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

Histopathological examination of tissues starts with surgery, biopsy, or autopsy. The tissue is removed from the body or plant, and then placed in a fixative to keep it from decaying. The most common fixative used is 10% neutral buffered formalin, which contains a special type of formaldehyde in water.

Preparation for histology

Main article: Histology

When doctors want to study how diseases affect tissues, they prepare samples for viewing under a microscope. This can be done by using special chemicals or by freezing the sample quickly.

If a large piece of tissue is taken during surgery, a specialist will choose the most important part to examine. This part is placed in a small plastic case to keep it safe. Sometimes, a special material called agar is used to help keep the tissue in the right position.

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 special tests that help find disease clues. Smaller samples need less time to fix.

Processing

The tissue is dried using different strengths of alcohol. Finally, a liquid called xylene is used before the tissue is placed in warm wax. This 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, which can help 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 often use special colors to make different parts stand out. These colors help show what’s inside the cells.

One of the most common ways to color these slides uses two special dyes: hematoxylin and eosin, which people often call 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.

Some tests even use special proteins to find exact pieces inside cells, like certain fats or sugars. These methods help scientists learn more about how cells work and what might be wrong when someone is sick.

Interpretation

Histological slides are looked at under a microscope by a pathologist, a doctor who has special training. The pathologist creates a pathology report that explains what they see and their thoughts on the health issue. For cancer, this report is very important because it tells doctors if the cancer has been fully removed during surgery. They use special methods to check the tissue carefully.

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

Major histopathologic architectural patterns include:

• Nests: islands of cells of similar type.
• Acinar or tubular: Each acinus consists of cells that surround a lumen.
• Trabecular, elongated (rod-shaped) groups of cells.
• Papillary: Protuberances of epithelioid cells around fibrovascular cores.
• Micropapillary: Papillary tufts without fibrovascular cores
• Fascicular: Generally the same cell type throughout, but some form band-like groups that are aligned in the same direction.
• Woven or storiform: Elongated cells or nuclei wherein small bundles are aligned in an otherwise haphazard pattern.
• Solid: More or less the same cell type throughout, with no spaces between, and no other particular pattern.
• Cribriform: Solid with multiple clear spaces.
• Whorled: Multiple concentric objects, or spiral-shaped
• Cartwheel pattern: Center points that radiate cells or connective tissue outward

Major nuclear patterns include:

• Monomorphic when having relatively similar sizes and shapes.
• Pleomorphic when having different sizes and shapes.
• Fine chromatin when inconspicuous (essentially only nucleoli seen in the nuclei), versus coarse chromatin.
• Sometimes "heterochromatic" versus "euchromatic" nuclei are used for visual appearance.
• Granular "salt-and-pepper" chromatin.