Formalin-fixed paraffin embedded (FFPE) is a method of preparation and preservation for biospecimens that are a staple of therapeutic and research applications for many decades. It aids experimental research, examination, and drug or diagnostic development. A sample is preserved by fixing it in formalin or formaldehyde to help preserve the structures in the tissue. It is then embedded in paraffin wax, sectioned, and mounted on a microscopic slide for further examination. The biospecimens that can be subjected through the FFPE method may be obtained from both normal or diseased tissues and human or any other living organism.
Once the tissue sample is obtained from the host, the tissue is immediately immersed in 10 percent neutral buffered formalin for 18 to 24 hours. Once ready, dehydrate and clear the tissue using increasing concentrations of ethanol. It is then embedded into paraffin. Samples should be handled with care to ensure the maintenance of quality. A certified pathologist helps ensure the quality of the sample. Once the sample is completed, it is stored in tissue banks such as research centers, universities, and hospitals. It is also important to keep available data or information such as origin, stage of the disease, donor age, and etcetera that is associated with the biospecimen. Another crucial point is to store the legal documents and signed consent forms for the tissue samples as it can affect the usability of the samples in clinical trials or research.
FFPE tissues are commonly used in immunohistochemistry (IHC). The information obtained from IHC can be vital especially in the detection of disease. Some of the applications of FFPE include:
Hematology – FFPE tissues can be used for the study of blood and related disorders which can be important in genetics, tissue regeneration, and toxicology.
Oncology – FFPE tissues are key to cancer research as the presence of specific proteins can help with assessment and diagnosis of cancer. The minimum tumor content in FFPE tumor tissues is generally 60 percent.
Immunology – FFPE tissues are useful in the analysis of the immune system response in both diseased and healthy states. It can help with the development of treatment.
Comparative – FFPE tissues that are both healthy and diseases are necessary for comparative purposes.
Tissue microarray (TMA) is an innovation that is expected to overcome issues where the validation of markers in standard histopathological techniques are:
Especially when multiple markers are required to be tested on various specimens. This is due to the high-throughput molecular biology design where it allows simultaneous assessment of expression of interesting candidate-related genes and gene products on hundreds of biospecimens. The TMA technique also allows:
Parallel molecular profiling of proteins, DNA, and RNA
Large scale analyses using IHC, RNA in situ hybridization, and fluorescence in situ hybridization (FISH) at significantly lower costs and lesser time.
The TMA Technique
The technique uses composite paraffin blocks (such as FFPE specimens) that are constructed through the extraction of cylindrical cores from various paraffin blocks which are then re-embedded into a recipient or microarray block at specific array coordinates.
A TMA instrument is used to obtain a tissue core from the donor block.
The core is placed in the recipient block at specifically assigned coordinates and recorded on a spreadsheet.
This block is then sectioned using a microtome, mounted, and analyzed.
Each block can be cut into 100 to 500 sections.
TMA Advantages and Applications
There are many advantages of TMA compared to other techniques. Using the TMA technique, analysis of an entire cohort of cases is made possible just by staining one to two master slides. Other advantages include:
Amplification of a scarce resource
Simultaneous analysis of many specimens
Preservation of original block and conservation of valuable tissue
Decreased use of assay volume
Shorter duration and more cost-effective
TMA has been proven to be an efficient and effective tool in the assessment of quality assurance programs. A TMA block can be created from various tissue specimens, sectioned, and distributed to different labs that perform molecular tests and immunostaining. Therefore, it can facilitate the standardization of FISH, IHC, and other molecular assays so results would be reproducible. Other applications include:
Internal quality control
Optimization of diagnostic reagents
Facilitation of rapid translation of molecular discoveries to clinical applications
Clinical validation of histopathological specimens
One main criticism of the technique is that the cores used for TMA may not be representative of the entire tumor especially in heterogeneous cancers like Hodgkin lymphoma and prostate adenocarcinoma. However, there are many studies that have shown high concordance between TMA spots and whole sections in IHC of multiple tumor types. Another minor criticism would be the absence of one or more core sections. This can be addressed by the statistical power of analysis ranging from hundreds to thousands of cases as it eliminates the variability of a single data point in the conclusion.
Jawhar NMT. Tissue microarray: a rapidly evolving diagnostic and research tool. Ann Saudi Med. 2009; 29(2): 123-127.
Tissue microarray. Wikipedia. Accessed 4/30/2019. https://en.wikipedia.org/wiki/Tissue_microarray#Procedure
What is FFPE tissue and what are its uses. BioChain. Accessed 4/30/2019. https://www.biochain.com/general/what-is-ffpe-tissue/