Ecological balance  - astable balance in the numbers of each species in an  ecosystem¹. In the ecosystem of the mouth this balance is brought about by competition and cooperation between the different organism and the hosts defences which tend to control population size.

    Ecosystem - a stable environment in which live a large number of different forms of life, each affecting the other. Example are a forest, desert, tidal area, soil, oral cavity, gut².

    Ectoderm - the outer of the three cell layers which form, as the clump³ of early embryonic⁴ cells begins to differentiate⁵. The ectoderm will form the epidermis⁶ of the skin and the nervous system. The other two layers are the mesoderm and the endoderm.

    Ectomesenchyme - a name given to dental MESENCHYME which reflects its partly ectodermal origin.

    Eicosanoids - are a class of hormones⁷ which are all made from phospholipids. They include prostaglandins, thromboxanes, and leucotriens.

    Elastic⁸ fibres -are long, thin, ribbons-like fibres, sometimes even sheet-like. They are composed of a central core of elastin, a rubbery protein, surrounded by glycoprotein microfilaments. Elastin is found all over the body but particularly in the walls of blood vessels⁹ and in our vocal¹⁰ chords.

    Electron - the negatively charged elements of an atom which circle the nucleus¹¹. If an electron is lost the atom becomes a relatively¹² positively¹³ charged ion. It has been ionised .

    Electron Transport System- Hydrogen ions produced during the 3 preparatory steps of aerobic¹⁴ respiration¹⁵ are carried by nicotinamide adenine dinucleotide (NAD). The hydrogen ion plus one electron form NADH, which is taken to the electron transport system. This transport system is run by a series of 5 molecules¹⁶. The first, removes the two electrons from NADH,( one comes from the hydrogen atom, leaving behind a hydrogen ion). These two electrons, bounce from the first molecule¹⁷ in the transport system to the second, third, forth¹⁸ and then last one, cytochrome oxidase, which finally places the electrons onto oxygen gas O2. The electron rich oxygen atoms are attractive to the hydrogen ions and they combine to form water. (Oxygen in the process of aerobic respiration acts therefore as an electron acceptor). In the process of bouncing "downhill" the electrons have released sufficient energy to power up a small battery. This battery has been made by pumping hydrogen ions out of the inner membrane¹⁹ of the mitochondria. The collection of hydrogen ions outside, piles up and their electrical pressure mounts. They want to get back across the membrane, and are allowed, one at a time to pass back through the enzyme²⁰ ATP synthase. This enzyme sits like a water wheel in the cell membrane, turned by the passage of hydrogen ions. Its turning wheel builds an ATP molecule in every turn. The wheel may be going at about 200 revolutions per second, powering the synthesis of an ATP molecule with each turn. ATP formed in this way takes a while but can be sustained to fuel the body during aerobic exercise. When the demand for power exceeds this rate, the cells have to rely on anaerobic²¹ respiration.

    Electrostatic - a force generated by differences in electric charge of two particles.

    Enamel²² prisms - rod-like bundles of hydroxyapatie crystals which are orientated²³ at right angles to the tooth surface. Each prism can be traced from the outside of the enamel all the way to the dentine junction²⁴.

    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 reptiles²⁵ in which the enamel prisms are haphazardly²⁶ arranged; in contrast enamel prism are parallel to each other and orientated at right angle to the tooth surface.

    Endocrine glands²⁷ - the secretion²⁸ 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 membranes²⁹ which is continuous with the nuclear membrane. It transports products of cell synthesis to the golgi apparatus³⁰. Described as rough endoplasmic reticulum when there are many ribosomes attached.

    Endorphins - a neuropeptide which has specific binding³¹ sites on nerve cells called opiate receptors. When the receptor is activated³² 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 flattened³³ into a pavement stone shape and are usually two or three layers thick.

    Endotoxins - the contents and cell wallsof dead bacteria which may be toxic³⁴ 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 parasites³⁵. 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. Enzymes³⁶ are sensitive to their environment, especially to excessive temperature or pH.

    Epidermal³⁷ growth factor - a cytokine that stimulates³⁸ epithelial cell proliferation.

    Epinephrine - a neurotransmitter substance found at all adrenergic synapses³⁹ (nor epinephrine or epinephrine). It is the most common neurotransmitter in the nervous system, in particular at ganglion cells of the sympathetic nervous system.

    Epitaxy - the initiation⁴⁰ of crystalformation in a saturated⁴¹ solution by providing a template against which crystal can form. There are specific sites on collagen molecules which appear to function as templates against which hydroxyapatite crystals form.

    Epithelial attachment⁴²- the cuff⁴³ of junctional epithelium which joins the gingival sulcus epithelium to the enamel of the tooth. Apical migration⁴⁴ of the epithelium down onto the cementum may occur due to ageing or periodontal disease. Loss of attachment produces a periodontal pocket and a new habitat for anaerobic oral bacteria.

    Epithelium - a layer of cells which forms alining for a tube or the covering for an organ or the whole body.

    Eucaryote - a cell in which the genetic⁴⁵ material is confined to the nucleus, in distinction to a procaryote in which the genetic material is dispersed⁴⁶ throughout the cell. Other distinctions of eucarytoic cells are the presence of organelles such as the golgi apparatus, endoplasmic reticulum, lysosomes and mitochondria

    Exocrine glands - the secretion passes intoa duct like sweat, saliva⁴⁷ and mucous⁴⁸.

    Extinct - a plant or animal species may entirely⁴⁹ cease to exist. Recent examples are the dodo, a large flightless bird which used to live as recently as two hundred years ago, on the island of Mauritius. There are today many species of birds, flowers, fish, insects, large mammals, including certain types of whale, which are threatened with extinction⁵⁰, most as a result of human activity. Happily, the smallpox⁵¹ virus is about to become extinct

    Extra cellular⁵² matrix - the supporting surrounding material of a cell including ground substance and fibres.

    Extrinsic⁵³ fibres - refers to those fibres of cementum which are continuous with periodontal ligament fibres. Extrinsic fibres have been trapped in cementum during its formation in order to anchor them. see also intrinsic fibres, and sharpey's fibres.

    Exudate - the fluid plasma⁵⁴ which leaks out of blood vessels due to an increase in capillary⁵⁵ permeability. The increased permeability, is caused by histamine, and bradykinin, which are released in response to tissue damage. The formation of an exudate is the first step in the process of inflammation.

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