Histology is the microscopic study of tissues, forming the foundation for diagnosing a wide range of diseases. Histology results originate from tissue samples, typically obtained through a biopsy or surgical excision, and are recorded in a formal pathology report. These findings provide clinicians with the detailed information needed to confirm a diagnosis, determine the severity of a condition, and plan appropriate treatment.
From Sample to Slide: The Histology Process
The preparation of a tissue sample for microscopic analysis is a multi-step laboratory procedure that preserves the specimen’s structure. The first step, fixation, immediately halts tissue decay, commonly using a chemical solution like 10% neutral buffered formalin. This process chemically stabilizes cellular structures, preventing the tissue from changing its appearance after removal.
Following fixation, the sample undergoes processing where water is removed and replaced with a rigid medium for thin slicing. This involves gradually dehydrating the tissue with increasing concentrations of alcohol, followed by a clearing agent like xylene. The tissue is then embedded in molten paraffin wax, which solidifies into a block that can be precisely cut without tearing the structure.
A specialized instrument called a microtome is used for sectioning, slicing the wax-embedded block into sections about five micrometers thick. These transparent sections are floated onto glass slides and stained to make the structures visible under a microscope. The most common method is Hematoxylin and Eosin (H&E), where hematoxylin stains cell nuclei blue and eosin stains other structures pink, creating the necessary contrast for cellular detail.
The Role of the Pathologist
The pathologist is a medical doctor specializing in diagnosing disease by examining tissues, organs, and body fluids. They translate the prepared tissue slide into a formal medical diagnosis for the treating physician and the patient. Their responsibility begins with the macroscopic, or “gross,” examination of the specimen, where they measure, describe, and select the most representative areas for processing.
Once the stained slides are ready, the pathologist correlates the microscopic findings with the patient’s clinical history. They analyze the cellular architecture, looking for abnormal cell shapes, altered tissue organization, and signs of inflammation. If the initial H&E stain is not conclusive, the pathologist may order special stains or advanced molecular tests to identify specific proteins, genetic markers, or infectious agents. The pathologist authors the final report, which provides the definitive diagnosis guiding subsequent patient care.
Deciphering the Histology Report
The histology report communicates the pathologist’s findings and diagnosis to the medical team. One of the first sections is the Gross Description, which details the specimen as it appeared to the naked eye, including its size, weight, color, and texture. This provides an objective record of the physical specimen before it was processed for microscopic review.
The Microscopic Description follows, presenting the pathologist’s detailed observations of the tissue under the microscope. This narrative describes the cellular features, cell organization, and any abnormalities, functioning as the evidence supporting the final conclusion. This section often contains technical terms and explains the observed cellular and structural changes.
The most anticipated part of the document is the Diagnosis, a concise statement providing the final medical conclusion. For surgical excisions, especially those involving tumors, the report includes a section on Surgical Margins. This refers to the rim of healthy tissue surrounding the main specimen removed by the surgeon.
A “negative” or “clear” margin means no diseased cells were found at the edge of the removed tissue, suggesting the entire lesion was successfully taken out. Conversely, a “positive” or “involved” margin indicates that diseased cells were present at the edge of the specimen, raising the possibility that abnormal tissue remains. The margin status directly influences whether further surgery or treatment is necessary.
Common Diagnostic Outcomes
Histology results fall into three broad categories that determine clinical significance and next steps. The first outcome is a Normal or Reactive Change, meaning the tissue is structurally healthy or shows a temporary response to injury or infection. Examples include inflammation reacting to a minor injury or foreign substance, or a healing process proceeding as expected.
A second outcome is a Benign Finding, which refers to a non-cancerous growth or condition. Benign tumors are characterized by cells that closely resemble normal tissue and are confined to a single, well-defined area, often surrounded by a capsule. These growths are slow-growing and do not invade surrounding tissues or spread to distant parts of the body.
The third category is a Malignant Finding, which represents cancer. Malignant cells display highly abnormal features, such as irregular shapes, large or multiple nuclei, and a rapid, uncontrolled rate of division. These cells infiltrate and destroy local tissue and spread through the bloodstream or lymphatic system in a process known as metastasis.
For malignant tumors, the report includes an assessment of tumor grading, which describes how abnormal the cancer cells look under the microscope. Grading is assigned on a scale, such as Grade 1 through 4. A lower number indicates “well-differentiated” cells that resemble normal tissue, suggesting a less aggressive tumor. A higher grade means the cells are “poorly differentiated” or “undifferentiated,” appearing highly abnormal and indicating a more aggressive disease. Tumor staging, which evaluates the size and spread of the cancer, is a separate process combining histology findings with imaging and clinical information.