Webster's New World Medical Dictionary

plasticity, brain
See 
brain plasticity.
platelet
An irregular, disc-shaped element of
blood that assists in blood clotting. During normal
blood clotting, platelets group together (aggregate).
Although platelets are often classified as blood cells,
they are actually fragments of large cells called
megakaryocytes. Also known as thrombocyte. See
also 
blood cell.
platelet count
The calculated number of
platelets in a volume of blood, usually expressed as
platelets per cubic millimeter (cmm) of whole
blood. Platelets are the smallest cell-like structures
in the blood and are important for blood clotting
and plugging damaged blood vessels. Platelet
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platelet count
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counts are usually done by laboratory machines that
also count other blood elements such as white and
red cells. Platelets can also be counted with the help
of a microscope. Normal platelet counts are in the
range of 150,000 to 400,000 per microliter These
values may vary slightly among different laborato-
ries. See also 
CBC.
pleiotropic
Producing or having multiple effects
from a single gene. For example, the Marfan gene is
pleiotropic, potentially causing such diverse effects
as long fingers and toes (arachnodactyly), disloca-
tion of the lens of the eye, and dissecting aneurysm
of the aorta.
pleomorphic
Many-formed. For example, a
pleomorphic tumor would be a growth that is com-
posed of different types of tissues. Also known as
protean.
plethoric
Florid, red-faced. Persons with poly-
cythemia vera commonly have a plethoric facial
appearance.
pleura
One of the two membranes around the
lungs. These two membranes are called the visceral
and parietal pleurae. The visceral pleura envelops
the lung, and the parietal pleura lines the inner
chest wall. There is normally a small quantity (about
3 to 4 teaspoons) of fluid that is spread thinly
between the visceral and parietal pleurae. The pleu-
ral fluid acts as a lubricant between the two mem-
branes.