Tissue Microarrays

DNA microarrays provide invaluable tools for defining the global expression profile of a particular tissue; however, these studies are limited in that they are relatively expensive and time-consuming and rely on fresh frozen tissue that often is not readily available. Tissue microarrays (TMAs) allow the high-throughput analysis of paraffin-embedded tissues. This technique, originally described by Kononen et al.,5 allows hundreds of tissue cores to be placed in a single paraffin block known as a TMA. Two instruments for the production of TMA blocks are available from Beecher Instruments (Sun Prairie, WI) and from Chemicon International (Advanced Tissue Arrayer,

Figure 29-2. Tissue microarray construction and analysis. Tissue specimens embedded in paraffin blocks are sampled with a metallic punch,and the tissue cores are transferred to an empty paraffin block to produce a tissue microarray. Sections taken from the tissue microarray block will contain sections of all the sampled tissues, which can be studied simultaneously by immunohistochemistry (IHC), in situ hybridization (ISH), or fluorescent in situ hybridization (FISH).

Figure 29-2. Tissue microarray construction and analysis. Tissue specimens embedded in paraffin blocks are sampled with a metallic punch,and the tissue cores are transferred to an empty paraffin block to produce a tissue microarray. Sections taken from the tissue microarray block will contain sections of all the sampled tissues, which can be studied simultaneously by immunohistochemistry (IHC), in situ hybridization (ISH), or fluorescent in situ hybridization (FISH).

Temecula, CA). The construction of a TMA (Figure 29-2) involves assembling a paraffin block containing hundreds of tissue cores (0.6 mm in diameter) derived from different "donor" blocks. Since the cores are very small, minimal damage is done to the original block. The technology is based on precision micrometers, which move a needle (0.6mm to 2 mm in diameter) in the x- and y-axis. Micrometer drives are used to position the punch assembly with respect to the donor and recipient blocks. Once the TMA block is created, at least 50 histologic sections can be produced from the TMA block for either immunohisto-chemisty or in situ hybridization (fluorescent, radioactive, or nonradioactive probes).

TMAs can be created from single or multiple tumor types, different grades or stages of tumors, using human, mouse, or other animal tissues, or even cultured cell lines. They do present a significant challenge of data management, which is being addressed with new technologies. Technologies also are available to produce TMA blocks from frozen tissues.6,7 Several scanners based on digital imaging and automated interpretation of immunohisto-chemical results have been commercially developed, including the BLISS imaging system (Bacus Laboratories, Lombard, IL), the ScanScope slide scanner (Aperio Technologies, Vista, CA), the ACIS II (Clarient, San Juan

Capistrano, CA), and the Ariol system (Applied Imaging, San Jose, CA). The results from TMAs have been validated in several studies, which have shown that given adequate redundancy, immunostained TMAs are equivalent to standard whole sections.810

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