skill and experience as well as attention to detail, because too
much pressure produces unacceptable distortion of the cells.
Air-drying and unevenness in the mounting medium are also
Cytocentrifugation has advantages in that the preparatory
method is simpler and less time consuming than filters. Air-dried
slides also can easily be made for staining with various stains as
well as immunohistochemical methods. The main disadvantage
is that some degree of cell loss is practically inevitable.
Choice of method of preparation should be determined by
the number and types of cases of CSF processed in an individual
laboratory as well as the overall specimen volume and distribu-
tion. For example, in a small laboratory not accustomed to pre-
paring filters, cytocentrifugation is usually the method of choice.
Filters are more appropriately used in a laboratory in which a
high volume of CSF samples are processed or one in which fil-
ters are routinely used for other specimen types. Similarly, in
laboratories where thin preparations are routinely used for other
types of body cavity fluids, it may be appropriate to apply this
method to CSF as well. In our laboratory, both membrane fil-
ters and cytocentrifuged slides are prepared on each case.2 Milli-
pore filters give the best cell yield, whereas the cellular detail on
Papanicolaou-stained cytocentrifuged slides is often superior.
Air-dried slides prepared by this method are used for Diff-Quik
staining and immunohistochemistry.
Fine-Needle Aspiration Biopsy
The method of preparation of samples obtained by needle aspi-
ration depends on the size and consistency of the tissue. Large
pieces of firm tissue are best processed for histologic evalu-
ation as cell blocks. Small fragments of firm tissue are often
used for squash preparations, and loosely aggregated cells may
be smeared.3,4 In any case, additional cellular material may be
obtained by washing the needle with a balanced salt solution,
which may be used for filters or cytocentrifuged preparations.
It is critical that the observer be familiar with the presentation
of common brain lesions in the type of preparation being exam-
ined. For example, because the perceived cellularity is one of the
most important parameters in distinguishing normal brain cells
from gliosis and low-grade astrocytomas, experience in judging
the thickness of the smeared or squashed material is necessary
in making this determination.
These preparations may be stained with various methods.
Hematoxylin and eosin are standard for cell blocks, whereas the
Papanicolaou stain is generally applied to other types of slides.
In addition, it is frequently helpful to prepare air-dried slides,
which may be used for immunohistochemistry, particularly in
high-grade tumors in which malignant gliomas must be distin-
guished from metastasis and lymphoma.
Normal Cerebrospinal Fluid and Histology
The CSF is formed in the ventricles of the brain, and it travels
posteriorly to exit into the SAS, which covers the brain and spi-
nal cord. The SAS is lined externally by the thin arachnoid mem-
brane composed of meningothelial cells and internally by the
pial surface of the brain. The only cellular elements normally
seen in the SAS are blood vessels and strands of loose connective
tissue. Although CSF is theoretically acellular, small numbers of
lymphocytes and occasional monocytes are seen in most sam-
ples of CSF (Fig. 16.1). Based on evaluation of samples from
patients in whom no neurologic disorders are found, as well
as from patients with seizures and those undergoing myelog-
raphy, cell counts of less than 5 or 10 cells/mm3 are considered
to be normal in adults. Slightly higher cell counts may be seen
in neonates, although there are seldom opportunities to exam-
ine fluid from neurologically uninvolved individuals in this age
The composition of the cellular component and the mor-
phology of the individual cells are critical factors in deter-
mining whether the cells in a sample of CSF are normal or
pathologic. Mature lymphocytes of small or intermediate size
and a few monocytes, presumably derived from blood, are the
cell types seen in normal CSF (see Fig. 16.1). The presence of
polymorphonuclear leukocytes and of enlarged atypical lym-
phocytes or monocytes that have become phagocytic, as well
as an increase in absolute cell number, all are indications that
a pathologic condition exists. In some reports, these atypical,
reactive elements have been termed pia arachnoid cells, sug-
gesting that they are derived from proliferations of the lining
cells of the SAS in a manner analogous to the reactive mesothe-
lium of effusions. The demonstration that these cells express
either lymphoid or monocytic antigens, however, and their
morphologic resemblance to inflammatory cells rather than
Fig. 16.1 Normal components of cerebrospinal fluid (CSF) include lymphocytes and monocytes ((A) Papanicolaou xHP; (B) Diff-Quik x HP).