Interpreting an H&E (hematoxylin and eosin) stained slide starts with understanding what the two dyes are showing you. Hematoxylin stains nuclei blue-purple, while eosin stains cytoplasm and extracellular proteins in shades of pink. Once you can reliably distinguish those two color channels and what they reveal, you can begin reading tissue architecture, identifying cell types, and spotting abnormalities.
How the Two Dyes Work
Hematoxylin is a basic dye extracted from the logwood tree. It carries a positive charge, which means it binds to negatively charged (acidic) structures in cells. The most important of these are the phosphate groups on DNA and RNA. Because nuclei are packed with DNA, they take up hematoxylin strongly and appear blue to dark purple. Structures that attract hematoxylin are called “basophilic.”
Eosin is an acidic dye with a negative charge. It binds to positively charged (basic) structures: most cytoplasmic proteins, mitochondria, ribosomes, collagen, and muscle fibers. These structures turn varying shades of pink and are called “eosinophilic.” The interplay between these two stains gives every structure on the slide a place on a blue-to-pink color spectrum, which is the foundation of everything you’ll interpret.
What Each Color Tells You
The blue-purple signal is your guide to nuclei. Every cell on the slide should have at least one visible nucleus (with the exception of mature red blood cells, which lack nuclei entirely). The intensity of the blue varies: cells that are metabolically active or rapidly dividing tend to have darker, more prominent nuclei because they contain more DNA and RNA. Areas dense with inflammatory cells or tumor cells often appear strikingly blue at low magnification simply because of the sheer number of nuclei packed together.
The pink signal covers almost everything else. Cytoplasm generally appears light pink. Collagen fibers in connective tissue stain a slightly deeper, glassy pink and often have a wavy, fibrous texture. Muscle fibers pick up eosin in a brighter, more uniform pink, and you can distinguish them from collagen by their parallel, organized arrangement and their elongated nuclei pushed to the cell periphery. Red blood cells are intensely eosinophilic, appearing bright orange-pink, and they lack nuclei, making them easy to spot inside blood vessels or areas of hemorrhage.
Some structures fall between the two extremes. Mucin and ground substance can appear pale blue or almost clear. Cartilage matrix often has a glassy, pale purple quality because of its high content of negatively charged molecules. Calcium deposits may appear deep purple-blue. Learning to read these intermediate tones takes practice, but they become intuitive once you’ve spent time at the microscope.
Reading a Slide From Low to High Power
Start at the lowest magnification your microscope offers, typically 2x or 4x. At this level, you’re not looking at individual cells. You’re assessing the overall tissue architecture: Is the tissue organized into recognizable structures like glands, vessels, or layers? Is there a region that looks different from the rest in color or density? Areas that appear unusually blue at low power suggest high cellularity, meaning lots of nuclei crowded together. This could indicate inflammation, a tumor, or simply a lymph node doing its job.
Move to 10x to examine the relationship between structures. At this magnification you can start distinguishing epithelial layers from underlying connective tissue, identify blood vessels by their round lumens lined with flat endothelial cells, and see whether glands maintain their normal shape or appear distorted. Look for the boundary between different tissue types. In skin, for instance, you should see a clear line where the blue-rich epidermis meets the pink-dominant dermis below.
Switch to 20x or 40x when you need to evaluate individual cells. This is where nuclear detail becomes visible and where you make judgments about whether cells look normal or abnormal.
Nuclear Features That Matter
The nucleus is the single most important structure in H&E interpretation, especially when you’re evaluating for disease. Normal nuclei tend to be round or oval, uniform in size, and evenly stained. When pathologists assess a tissue sample for malignancy, they focus on a set of nuclear changes collectively called “nuclear atypia.”
Enlarged nuclei are one of the most reliable warning signs. Compare the size of the nucleus to the cytoplasm around it. In most normal cells, the nucleus occupies a relatively small fraction of the cell. When that ratio shifts and the nucleus takes up most of the cell’s volume, it suggests abnormal growth. Hyperchromasia, where nuclei stain unusually dark, indicates increased DNA content and is another hallmark of malignancy. The chromatin pattern matters too: normal nuclei have fine, evenly distributed chromatin, while abnormal nuclei often show coarse, clumped chromatin or irregular clearing.
Prominent nucleoli (the small, round, darker dots within a nucleus) suggest the cell is actively producing proteins at a high rate. One or two small nucleoli can be normal in metabolically active cells, but large, conspicuous, or multiple nucleoli raise concern. Finally, look at how nuclei are arranged relative to each other. Normal epithelial cells line up in an orderly fashion with their nuclei oriented in the same direction (this is called polarity). Loss of that orderly arrangement, where nuclei point in random directions or pile on top of each other, is a sign of disorganization that pathologists use to grade dysplasia and cancer.
