G-14
Glossary
Menopause
Period of life when, prompted by hormonal changes, ovula-
tion and menstruation cease.
Menstruation
(men
0
stroo-a
9
shun) Te periodic, cyclic discharge of
blood, secretions, tissue, and mucus from the mature female uterus in the
absence of pregnancy.
Merocrine glands
(mer
9
o-krin) Glands that produce secretions inter-
mittently; secretions do not accumulate in the gland.
Mesencephalon
(mes
0
en-sef
9
ah-lon) One of the three primary vesicles
of the developing brain; becomes the midbrain.
Mesenchyme
(meh
9
zin-kīm) Common embryonic tissue from which all
connective tissues arise.
Mesenteries
(mes
0
en-ter
9
ēz) Double-layered extensions of the perito-
neum that support most organs in the abdominal cavity.
Mesoderm
(mez
9
o-derm) Primary germ layer that forms the skeleton
and muscles of the body.
Mesothelium
(mez
0
o-the
9
le-um) Te epithelium found in serous mem-
branes lining the ventral body cavity and covering its organs.
Messenger RNA (mRNA)
Long nucleotide strands that reflect the exact
nucleotide sequences of the genetically active DNA and carry the DNA’s
message.
Metabolic rate
(mĕt
0
ah-bol
9
ik) Energy expended by the body per unit
time.
Metabolic water (water of oxidation)
Water produced from cellular
metabolism (about 10% of our body’s water).
Metabolism
(mĕ-tab
9
o-lizm) Sum total of the chemical reactions occur-
ring in the body cells.
Metaphase
Second stage of mitosis.
Metastasis
(mĕ-tas
9
tah-sis) Te spread of cancer from one body part or
organ into another not directly connected to it.
Metencephalon (aFerbrain)
A secondary brain vesicle; anterior portion
of the rhombencephalon of the developing brain; becomes the pons and
the cerebellum.
MHC (major histocompatibility complex) proteins
Molecules on the
outer surface of the plasma membrane of all cells; help the immune sys-
tem distinguish self from nonself. ± cells recognize antigens only when
combined with these proteins.
Microfilaments
(mi
0
kro-fil
9
ah-ments) Tin strands of the contractile
protein actin.
Microglial cells
(mi-kro
9
gle-al) A type of CNS supporting cell; can
transform into phagocytes in areas of neural damage or inflammation.
Microtubules
(mi
0
kro-tu
9
būlz) One of three types of rods in the cyto-
skeleton of a cell; hollow tubes made of spherical protein that determine
the cell shape as well as the distribution of cellular organelles.
Microvilli
(mi
0
kro-vil
9
i) ±iny projections on the free surfaces of some
epithelial cells; increase surface area for absorption.
Micturition
(mik
0
tu-rish
9
un) Urination, or voiding; emptying the bladder.
Midbrain (mesencephalon)
Region of the brain stem between the dien-
cephalon and the pons.
Midsagittal (median) plane
Specific sagittal plane that lies exactly in the
midline.
Milliequivalents per liter (mEq/L)
Te units used to measure electro-
lyte concentrations of body fluids; a measure of the number of electrical
charges in 1 liter of solution.
Mineralocorticoid
(min
0
er-al
0
ō-kor
9
tih-koyd) Steroid hormone of the
adrenal cortex that regulates Na
1
and K
1
metabolism and fluid balance.
Malignant
(muh-lig
9
nent) Life threatening; pertains to neoplasms that
spread and lead to death, such as cancer.
Malignant melanoma
(mel
0
ah-no
9
mah) Cancer of the melanocytes; can
begin wherever there is pigment.
Mammary glands
(mam
9
mer-e) Milk-producing glands of the breast.
Mandible
(man
9
dĭ-bl) Lower jawbone; U shaped, largest bone of the
face.
Mass number
Sum of the number of protons and neutrons in the
nucleus of an atom.
Mast cells
Immune cells that function to detect foreign substances in
the tissue spaces and initiate local inflammatory responses against them;
typically found clustered deep to an epithelium or along blood vessels.
Mastication
(mas
0
tĭ-ka
9
shun) Chewing.
Meatus
(me-a
9
tus) External opening of a canal.
Mechanical advantage (power lever)
Condition that occurs when the
load is close to the fulcrum and the effort is applied far from the fulcrum;
allows a small effort exerted over a relatively large distance to move a
large load over a small distance.
Mechanical disadvantage (speed lever)
Condition that occurs when the
load is far from the fulcrum and the effort is applied near the fulcrum;
the effort applied must be greater than the load to be moved.
Mechanical energy
Te energy directly involved in moving matter; e.g.,
in bicycle riding, the legs provide the mechanical energy that moves the
pedals.
Mechanoreceptor
(meh
0
kĕ-no-re-sep
9
tor) Receptor sensitive to me-
chanical pressure such as touch, sound, or exerted by muscle contraction.
Medial
(me
9
de-ahl) ±oward the midline of the body.
Median (midsagittal) plane
Specific sagittal plane that lies exactly in the
midline.
Mediastinum
(me
0
de-ah-sti
9
num) Te medial cavity of the thorax con-
taining the heart, great vessels, and trachea.
Medulla
(mĕ-dul
9
ah) Central portion of certain organs.
Medulla oblongata
(mĕ-dul
9
ah ob
0
long-gah
9
tah) Inferiormost part of
the brain stem.
Medullary cavity
Central cavity of a long bone. Contains yellow or red
(bone) marrow.
Meiosis
(mi-o
9
sis) Nuclear division process that reduces the chromo-
somal number by half and results in the formation of four haploid (
n
)
cells; occurs only in certain reproductive organs.
Melanin
(mel
9
ah-nin) Dark pigment formed by cells called melanocytes;
imparts color to skin and hair.
Melatonin
(mel
0
ah-to
9
nin) A hormone secreted by the pineal gland;
secretion peaks at night and helps set sleep-wake cycles; also a powerful
antioxidant.
Membrane potential
Voltage across the plasma membrane.
Membrane receptors
A large, diverse group of integral proteins and gly-
coproteins that serve as binding sites for signaling molecules.
Memory cells
Members of ± cell and B cell clones that provide for im-
munological memory.
Menarche
(mĕ-nar
9
ke) Establishment of menstrual function; the first
menstrual period.
Meninges
(mĕ-nin
9
jēz) Protective coverings of the central nervous sys-
tem; from the most external to the most internal, the dura mater, arach-
noid mater, and pia mater.
Meningitis
(mĕ-nin-ji
9
tis) Inflammation of the meninges.
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