Pathology slides are thin slices of biological tissue mounted on glass, which allow medical professionals to examine cellular structures under a microscope. These slides transform a small piece of tissue, often obtained through a biopsy or surgery, into a visual representation that aids in disease diagnosis. Pathologists, who are specialized physicians, use this physical evidence to identify signs of infection, inflammation, and cellular abnormalities like cancer. The preparation of these specimens is a precise laboratory process that preserves the tissue’s structure, making microscopic evaluation possible and guiding treatment decisions.
Creating the Pathology Slide
The creation of a pathology slide begins with fixation, a process that prevents the tissue from decaying and preserves its structure. Most specimens are immersed in a 10% neutral buffered formalin solution, which chemically cross-links proteins within the cells. This preservation step typically requires the tissue to be in the fixative for 6 to 24 hours to ensure thorough penetration.
After fixation, the tissue undergoes processing to prepare it for sectioning, which involves removing all the water. The tissue is passed through a series of increasing concentrations of alcohol, a process called dehydration, to replace the water with alcohol. Next, a clearing agent like xylene is introduced to remove the alcohol, as it is miscible with the final embedding medium.
The tissue is then placed into a mold and infiltrated with molten paraffin wax, known as embedding. As the wax cools and solidifies, it forms a solid block that provides the necessary support for the soft tissue. This solid block is then mounted onto a specialized instrument called a microtome.
The microtome uses a sharp blade to cut the wax block into extremely thin sections, often measuring only 3 to 5 micrometers thick. These delicate ribbons of tissue are carefully floated onto a warm water bath to remove wrinkles, and then picked up onto a glass slide. Because the tissue is naturally transparent, the final step involves staining the slice to add contrast and color to cellular components.
Interpreting the Visual Evidence
The stained slide is then ready for microscopic examination by a pathologist. The most common staining method used is the Hematoxylin and Eosin (H&E) stain, which employs two different dyes to color various cell parts. Hematoxylin is a basic dye that stains structures containing nucleic acids, such as the cell nucleus, a purplish-blue color.
Eosin is an acidic dye that stains proteins in the cytoplasm and the extracellular matrix a vibrant pink or red. This two-color contrast allows the pathologist to clearly distinguish the cell’s nucleus from its cytoplasm. Pathologists examine the slide for changes in cellular size, shape, and arrangement, comparing the specimen to the appearance of healthy tissue.
For instance, signs of malignancy often include nuclei that are abnormally large, irregular in shape, or stained more intensely blue than normal cells. The spatial relationship between cells and the overall tissue pattern are also evaluated to identify disease processes like inflammation, infection, or the disorganized growth associated with cancer.
Transitioning to Digital Pathology
The traditional practice of viewing physical glass slides through a microscope is increasingly being supplemented by digital pathology. This modern approach involves using a specialized, high-resolution scanner to capture the entire physical slide. The result is a whole slide image (WSI), a digital file that can be viewed on a computer screen.
Whole slide imaging allows pathologists to examine the tissue virtually, with the ability to zoom in and out across the entire specimen. This digitization significantly improves workflow by enabling remote consultation and collaboration among specialists, eliminating geographical barriers. Digital slides are also easily archived and stored without the risk of physical degradation or breakage.
The digital format also supports the integration of artificial intelligence (AI) tools, which can analyze the images to automate certain tasks. AI algorithms can assist in quantifying cellular features, identifying rare events, and standardizing the grading of tumors. The digital image is becoming the primary tool for review, storage, and advanced analysis in the field.