PRINCIPLES OF TOXICOLOGY

566 GLOSSARY
mitochondria Small spherical or rod-shaped components (organelles) found in the cytoplasm of
cells, enclosed in a double membrane. They are the principal sites of energy generation (ATP) and
they contain the enzymes of the Krebs and fatty acid cycles and the respiratory pathways.
Mitochondria contain an extranuclear source of DNA and have genetic continuity.
mitosis The process in cell division by which the nucleus divides, typically consisting of four
stages—prophase, metaphase, anaphase, and telophase—followed by cytokinesis and normally
resulting in two new cells, each of which contains a complete copy of the parental chromosomes.
mixed-function oxidase system (MFO) A nonspecific, multienzyme complex on the smooth
endoplasmic reticulum of cells in the liver and various other tissues. These enzymes constitute the
important enzyme system involved in phase I reactions (i.e., oxidation/reduction reactions). Also
called cytochrome P450 enzymes.
monclonal antibody A homogeneous antibody that is produced by a clone of antibody-forming cells
and that binds with a single antigenic determinant.
monocyte A mononuclear phagocytic leukocyte with an ovoid or kidney-shaped nucleus, containing
lacy, linear chromatin, and abundant gray-blue cytoplasm fitted with fine, reddish and azure
granules.
morbidity The rate of sickness or ratio of sick persons to well persons in community.
multiple myeloma A malignant proliferation of plasma cells in bone marrow causing numerous
tumors and characterized by the presence of abnormal proteins in the blood.
muscarine A highly toxic alkaloid C9H20NO2, related to the cholines, derived from the red form of
the mushroom Amanita muscaria.
mutagen Any substance causing genetic mutation.
mutagenesis The induction of those alterations in the information content (DNA) of an organism or
cell that are not due to the normal process of recombination. Mutagenesis is irreversible and is
cumulative, in the event of increased mutation rates or decreased selection pressures.
mutagenic tests Test of an agent to determine effects on the faithful replication of genetic material.
The genetic damage can occur in both somatic and germinal cell lines.
mutation A permanent offspring-transmissible change in genetic material or structure. Such changes
may manifest themselves as altered morphology or altered ability to direct the synthesis of proteins.
myalgia Muscular pain or tenderness, especially when diffuse and nonspecific.
myasthenia gravis A disease characterized by progressive fatigue and generalized weakness of the
skeletal muscles, especially those of the face, neck, arms, and legs, caused by impaired transmission
of nerve impulses following an autoimmune attack on acetylcholine receptors.
myelin sheath The insulating envelope of myelin that surrounds the core of a nerve fiber or axon
and facilitates the transmission of nerve impulses. In the peripheral nervous system, the sheath is
formed from the cell membrane of the Schwann cell and, in the central nervous system, from
oligodendrocytes. Also called medullary sheath.
myeloid leukemia Leukemia arising from myeloid tissue (bone marrow) characterized by unrestrained
growth of the granular, polymorphonuclear leukocytes and their precursors.
myelotoxin A cytotoxin that causes destruction of bone marrow cells.
myoclonus A sudden twitching of muscles or parts of muscles, without any rhythm or pattern,
occurring in various brain disorders.
myotonia Tonic spasm or temporary rigidity of one or more muscles, often characteristic of various
muscular disorders.
narcosis A condition of deep stupor or unconsciousness produced by a drug or other chemical
substance.
nasopharyngeal region The part of the pharynx lying above the level of the soft palate (also known
as the postnasal space).