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Methodology Guidelines
Fixation
98
Manual de operaciones de la Cytospin 4
excessive speed or time of cytocentrifugation.
Occasionally one will see a specimen that has a pattern
of cell deposition around the periphery of the deposition
spot, with a loss of cells in the centre. This effect is due to
an excess amount of residual fluid in the centre of the cell
deposition area when the specimen is fixed. Because the
cells in the centre of the area are quite wet, they wash off the
slide as it is immersed into the fixative. The solution to this
problem is to allow longer time for the slide to dry prior to
fixation, and to be exceptionally gentle during immersion of
the slide into the fixative.
Fixation is used to preserve cell samples, to render them
more easily stained, and to produce characteristic patterns
of cell structure that are used to distinguish cell types. Cells
continue their natural living processes after being removed
from body sites. Since they no longer have their normal
blood supply and other supporting environment, they will
begin to degenerate as they run out of required nutrients and
gases and begin to build up waste products. As these events
continue, the cell activates internal repair mechanisms that
eventually result in the cell digesting itself. This is called
autolysis. The rate at which autolysis progresses is different
for different cell types, but does mean that samples should
be processed as quickly as possible. Autolysis can be slowed
significantly by refrigeration, and samples may be held for
some period of time at refrigerator temperature. Where
practical however, specimens should be fixed or processed as
soon as possible.
Fixatives are chemical agents that both kill the cell and
stabilise its structure. The 'killing' also inactivates many
of the enzyme systems of the cell, particularly those
associated with autolysis. Fixatives therefore also function as
preservatives, and well-fixed cytological samples essentially
last indefinitely. Many specific chemicals can be used as
fixatives, and each has specific properties that are desirable
for certain types of study. These fixatives include those that
produce chemical cross-links within the tissues, such as
formalin, and those that precipitate cellular components
such as the alcohols. By far the most common fixative used
A78310250ES 5.1ª Edición
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