What are tissue microarrays?

Tissue microarrays are produced by a method of re-locating tissue from conventional histologic paraffin blocks such that tissue from multiple patients or blocks can be seen on the same slide. This is done by using a needle to biopsy a standard histologic sections and placing the core into an array on a recipient paraffin block. This technique, originally described by in 1987 by Wan, Fortuna and Furmanski in Journal of Immunological Methods. They published a modification of Battifora's "sausage" block technique whereby tissue cores were placed in specific spatially fixed positions in a block. The technique was popularized by Kononen and colleagues in the laboratory of Ollie Kallioneimi after a publication in Nature Medicine in 1998. This technology should not be confused with DNA microarrays where each tiny spot represents a unique cloned cDNA or oligonucleotide. In tissue microarrays, the spots are larger and contain small histologic sections from unique tissues or tumors.

Figure 1 shows an example of a tissue microarray and its construction. The arrays are assembled by taking core needle “biopsies” from specific locations in pre-existing paraffin-embedded tissue blocks and re-embedding them in an arrayed “master” block, using techniques and an apparatus developed by Konenen et al. In this way, tissue from 600 specimens can be represented in a single paraffin block.

Figure 2 shows an overview of a completed colon cancer array and higher magnification views of spots from this array and a melanoma array. Stains shown in these examples include hematoxylin and eosin and DAB based-immunoperoxidase. The current Beecher Instruments arraying device is designed to produce sample circular spots that are 0.6mm in diameter at a spacing of 0.7-0.8mm. The surface area of each sample is 0.282 mm2, or in pathologists’ terms, about the size of 2-3 high power fields. The number of spots on a single slide is variable depending on the array design, the current comfortable maximum with the 0.6 mm needle is about 600 spots per standard glass microscope slide. New technologies are under development that may allow as many as 2000 or more sections per slide. Using this method, an entire cohort of cases can be analyzed by staining just one or two master array slides, instead of staining hundreds of conventional slides, yet each spot on the array is similar to a conventional slide in that complete demographic and outcome information is maintained for each case so that rigorous statistical analysis can be done as rapidly as the arrays are analyzed.