Histopathology

Histopathology is the study of tissues under a microscope to learn about diseases. The word comes from ancient Greek words meaning "tissue," "suffering," and "study of." When doctors want to understand if someone has a disease, they often take a small piece of tissue from the body, called a biopsy. This piece is then looked at very closely by a special doctor called a pathologist. By examining the tissue this way, doctors can see changes that happen when the body is sick. This helps them decide how to treat the person properly. Histopathology is different from cytopathology, which looks at single cells or tiny pieces of tissue instead of larger pieces.

Collection of tissues

Histopathology looks at tissues. It starts with surgery, a biopsy, or an autopsy. The tissue is taken from a body or a plant. It is then put in a fixative to stop it from decaying. The most common fixative is 10% neutral buffered formalin. This contains a special type of formaldehyde in water.

Preparation for histology

Main article: Histology

When doctors study how diseases change tissues, they prepare samples to look at under a microscope. They use special chemicals or quickly freeze the sample.

If a large piece of tissue is taken during surgery, a specialist picks the most important part to look at. This part is placed in a small plastic case to keep it safe. Sometimes, a material called agar is used to help hold the tissue in place.

Chemical fixation

Main article: Fixation (histology)

One common way to prepare tissue is by using chemicals. A solution called formalin is often used because it works well with tests that help find signs of disease. Smaller samples need less time to prepare.

Processing

The tissue is dried using different strengths of alcohol. Then, a liquid called xylene is used before the tissue is placed in warm wax. The wax helps keep the tissue steady so very thin slices can be cut and placed on a glass slide. These slides are then stained to show details under the microscope.

Frozen section processing

Main article: Frozen section procedure

Another method involves freezing the tissue quickly. This makes it possible to get results faster during surgery, helping doctors decide what to do next. However, this method doesn’t give as clear a picture as the wax method.

Staining of processed histology slides

Main article: Staining

When scientists look at tiny pieces of tissue under a microscope, they use special colors to make different parts easier to see. These colors help show what’s inside the cells.

One common way to color these slides uses two special dyes: hematoxylin and eosin, often called H&E. Hematoxylin makes the centers of cells (called nuclei) look blue, and eosin makes the rest of the cell and the material around it look pink. Scientists can use many other colors and methods too, depending on what they want to see.

These methods help scientists learn more about how cells work and what might be wrong when someone is sick.

Interpretation

Histological slides are viewed under a microscope by a pathologist, a doctor with special training. The pathologist makes a pathology report describing what they see and their ideas about the health problem. For cancer, this report is very helpful because it tells doctors if the cancer was fully removed during surgery. They use special ways to study the tissue carefully.

When looking at tissue under the microscope, pathologists can see different patterns and features. They start by looking at the overall layout, then how groups of cells are arranged, and finally the tiny details inside each cell. These patterns help doctors learn what is happening inside the body.

Major histopathologic architectural patterns include:

• Nests: islands of cells of similar type.
• Acinar or tubular: Each acinus has cells surrounding a space.
• Trabecular, long (rod-shaped) groups of cells.
• Papillary: bumps of cells around core structures.
• Micropapillary: Small bumps of cells without core structures.
• Fascicular: The same cell type, sometimes forming band-like groups.
• Woven or storiform: Long cells or nuclei in small bundles arranged in a mixed pattern.
• Solid: The same cell type with no spaces between, and no special pattern.
• Cribriform: Solid with many clear spaces.
• Whorled: Multiple concentric objects, or spiral-shaped.
• Cartwheel pattern: Center points that send out cells or connective tissue.

Major nuclear patterns include:

• Monomorphic when cells have similar sizes and shapes.
• Pleomorphic when cells have different sizes and shapes.
• Fine chromatin when small and hard to see, versus coarse chromatin.
• Sometimes "heterochromatic" versus "euchromatic" nuclei are used for how they look.
• Granular "salt-and-pepper" chromatin.