A biopsy is a medical procedure that involves removing a small sample of tissue or cells from the body for detailed examination. This sample is then subjected to histology, the foundational laboratory method for studying the microscopic structure of tissues. The purpose of this process is to provide a definitive diagnosis, such as identifying if a growth is benign or malignant, or determining the cause of inflammation or infection. This visual evidence guides subsequent treatment decisions by allowing medical professionals to understand the disease at a cellular level.
Methods for Tissue Collection
The method used to acquire a tissue sample is selected based on the location, size, and nature of the area of concern. Fine-needle aspiration (FNA) is the least invasive technique, utilizing a very thin, hollow needle to draw out a small collection of cells and fluid. FNA is typically used for easily accessible masses, such as in the breast or thyroid, but it retrieves cells rather than the intact tissue architecture.
A core needle biopsy uses a larger, hollow needle to extract a small cylinder of tissue, often guided by imaging techniques like ultrasound or CT scans. This method provides an intact piece of tissue that preserves the relationship between the cells. Endoscopic biopsies are performed during procedures like a colonoscopy, where specialized tools snip small pieces of tissue from internal organ linings.
When a larger sample is necessary, a surgical approach is used, categorized as incisional or excisional biopsies. An incisional biopsy removes only a portion of the suspicious mass, while an excisional biopsy removes the entire abnormal area. Surgical methods typically require local or general anesthesia and a longer recovery period than needle-based sampling.
Preparing the Sample for Analysis
Once a tissue sample is collected, it immediately enters the histology laboratory for preparation. The first and most time-sensitive step is fixation, which involves immersing the tissue in a stabilizing solution, typically 10% neutral buffered formalin. Fixation halts cellular decay and cross-links proteins, preserving the cellular structure and architecture as they were at the moment of collection.
Following fixation, the tissue must be prepared for slicing by removing all water through dehydration and clearing. The tissue is run through increasing concentrations of alcohol to extract water, as the subsequent embedding material is incompatible with aqueous solutions. Next, a clearing agent, often xylene, is introduced to remove the alcohol and prepare the tissue for the final embedding medium.
The tissue is then embedded into a solid block of paraffin wax, which acts as a rigid support structure. This wax block is cooled and hardened, making the delicate tissue firm enough for precision slicing. The block is mounted onto a microtome, which uses an extremely sharp blade to cut sections, usually 3 to 8 micrometers thick. These thin slices are floated onto a warm water bath to flatten before being mounted onto a glass slide.
Since the fixed tissue is largely colorless, it must be stained to make the internal cellular components visible under a microscope. The most common technique is the Hematoxylin and Eosin (H&E) stain, which imparts distinct colors to different cellular structures. Hematoxylin stains cell nuclei deep blue, while Eosin stains the cytoplasm and extracellular components pink. This contrast allows the pathologist to differentiate between cell types and structural patterns for diagnostic review.
Interpreting the Findings
The completed glass slide is sent to a pathologist, a physician who specializes in translating the visual information of the stained tissue into a formal medical diagnosis. The pathologist examines the slide under a microscope, looking for alterations in the normal arrangement and appearance of cells, known as cellular architecture. Recognizing subtle shifts from normal tissue patterns allows them to identify diseases like cancer, infection, or chronic inflammation.
If a malignancy is identified, the pathologist provides details including the tumor grade, which measures how abnormal the cancer cells look and how quickly they are likely to grow. They also contribute information for the disease stage, describing the extent of the disease, such as its size and whether it has spread to other tissues. This information is crucial for the treating physician to select the most appropriate therapy.
The pathologist compiles all findings into a structured pathology report, which serves as the formal diagnostic document. This report typically begins with a gross description of the sample, followed by the detailed microscopic findings. Specialized tests, such as immunohistochemistry, may be mentioned if used to confirm findings by staining for specific proteins. The document concludes with the final diagnosis, informing patient care.