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in a position in the TMA block to help in orientation. We use dif-
ferent normal tissues in different positions of the TMA to allow the
identification of every column based on their morphology. The
normal tissues, such as liver, kidney, thyroid, skeletal muscle, or
colonic mucosa, could act as an orientation core and be placed in
specific positions outside each of the TMA columns.There is a limit on the number of patients’ samples that could be
put in one TMA block. One of the problems regarding the use of
TMA is that 0.6-mm biopsies of tumor specimens on an array may
not be representative of the whole tumor specimen because of tis-
sue heterogeneity. On the other hand, to increase the representa-
tion of the tissue chosen from the “donor” tissue, we could increase
the number of cores per patient. The number of cores used per
patient may also depend on the nature of the tissue (heterogeneity)
as well as the number of tests. We need to strike a balance between
these factors in designing our individual TMA block. We could use
one core per patient if the tissue is homogeneous and the tests are
easy to do and reproducible. In research with uncertain outcome,
we recommend to increase the number of cores (around 2–3 cores)
for each patient.
We need to plan the number of samples that will be arrayed
and create a design accordingly for the microarray block. In some
instances, the space of the paraffin wax can be used to a maximum
such as using 40 mm × 25 mm area with array 300–500 samples
[ 13 ]. On the other hand, we could use a smaller area in the block
(30 mm × 20 mm) to do TMA of 60–120 samples.
We could take three cores from one patient and put them side
by side in one TMA block. The spacing between the centers of two
nearby specimens on the array will vary according to the size of
punch used. For each core of tissue in the TMA, the sample is
0.6 mm in diameter. For TMA with many samples, the spacing
between the samples is 0.8 mm. If we would like to construct a
TMA, which house less samples, we could use a wider spacing of
1.0 mm (or 2.0 mm when using 1.5 mm punches). The advantage
of having wider space between each core is easier to add additional
samples if needed. In addition, a TMA with less samples allows
more space at the edges of the block. It is important to leave suf-
ficient space at the edges of the array block (minimum 5–6 mm
margins) to avoid cracking of the paraffin before and during the
cutting of the TMA.3 Methods
There are three methods of tissue microarray. There are many
models for each of these three methods. There are advantages of
each of these methods and the choice depends on the operator
preference (see Note 2).2.4 Design of TMA
Nassim Saremi and Alfred K. Lam