Tissue microarray

Introduction

∗ This is an abridged version of this topic in Chapter 6 of the 7 ^th edition of this text

Tissue microarray (TMA) is a method used to evaluate numerous samples of tissue in a short time. First introduced by and further developed by it is used to examine several histological sections at the same time by arraying them in a single paraffin block. TMA uses multiple tissue samples arranged in a single paraffin block using precision tools to prepare the recipient block.

Histological techniques play an important role in molecular biology and TMA has become a useful diagnostic tool, it conserves tissue samples saving time for both research and clinical work. TMA has applications in clinical pathology serving as quality control for new antibodies. The production of antibodies is an expensive and lengthy process, and TMA is a unique tool which aids the streamlining of the cumbersome validation and quality control of archival tissue as well as daily immunohistochemistry (IHC) controls.

TMA enables the study and evaluation of many diseases. A hollow needle is used to take numerous tissue core samples from specific areas of a pre-existing block of tissue and then these are placed in a single array block. Sections are taken from this block, the exact number depending on the size of the cores and the experience of the technologist, ultimately producing a single slide containing hundreds of tissue cores for review. The technique can be used for a wide range of staining procedures, including IHC, in situ hybridization (ISH), fluorescent in situ hybridization (FISH), special stain control samples and quality control sections for H&E. Only a small amount of reagent is used to analyze each slide, making TMA cost-effective, particularly with IHC and ISH techniques. TMAs have been widely used in IHC for quality control and assurance. They may demonstrate the antibody thresholds on a single slide which optimizes where the high and low signal intensities are seen.

Types of tissue microarrays

Prevalence TMAs are assembled from tumor samples of one or several types without attached clinical and pathological information. They are used to determine the prevalence of a given alteration in a specific area of interest in a tumor.

Progression TMAs contain samples of different stages of one tumor type and are used to discover associations between tumor genotype and phenotype. For example, a breast cancer progression TMA could contain samples of normal breast from patients with and without a history of breast cancer, different non-neoplastic breast diseases, ductal and lobular carcinoma in situ, invasive cancer of all stages, grades and histological subtypes, as well as metastases and recurrences after initially successful treatment.

Prognosis TMAs contain samples from tumors available with clinical follow-up data and represent a fast and reliable method for the evaluation of clinical importance of newly detected disease-related genes. Validation studies using prognosis TMAs can establish the associations between molecular findings and clinical outcomes.

Experimental TMAs are constructed from cell lines or samples from TMA archives for testing new antibodies and looking for gene targets.

Designing the grid

The design of the grid varies, depending on the purpose of the array, and needs considerable thought before tissue transfer occurs. The pathologist and technologist determine the guidelines according to the purpose and utilization of each specific laboratory. An array uses a series of 50 or more samples, set into one or several blocks. It is important to plan and record in advance how many samples will be arrayed, and to create a map or grid sheet. A large number of samples (high density) can be arrayed in a 37 × 24 × 5 mm block and a smaller number of samples (low density) in a 24 × 24 × 5 mm block.

Normal tissue controls and control cell lines are placed in columns between the tumors and normal tissue can be sited asymmetrically at one end of the block. Placing a notch at the end of the cassette block helps to confirm later that the orientation of the block is correct. Archived blocks can be used as a source of control tissue without destructive sampling. Making the tissue array is a multistep project. Selecting the slides, collecting the blocks and designing the grid consume the time, rather than the array process itself. Standardizing the construction of the grid makes it easier to follow, but it can still take several weeks before the array process begins. The organization of the blocks and slides is critical throughout the TMA process.

Fixation and processing of tissues and controls (see Chapter 4 , Chapter 6 )