Macrophages - cells derived¹ from monocytes which have the ability to phagocytose foreign particles and dead tissue and to move through tissue, or to remain fixed² in one place. There are many macrophages in the spleen where they remove dead red blood cells from the circulation.

    Major salivary³ glands⁴ - are three large glands on each side of the face; the parotid, submandibular and submaxillary.

    Malleus - one of the three bones of the inner ear. The others are the stapes and the incus.

    Mastication⁵ - the process of preparing food for swallowing and digestion⁶ by chewing it.

    Matrix - comes from the Latin word "mater"meaning mother. It is a structure which encloses or holds something within it. Cells are held or enclosed in a matrix of fibres, water and large molecules⁸ called the ground substance.

    Matrix vesicles. - small bubble-like structures containing calcium-binding phospholipids and alkaline phosphatase. Crystals of hydroxyapatite from inside the vesicle which ruptures⁹ and releasing crystals into the surrounding osteoid or predentine so as to start mineralising it.

    Mechano-receptors- sensory¹⁰ receptors which respond when mechanically deformed¹¹ by pressure, tension, vibration¹² or touch.

    Mesenchyme - dental mesenchyme is tissue derived from the mesoderm of the embryo¹³ and which has been infiltrated¹⁴ and highly influenced by migrating cells from theneural crest¹⁵.

    Mesial drift - a gradual movement of all the posterior teeth in a mesial direction. It occurs only if there has been interproximal wear between the teeth. The drift is not a passive one however, as it has been shown that during chewing, the bite force has a a mesial component¹⁶.

    Mesial - towards the midline.

    Mesoderm - the middle of the three cell layers which form, as the clump¹⁷ of early embryonic¹⁸ cells begins to differentiate¹⁹. The mesoderm will form the muscles, blood system, connective tissue, including bone and dentine, the kidneys and the dermis²⁰ of the skin. The other two layers are the ectoderm and theendoderm.

    Messenger RNA - a ribose nucleic acid acid which caries the DNA²¹ code in matching nucleotides, from the nucleus²² to the ribosome of the cell.

    Metabolise - to obtain energy by breaking down a molecule⁷ into smaller components²³, as in respiration²⁴ or to store energy by building a more complex molecule out of smaller components. It always takes place in a number of stages, controlled by enzymes²⁵. Each step in the process follows a predictable "metabolic²⁶" pathway for that reaction.

    Microbiology - the study of the microscopic²⁷ forms of life.

    Microfilaments - are the smallest filaments²⁸ of the cytoskeleton. The filaments are made of hundreds of actin molecules, stacked in a line. They can be quickly broken down or extended. Actin filamanents give the cell its shape and help to change it. When cells move, in embryology and repair, or just during the continual patrol of lymphocytes, they must hold on to something in order to crawl. The filaments serve to anchor one part of the cell, via fibronectin to the cell matrix, so the rest of the cell can pull itself towards the anchor. Lymphocytes move and scavenge by sticking out arms and feet to help them crawl and engulf³⁰ foreign particles. Muscle cells change their shape by using stacks of actin filaments as a ladder on which myosin climbs.

    Micron - 1000th part of a millimetre.

    Microorganisms - single celled animals which may range from the very small viruses, through bacteria and fungi³¹ to almost visible protozoa.

    Microtubules - are the largest filament²⁹ in the cytoskeleton. They are the hollow tubes along which cell products are conducted long distances. The system is not unlike a railway network around the cell, sometimes involving long distances. For example, neurones transport out neurotransmitter substances along the axons to distant synapses³², inside microtubules.

    Minor³³ salivary glands - are microscopic glands under the surface of the oral mucosa. They are found throughout the lining³⁴ mucosa of the mouth including the tongue.

    Mitochondria - a cell organelle found in eucaryotic cells which produces ATP as a product of the kreb cycle and the electron transport system Cells requiring large amounts of energy, such as secreting³⁵ odontoblasts, have large numbers of mitochondria. Mitochondria are self replicating³⁶ and contain their own DNA for this purpose.

    Mitosis- the division of a cell into two daughter cells, each of which is identical.

    Mitotic activity - the rate of mitosis, and hence cell division. The mitotic activity of basal cells in an epithelium must match the rate of desquamation.

    Molecules - a combination of atoms joined together in fixed proportions.

    Monocytes - remain in the blood only a short time before they migrate into the tissues particularly where dead tissue must be removed, where they are calledmacrophages.

    Morphogenesis - the process in which tissue shapes and organ structures are developed during embryology.

    Morphogenic fieled - an environment in which the shape or pattern of a developing organ is determined³⁸.

    Motoneurone - nerve cells with their cell bodies in the brain stem or spinal³⁹ cord, which transmit impulses along their axons to effector organs, including endocrine, exocrine glands and muscles fibres. The axons of most motoneurones have many branches, each of which ends at a neuromuscular junction⁴⁰. The group of muscle cells innervated by one motoneurone is called a motor unit.

    Mucoperiosteum - a type of oral mucosa which has a fibrous lamina propria, no submucosa, and is attached to the underlying⁴¹ periosteum of bone. The attached gingiva is a mucoperiosteum.

    Mucous⁴² - a secretion⁴³ which is viscous⁴⁴ and slimy due to the presence ofglycoproteins.

    Mutation⁴⁵ - a change in the order of nucleotide bases on a gene³⁷, which alters the configuration⁴⁶ of the protein produced, and thus may alter the behaviour of the cell. A mutation may cause a cell to die, or become cancerous. Mutations in bacteria and viruses help them to evade⁴⁷ detection by their hosts.

    Myelin - the fatty covering of myelinated nerves which appears white to the naked eye. The parts of the brain and spinal cord, in which myelinated nerves run, has therefore been called the "white" matter as distinct from the "grey" matter composed of nerve cells. Myelin also contains about 20 % of proteins whose prime role is to mediate⁴⁸ adhesion between adjacent Schwann cells. These cell membrane⁴⁹ glycoproteins are also members of the immunoglobulin family of cell surface proteins. Defects in the these surface proteins may cause them to act as antigens to the immune system. The disease multiple sclerosis is caused by antibodies to the myelin proteins, which results in inflammation and loss of myelin.

    Myelinated - nerves axons which are completely wrapped in a sheath of myelin by schwann cells. One cell wraps about a millimetre of nerve axon. Myelinated nerve axons carry impulses faster then unmeyelinated nerves as the impulse jumps across the myelin sheath of each adjacent Schwann cell to that of the next.