A pathologist is a medical doctor who specializes in diagnosing diseases by analyzing body fluids, cells, and tissues. When it comes to the intestines, this role involves the detailed examination of samples, which may range from tiny biopsies collected during an endoscopy to large sections of the bowel removed during surgery. The analysis of these samples provides the definitive diagnosis for a wide range of conditions, from inflammatory disorders to cancer. The pathologist’s findings are then translated into a report that guides the patient’s gastroenterologist and surgeon in determining the correct course of treatment.
Preparing Intestinal Tissue for Examination
The journey of an intestinal sample, whether a small pinch of tissue or a major surgical resection, begins with the pre-analytic phase. Immediately upon removal, the tissue must be placed into a fixative, most commonly a 10% buffered formalin solution, to prevent cellular breakdown and preserve structure. This fixation process chemically stabilizes the cells and proteins, which is necessary for all subsequent steps.
For larger surgical specimens, a pathologist performs a “gross examination,” where the specimen is physically described, measured, and weighed. They carefully examine the tissue margins, often applying colored inks to mark the edges of the removed section, which indicates whether the entire disease, like a tumor, has been excised. Selected areas of tissue, representative of the disease and the normal surrounding tissue, are then sampled for further processing.
Following fixation, the tissue undergoes processing, where water is removed through a series of alcohol baths. The dehydrated tissue is then cleared with a substance like xylene before being infiltrated and embedded into a block of molten paraffin wax. Once the wax cools and solidifies, it provides a firm medium that allows a specialized machine called a microtome to cut extremely thin slices, typically between three and five micrometers thick, which are then mounted onto glass slides.
Microscopic Analysis and Initial Interpretation
The prepared slices are then subjected to the fundamental diagnostic technique: Hematoxylin and Eosin (H&E) staining. This process uses two contrasting dyes to make the transparent tissue visible under the microscope. Hematoxylin is a basic dye that stains acidic components, primarily the cell nuclei, a purplish-blue color, while Eosin is an acidic dye that stains basic components, such as the cytoplasm and extracellular matrix, various shades of pink.
The pathologist reviews the stained slide to assess the overall architecture of the intestinal wall, observing the integrity of the mucosa, submucosa, and muscular layers. They look for any disorganization, such as crypt distortion or villous blunting, which are hallmarks of disease. Initial interpretation focuses on cellular changes, including inflammatory cells, signs of atrophy, or increased cell division.
Specific visual findings, such as the pattern of inflammatory cell infiltration, can point the pathologist toward an initial diagnosis. For instance, the presence of lymphocytes and plasma cells in the lamina propria is normal, but an abnormal increase in these cells or the appearance of neutrophils suggests an active inflammatory process. The pathologist correlates these microscopic observations with the patient’s clinical history to form a preliminary diagnostic picture.
Key Conditions Identified in Intestinal Samples
The pathologist’s detailed analysis allows for the differentiation of the major categories of intestinal disease based on distinct patterns of injury and cellular change. Malignancy, particularly colorectal cancer, is identified by cellular atypia, meaning the cells appear abnormal and grow in a disorganized, invasive manner. Pre-cancerous lesions, like adenomatous polyps, show varying degrees of dysplasia, where the epithelial cells are structurally abnormal but have not yet invaded the underlying tissue.
Inflammatory Bowel Diseases (IBD) are distinguished by their unique patterns of inflammation. Ulcerative colitis typically presents with inflammation that is continuous and limited to the mucosa and submucosa of the colon. Crohn’s disease is characterized by “skip lesions,” or patches of disease interspersed with healthy tissue, and transmural inflammation, meaning the inflammation extends through all layers of the intestinal wall. The presence of non-caseating granulomas is a specific microscopic feature that strongly suggests a diagnosis of Crohn’s disease.
Infectious enteritis, caused by organisms like bacteria or parasites, is diagnosed when the pathologist identifies specific tissue damage patterns or the causative agent itself. For example, some bacterial infections cause acute inflammation with surface damage, while certain parasitic infections can be identified by the physical presence of the organism within the tissue section.
Specialized Testing for Precision Diagnosis
When the routine H&E analysis does not provide a definitive answer, specialized testing is employed. One common advanced tool is Immunohistochemistry (IHC), which uses antibodies to visualize specific proteins. By targeting proteins, such as those related to cellular origin or proliferation markers, IHC can confirm the type of tumor, identify the source of a metastatic cancer, or reveal the loss of mismatch repair proteins, which is a sign of Lynch syndrome.
Molecular and genetic testing represents the cutting edge of precision diagnosis, especially in oncology. This testing analyzes the DNA and RNA extracted from the intestinal tissue to look for specific gene mutations, such as in KRAS, NRAS, or BRAF, that are common in colorectal cancer. The results of these tests are paramount for guiding targeted therapy, as certain drugs are only effective against tumors with specific genetic profiles. Understanding these molecular signatures allows the pathologist to provide prognostic information and predict treatment response.