Enamel prisms - rod-like bundles of hydroxyapatie crystals which are orientated at right angles to the tooth surface. Each prism can be traced from the outside of the enamel all the way to the dentine junction.
Enamel - the outer layer hard layer which covers the dentine around the crown of a tooth. Enamel consists of closely packed crystals of hydroxyapatite with very little organic material. A recognisable unit of structure in enamel is the enamel prism.
Enameloid - a type of enamel found in fish and reptiles in which the enamel prisms are haphazardly arranged; in contrast enamel prism are parallel to each other and orientated at right angle to the tooth surface.
Endocrine glands - the secretion passes into the blood stream, like insulin, epinephrine.
Endoderm - the inner of the three cell layers which form, as the clump of early embryonic cells begins to differentiate. The endoderm will form the gut system and its associated organs. The other two layers are the mesoderm and the ectoderm.
Endoplasmic reticulum - a system of inner cell membranes which is continuous with the nuclear membrane. It transports products of cell synthesis to the golgi apparatus. Described as rough endoplasmic reticulum when there are many ribosomes attached.
Endorphins - a neuropeptide which has specific binding sites on nerve cells called opiate receptors. When the receptor is activated by endorphins or morphine it reduces the excitability of the post synaptic cell. Peptide receptors are also found on lymphocytes which suggest an association between neuropeptides and the regulation of the immune response.
Endosteum - a layer of bone forming cells, osteoblasts which covers the entire surface of the internal aspect of cortical and spongy bone, separating it from the surrounding connective tissue. see also bone membrane.
Endothelial cells - the epithelial cells of the endothelium which lines blood vessels. The cells are flattened into a pavement stone shape and are usually two or three layers thick.
Endotoxins - the contents and cell wallsof dead bacteria which may be toxic to the host.
Enkephalins - similar in structure and action to endorphins.
Environment - describes the surroundings in which organisms live. Some physical features of an environment are fairly stable, like trees, rivers, mountains, houses, soil, teeth. Some physical features are changeable, like wind, water, light, pH, food supply. Others features are less predictable, such as the balance in the community of collaborators, competitors and parasites. All forms of life including bacteria in the mouth, have an environment, which has an important influence on their survival. Successful organisms manage to exploit their environment to the best advantage or to adapt to it, perhaps only after several generations, if it becomes a serious challenge to the species.
Enzyme - a protein that controls and helps a chemical reaction to take place, but is not used up in the process. Usually each enzyme is specific for a particular step in a reaction. Enzymes are sensitive to their environment, especially to excessive temperature or pH.