glossary G -15microscopy The use of a microscope to view objects, includ-
ing cells, that are not visible to the naked eye.
microtubule A cytoskeletal element with roles in cell shape,
motion, and growth and in the structure of cilia and flagella.
The largest element of the cytoskeleton.
microvillus (my-crow-vill-us) [L. villus, shaggy hair] A slen-
der extension of the cell surface that functions in absorption
or secretion.
midbrain A brain region that evolved as a coordination cen-
ter for reflex responses to visual and auditory input; together
with the pons and medulla oblongata, part of the brain stem,
which includes the reticular formation.
mineral An inorganic substance required for the normal
functioning of body cells.
mineralocorticoid Hormone secreted by the adrenal cortex
that mainly regulates the concentrations of mineral salts in
extracellular fluid.
mitochondrion (my-toe-kon-dree-ahn), plural: mitochon-
dria. Organelle that specializes in ATP formation; it is the site
of the second and third stages of aerobic respiration.
mitosis (my-toe-sis) [Gk. mitos, thread] Type of nuclear
division that maintains the parental chromosome number for
daughter cells. It is the basis of bodily growth and the repair
of tissue damage.
mixture A substance of two or more elements whose propor-
tions can and usually do vary.
molecule A unit of matter in which chemical bonding holds
together two or more atoms of the same or different elements.
monoclonal antibody [Gk. monos, alone] Antibody pro-
duced in the laboratory by a population of genetically
identical cells that are clones of a single “parent” antibody-
producing cell.
monomer A small molecule that is commonly a subunit of
polymers, such as the sugar monomers of starch.
monosaccharide (mon-oh-sak-ah-ride) [Gk. sakharon,
sugar] The simplest carbohydrate, with only one sugar
monomer. Glucose is an example.
morphogenesis (more-foe-jen-ih-sis) [Gk. morphe, form, and
genesis, origin] Processes by which differentiated cells in an
embryo become organized into tissues and organs.
morula A compact ball of sixteen embryonic cells formed
after the third round of cleavage.
motor neuron A neuron that delivers signals from the brain
and spinal cord that can stimulate or inhibit the body’s effec-
tors (muscles, glands, or both).
motor unit A motor neuron and the muscle fibers under its
control.
mRNA See messenger RNA.
mucous membranes (mucosae) Pink, moist membranes
that line the tubes and cavities of various body systems; most
absorb or secrete substances.menstruation Periodic sloughing of the blood-enriched
lining of the uterus when pregnancy does not occur.
mesoderm (meh-so-derm) [Gk. mesos, middle, and derm,
skin] In an embryo, a primary tissue layer (germ layer)
between ectoderm and endoderm. Gives rise to muscle;
organs of circulation, reproduction, and excretion; most of the
internal skeleton; and connective tissue layers of the gastroin-
testinal tract and integument.
messenger RNA (mRNA) A linear sequence of ribonucleo-
tides transcribed from DNA and translated into a polypeptide
chain; the only type of RNA that carries protein-building
instructions.
metabolic acidosis Lower than optimal blood pH caused by
diabetes mellitus.
metabolic alkalosis Higher than optimal pH in blood and
other body fluids.
metabolic pathway An orderly sequence of enzyme- driven
reactions by which cells maintain, increase, or decrease the
concentrations of particular substances.
metabolic syndrome A cluster of symptoms, including
slightly elevated blood sugar that increases the risk of devel-
oping type 2 diabetes.
metabolism (meh-tab-oh-lizm) [Gk. meta, change] All
controlled, enzyme-driven chemical reactions by which
cells acquire and use energy. Through these reactions, cells
synthesize, store, break apart, and eliminate substances in
ways that contribute to growth, survival, and reproduction.
metaphase Of mitosis or meiosis II, the stage when each
duplicated chromosome has become positioned at the
midpoint of the microtubular spindle, with its two sister chro-
matids attached to microtubules from opposite spindle poles.
Of meiosis I, the stage when all pairs of homologous chromo-
somes are positioned at the spindle’s midpoint, with the two
members of each pair attached to opposite spindle poles.
metastasis The process in which cancer cells break away
from a primary tumor and migrate (via blood or lymphatic
tissues) to other locations, where they establish new cancer
sites.
MHC marker Any of a variety of proteins that are self mark-
ers. Some occur on all body cells of an individual; others
occur only on macrophages and lymphocytes.
micelle (my-cell) Tiny droplet of bile salts, fatty acids, and
monoglycerides; plays a role in fat absorption from the small
intestine.
microevolution Changes in allele frequencies brought about
by mutation, genetic drift, gene flow, and natural selection.
microfilament [Gk. mikros, small, and L. filum, thread] One
of a variety of cytoskeletal components. Actin and myosin
filaments are examples.
micrograph Photograph of an image brought into view with
the aid of a microscope.
microorganism Organism, usually single-celled, too small to
be observed without a microscope.
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