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MALT  = Mucosa-associated lymphoid tissue.

M13 A bacteriophage that infects E.coli, derivatives of which are extensively used as cloning vectors.

Macromolecule  Any large, usually polymeric molecule (e.g., a protein, nucleic acid, polysaccharide) with a molecular mass greater than a few thousand daltons.

Major histocompatibility complex (MHC)  A set of genes found in all mammals whose products are primarily responsible for the rapid rejection of grafts between individuals, and funcion in signalling between lymphocytes and cells presenting antigen.

Malignant A term to describe a tumor that has become invasive or metastatic (i.e., colonizing other tissues). Usually progressive, leading to destruction of host cells and ultimately death of the host.

Malignant transformation The development of the ability to invade normal tissue without regulation in space or time. May also lead to metastatic growth (colonization of a distant site with subsequent unregulated invasive growth).

Manometer A “U”-shaped tube containing liquid, the levels of which in each limb of the “U” reflect the pressure difference between the ends.

Mapping (DNA)  Various techniques for determining the relative order of genes on a chromosome (genetic map), the absolute position of genes (physical map), or the relative position of restriction sites (restriction map).

Mast cells  cells found distributed near blood vessels in most tissues, that are full of granules containing inflammatory mediators. Tissue cell that releases histamine and other chemicals involved in inflammation.

Medium A mixture of inorganic salts and other nutrients capable of sustaining cell survival in vitro for 24 hours. Growth medium. That medium which is used in routine culture such that the cell number increases with time. Maintenance medium. A medium that will retain cell survival without growth (cell proliferation), e.g., a low-serum or serum-free medium used with serum-dependent cells to maintain cell survival without cell proliferation. Plural Media.

Megakaryocyte  Large bone marrow cell that gives rise to platelets.

Meiosis  In eukaryotes, a special type of cell division comprising two successive nuclear and cellular divisions with only one round of DNA replication resulting in production of four genetically nonequivalent haploid cells (gametes) from an initial diploid cell. Opp. Mitosis.

Melting temperature (Tm) The temperature at which a double-stranded DNA or DNA-RNA molecule denatures.

Membrane potential  Voltage difference across a membrane due to the slight excess of positive ions (cations) on one side and negative ions (anions) on the other resulting from the selective permeability of the membrane to different ions or from pumping of ions across the membrane by active transport.

Memory cells  Long-lived lymphocytes which have already been primed with their antigen, but have not undergone terminal differentiation into effector cells. They react more readily than naive lymphocytes when restimulated with the same antigen.

Menstruation  Flow of menstrual fluid from uterus and also called menstrual period.

Mesenchyme Loose, often migratory, embryonic tissue derived from the mesoderm, giving rise to connective tissue, cartilage, muscle, hemopoietic cells, etc., in the adult.

Mesoderm A germ layer in the embryo arising between the ectoderm and endoderm and giving rise to mesenchyme, which, in turn, gives rise to connective tissue, etc.

Metaphase  Mitotic stage at which chromosomes are fully condensed and attached to the mitotic spindle at its equator but have not yet started to segregate toward the opposite spindle poles.

Metastasis  Spread of tumor cells from their site of origin and establishment of areas of secondary growth.

MHC  = Major histocompatibility complex.

MHC restriction  A characteristic of many immune reactions in which cells cooperate most effectively with other cells with which they share an MHC haplotype.

Microfilaments  Cytoskeletal fibers (~7 nm in diameter) that are formed by polymerization of monomeric globular (G) actin, exhibit polarity, and associate with a variety of actin-binding proteins; also called actin filaments. Microfilaments play an important role in muscle contraction, cytokinesis, cell movement, platelet activation, and other cellular functions and structures.

Microinjection A method of introducing new DNA into a cell by injecting it directly into the nucleus.

Microtubules  Cytoskeletal fibers (24 nm in diameter) that are formed by polymerization of α,β-tubulin monomers and exhibit structural and functional polarity. They are important components of the cilium, flagellum, mitotic spindle, and other cellular structures. Vesicles and protein particles often are transported along microtubules in a process mediated by kinesin or dynein.

Microvillus (pl. Microvilli)  Small, membrane-covered projection on the surface of an animal cell containing a core of actin filaments and various associated proteins. Numerous microvilli are present on the absorptive surface of intestinal epithelial cells, increasing the surface area for transport of nutrients and forming the bruch border.

Mineral  Inorganic substance, that is, without carbon. Major minerals in body are calcium, phosphorus, potassium, sulfur, sodium, chloride, and magnesium.

Minimal medium A defined medium that provides only the minimum number of different nutrients needed for growth of a particular bacterium.

Missense mutation  Any mutation that alters a codon so that it codes for a different amino acid.

Mitogens  Substances which cause cells, particularly lymphocytes, to undergo cell division.

Mitosis  In eukaryotes, the process whereby a cell divides to produce two genetically equivalent daughter cells. Mitosis refers specifically to nuclear division, whereas cytokinesis refers to division of the cytoplasm. Both processes occur during the M phase of the cell cycle, which is conventionally divided into four stages: prophase, metaphase, anaphase, and telophase. Opp. Meiosis.

Monoclonal Derived from a single clone of cells. Monoclonal antibody. Antibody produced by a clone of lymphoid cells either in vivo or in vitro. In vitro the clone is usually derived from a hybrid of a sensitized spleen cell and continuously growing myeloma cell.

Morphogenesis The development of form and structure of an organism.

Motif  In proteins, a unit exhibiting a particular three-dimensional architecture that is found in a variety of proteins and usually is associated with a particular function. Many DNA-binding proteins contain one of a small number of DNA-binding motifs including the helix-loop-helix, homeodomain, leucine zipper, and zinc finger.

Motor end plate  Specialized region of muscle-cell plasma membrane that lies directly under axon terminal of a motor neuron.

Motor neuron  Somatic efferent neuron, which innervates skeletal muscle.

Motor unit  Motor neuron plus the muscle fibers it innervates.

Multicopy plasmid A plasmid with a high copy number.

Multigene family A number of identical or related genes present in the same organism, usually coding for a family of related polypeptides.

Muscle tone  Degree of resistance of muscle to passive stretch while attempting to relax.

Myelin sheath  Stacked specialized cell membrane that forms an insulating layer around vertebrate axons and increases the speed of impulse conduction. It is produced by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system.

Myeloma (Myeloid cell)  A tumor derived from myeloid cells. Used in monoclonal antibody production when the myeloma cell can produce immunoglobulin.

Myoblast  Embryological cell that gives rise to muscle fibers.

Myoepithelial cell  Specialized contractile cell in certain exocrine glands and it’s contraction forces gland’s secretion through ducts.

Myosin ATPase  Enzymatic site on globular head of myosin that catalyzes ATP breakdown to ADP and Pi, releasing the chemical energy used to produce force of muscle contraction.

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