting in altered function, from a more primitive parent cell to a more specialised group of daughter cells.

    diphyodont - only two sets of teeth, one deciduous and one permanent (from "di" = two,"phyo+ = generation and "dont" =teeth). see also polyphyodont

    displacement - of a tooth refers to its movement within the confines of the tooth socket. a tooth can be displaced more easily when forced in a lateral direction than when forced into the socket. continual or frequent displacement of a tooth may lead to it repositioning itself in the socket.

    dna - deoxyribosenucleic acid - a complex nucleic acid molecule which is used by cells to store genetic material as genes which control the structure of proteins and hence influencing all enzyme reactions. dna is coiled in a single closed loop in procaryotes, but coiled round other proteins to form a chromosome, and stored in the nucleus of eucaryotes

    duct - a tube which carries a secretion onto the surface of skin or mucosa.