The recent advances that have occurred in the human molecular genetics field have found that there are gene-based disease mechanisms in various areas of medicine. Studies regarding diagnostic and prognostic markers in many clinical specimens is vital in the translation of new findings from basic science to applications in clinical practice. With the increased use and advancements of new molecular biology methods, the research of progression and pathogenesis of diseases such as cancer are now revolutionized. Understanding the basic molecular mechanisms in the progression of normal tissues to cancerous or malignant tumors is crucial in the knowledge of the disease as it can lead to improved treatment, diagnosis, and cures. Some clinical studies have discovered various novel markers at the gene level where validation of these markers is necessary. However, it can be a time consuming, costly, and labor-intensive process especially if tested on several specimens.
Tissue microarray is a method used in the field of pathology to overcome issues where the validation of markers is:
It can be used to organize small amounts of tissue samples on a solid support. It is a method designed to allow the:
- Simultaneous assessment of gene expression on hundreds on tissue samples
- Parallel molecular profiling of tissue samples at DNA, RNA, and protein level
- Analysis of samples using fluorescence in situ hybridization (FISH), immunohistochemistry, and RNA in situ hybridization at lower costs and less time
Tissue Microarray Construction
Tissue microarrays can be constructed using composite paraffin blocks through the extraction of cylindrical core biopsies from donor blocks which are them embedded into a microarray or recipient block at specific array coordinates. Donor blocks are first retrieved and sectioned to produce the standard slides. These slides are then stained with hematoxylin and eosin. Once ready, the slides are examined by a certified pathologist who then marks the area of interest (usually an area with pathology such as cancer). Next, the samples can be arrayed. A tissue core can be acquired from the donor block using a tissue microarray instrument. This tissue core is then inserted into an empty recipient or paraffin block at a specific coordinate which is recorded on a spreadsheet. The sampling process is then repeated as many times as necessary from various donor blocks until many cores are placed in one recipient block. This results in the final tissue microarray block. A microtome is utilized to cut 5-micrometer sections from the blocks to produce slides necessary for immunohistochemical and molecular analyses.
Applications and Advantages
Tissue microarrays have many advantages over other techniques. Some of it include:
- Amplification of a scarce resource – After a standard histological section that is approximately 3 to 5 millimeters thick is used in primary diagnosis, the sections can further be cut 50 to 100 times yielding a total of 100 assays. In tissue microarrays, instead of 50 to 100 samples, it can produce material enough for 500,000 assays.
- Simultaneous analysis – Tissue microarrays allows the simultaneous analysis of many specimens as it provides high throughput data acquisition.
- Uniformity – In tissue microarrays, every tissue sample is treated uniformly. It can also be used in a variety of techniques such as fluorescent or chromogenic visualization, histochemical stains, tissue microdissection techniques, and more. Tissue microarray enables the analysis of the entire cohort on one slide standardizing the variables such as incubation times, antigen retrieval, washing procedure, reagent concentration, and temperature.
- Time and cost efficient – The tissue microarray method require small amounts of reagents for analysis. It is a method that is both time and cost efficient.
- Conservation of tissue samples – Tissue microarray is a technique that does not destroy the original block of the tissue sample.
Tissue Microarrays from Fresh Frozen Tissue
In tissue microarray, the method uses tissue samples from paraffin-embedded tissue donor blocks that are then placed into a recipient block. One of the challenges with paraffin-embedded tissue is the antigenic changes seen in proteins and degradation of mRNA due to the fixation and embedding process. Some researchers have modified the tissue microarray technique by using fresh frozen tissue that is embedded in optimal cutting temperature (OCT) compound. It is then arrayed into a recipient OCT block. Tissue samples are not fixed before the embedding process and the arrayed sections are assessed without fixation. The advantage of tissue microarrays from fresh frozen tissue is that:
- It works well for DNA, RNA, and protein analyses.
- Paraffin-embedded tissue arrays can be challenging for RNA in situ hybridization and immunohistochemistry analyses but tissue microarrays from fresh frozen tissue allow the optimal assessment by each technique.
- It has uniform fixation throughout the whole array panel.
- It is a technique that may have significant advantages in the assessment of certain genes and proteins as it improves both quantitative and qualitative results.
1) Jawhar NMT. Tissue microarray: a rapidly evolving diagnostic and research tool. Ann Saudi Med. 2009; 29(2): 123-127.
2) Fezjo MS, Slamon DJ. Tissue microarrays from frozen tissues – OCT technique. Methods Mol Biol. 2010; 664: 73-80.