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What are the advantages?
There are numerous
advantages to this technology including:
- Amplification of a scarce resource
- Experimental uniformity
- Decreased assay volume
- Does not destroy original block for diagnosis
A standard histologic section is about 3-5mm thick, with
variation depending on the submitting pathologist or tech.
After use for primary diagnosis, the sections can be cut
50-100 times depending on the care and skill of the sectioning
technician. Thus, on average, each archived block might yield
material for a maximum of 100 assays. If this same block
is processed for optimal microarray construction it could
routinely be needle biopsied 200-300 times or more depending
on the size of the tumor in the original block (Theoretically
it could be biopsied 1000's of times based on calculations
of area, but empirically, 200-300 is selected as a conservative
estimation) Then, once tissue microarrays are constructed,
they can be judiciously sectioned in order to maximize the
number of sections cut from an array. The sectioning process
uses a tape-based sectioning aid (from Instrumedics Inc.)
that allows cutting of thinner sections. Optimal sectioning
of arrays is obtained with about 2-3 µm sections. Thus instead
of 50-100 conventional sections or samples for analysis from
one tissue biopsy, the microarray technique could produce
material for 500,000 assays (assuming 250 biopsies per section
times 2000 2.5 µm sections per 5mm array block) represented
as 0.6 mm disks of tissue. Thus this technique essentially
amplifies (up to 10,000 fold) the limited tissue resource.
Using this technology, each tissue sample is treated in
an identical manner. Like conventional formalin-fixed paraffin
embedded material, tissue microarrays are amenable to a wide
range of techniques including histochemical stains, immunologic
stains with either chromogenic or fluorescent visualization,
in situ hybridization (including both mRNA ISH and FISH),
and even tissue micro-dissection techniques. For each of
these protocols, conventional sections can have substantial
slide to slide variability associated with processing 300
slides (for example, 20 batches of 15 slides). The tissue
microarrays allow the entire cohort to be analyzed in one
batch on a single slide. Thus reagent concentrations are
identical for each case, as are incubation times and temperatures,
wash conditions, and antigen retrieval, if necessary.
Another significant advantage is that only a very small
(a few µl) amount of reagent is required to analyze an entire
cohort. This advantage raises the possibility of use of tissue
microarrays in screening procedures (for example in hybridoma
screening), a protocol that is impossible using conventional
sections. It also saves money when reagents are costly.
Finally, there are occasions where the original block must
be returned to the patient or donating institution. In these
cases the block may be cored a few times without destroying
the block. Then upon subsequent sectioning, it is still possible
to make a diagnosis, even though tissue has been taken for
array-based studies. This is illustrated in the adjoining
figures.
   
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