Identifying Common Tissue Types
Epithelial tissue lines surfaces and forms glands. On H&E, it appears as sheets or tubes of closely packed cells sitting on a thin basement membrane. The cells have relatively little space between them, and their nuclei are prominent. Squamous epithelium (like skin or the lining of the mouth) appears as flattened layers, with cells becoming progressively flatter toward the surface. Glandular epithelium forms round or tubular structures with a central open space (lumen).
Connective tissue fills the spaces between other structures. It’s characterized by abundant pink-staining extracellular material, with relatively few scattered nuclei belonging to fibroblasts. Dense connective tissue like tendons or ligaments shows tightly packed, parallel collagen fibers with elongated fibroblast nuclei compressed between them. Loose connective tissue has a more open, wispy appearance with collagen fibers running in multiple directions.
Muscle tissue comes in three forms, all eosinophilic. Skeletal muscle fibers are large, multinucleated, and show cross-striations at high power. Smooth muscle cells are spindle-shaped with single, centrally placed, cigar-shaped nuclei. Cardiac muscle looks similar to skeletal muscle but has centrally placed nuclei and branching fibers. Distinguishing smooth muscle from dense collagen on H&E alone can be challenging since both stain pink. Smooth muscle cells have visible nuclei within the fibers, while collagen bundles typically have nuclei only at their edges. When the distinction is critical, pathologists use special stains or immunohistochemistry targeting smooth muscle actin.
Adipose (fat) tissue is easy to recognize. During slide preparation, the lipid in fat cells dissolves away, leaving behind large, empty-looking circles with a thin rim of pink cytoplasm and a flattened nucleus pushed to one side.
Signs of Inflammation and Injury
Inflammation shows up on H&E as collections of immune cells that wouldn’t normally be present in that tissue. Neutrophils have multi-lobed nuclei and are the hallmark of acute inflammation. They often cluster around areas of infection or tissue damage. Lymphocytes are small, round cells with dark, round nuclei and almost no visible cytoplasm. Collections of lymphocytes suggest chronic inflammation or an immune response. Macrophages are larger cells with abundant pale cytoplasm, sometimes containing ingested material.
Necrosis, or cell death, appears as areas where normal tissue architecture is destroyed. The nuclei in necrotic tissue undergo characteristic changes: they may shrink and become intensely dark (pyknosis), fragment into pieces (karyorrhexis), or dissolve and disappear entirely (karyolysis), leaving behind pink, featureless cytoplasmic “ghosts.” Recognizing these patterns helps you distinguish dead tissue from viable tissue.
Edema, or fluid accumulation, appears as expanded pale or clear spaces between cells and fibers, giving the tissue a washed-out, spread-apart look.
Common Staining Artifacts
Not every abnormality on a slide reflects biology. Artifacts introduced during tissue processing can mimic pathology or obscure important details, so recognizing them prevents misinterpretation.
Uneven staining, where parts of the section appear darker or lighter in alternating bands (sometimes called a “venetian blind” effect), usually results from the tissue being cut at inconsistent thickness on the microtome. Thin areas pick up less dye and look pale, while thick areas stain too intensely. The fix is simple: recut the section at a uniform thickness. If you’re interpreting a slide with this artifact, be cautious about reading too much into color intensity differences across the section.
Brown granular deposits scattered across the tissue can resemble formalin pigment, a dark precipitate that forms when tissue sits too long in unbuffered formalin fixative. However, a similar-looking artifact occurs when the tissue section dries out before a coverslip is applied. The nuclei in these areas take on a glossy, black, refractile quality with brown stippling that can obscure nuclear detail. This artifact can be corrected by removing the coverslip and remounting the slide.
Tissue folds appear as darker creased areas where the section folded over on itself during mounting. They’re easy to identify because the fold line creates an abrupt, linear change in staining intensity. Crush artifact, from forceps handling, compresses cells and distorts their morphology, making nuclei appear smeared or elongated in a streak. Both of these are mechanical problems, not biological findings.
Building Interpretation Skills
Reading H&E slides is a pattern recognition skill, and it improves with structured repetition. Start by learning what normal looks like for each organ system. You can’t reliably identify pathology if you don’t have a mental baseline for healthy tissue. Online virtual microscopy libraries from universities provide free access to annotated whole-slide images for exactly this purpose.
When you encounter an unfamiliar slide, work through a consistent checklist. Identify the organ or tissue type from its architecture at low power. Note the cell types present and whether they’re in normal proportions. Assess the nuclei for size, shape, chromatin pattern, and arrangement. Look for anything that breaks the expected pattern: abnormal cells, missing structures, excess cells, or architectural distortion. This systematic approach keeps you from fixating on one detail and missing the bigger picture.