What Is a p63 Stain and What Is It Used For?

The p63 protein is a molecule that aids in the development and maintenance of various tissues. It belongs to a family of proteins that act as transcription factors, meaning they regulate the activity of other genes. In medicine, a “stain” is a laboratory technique used to make specific cells or proteins visible within a tissue sample. This process, known as immunohistochemical staining, allows pathologists to identify the presence and location of substances like the p63 protein. A p63 stain is a diagnostic tool used to visualize the p63 protein in tissue specimens.

The Biological Function of p63 Protein

The p63 protein is a transcription factor with a role in the development and maintenance of epithelial tissues. These tissues form the lining of surfaces and cavities throughout the body, including the skin, glands, and urinary tract. The protein is particularly active in the basal cells of these tissues, which function as a reservoir of stem cells. These basal stem cells are responsible for regenerating the tissue by dividing and differentiating into more specialized cells.

The protein’s function is connected to this regenerative capacity. It helps maintain the self-renewal capability of these basal cells, ensuring a constant supply for tissue repair and homeostasis. The predictable presence of p63 in healthy tissues is what makes its absence, or presence in the wrong context, informative for diagnostics. By controlling the life cycle of these fundamental cells, p63 ensures that stratified epithelial tissues, which are composed of multiple layers, can properly form and sustain themselves.

The p63 Staining Procedure

The p63 stain is a laboratory method that falls under the category of immunohistochemistry (IHC). The process is performed on a small piece of tissue that has been obtained through a biopsy or surgical procedure. This tissue specimen is first preserved in formalin and then embedded in paraffin wax, allowing it to be sliced into extremely thin sections and placed on a glass microscope slide.

Pathologists apply a solution containing primary antibodies to the tissue section. These antibodies are engineered to recognize and bind exclusively to the p63 protein. A secondary antibody, which binds to the primary antibody, is then added. This secondary antibody is linked to an enzyme.

When a substrate solution is applied, the enzyme activates a chemical reaction that produces a colored precipitate at the site of the antibody binding. This color, most commonly brown, accumulates in the cell nucleus, which is the location of the p63 protein. The slide is also counterstained, often with hematoxylin, which colors other cell nuclei blue, providing a clear contrast for the pathologist to interpret under a microscope.

Diagnostic Uses of p63 Staining

Pathologists use the p63 stain as a diagnostic marker to differentiate between various cell types and tissue conditions, particularly in cancer diagnosis. One of the most frequent applications is in prostate pathology. Benign prostate glands are characterized by two cell layers: an inner luminal layer and an outer basal layer, where the basal cells consistently express p63. In contrast, prostate adenocarcinoma lacks this basal cell layer, so a p63 stain can help distinguish between benign and cancerous glands.

In breast pathology, p63 is used to identify myoepithelial cells, which form a p63-positive layer around the ducts and lobules. In conditions like ductal carcinoma in situ (DCIS), this myoepithelial layer remains intact. In invasive ductal carcinoma, however, the cancer cells have broken through this layer. A p63 stain highlights the presence or absence of this layer, aiding in the distinction between in situ and invasive disease.

The stain is also used in classifying tumors in other organs. Many squamous cell carcinomas, which can arise in the lung, skin, or head and neck, show strong and diffuse p63 expression. This characteristic helps to distinguish them from other types of cancer, such as adenocarcinoma, which are often p63-negative. The test is also used to help identify urothelial carcinoma, the most common type of bladder cancer, as these cells are p63-positive.

Analyzing p63 Stain Outcomes

The interpretation of a p63 stain depends on the context, including the tissue being examined and the specific diagnostic question. Pathologists evaluate not just the presence of the stain, but also its location, pattern, and intensity. A positive result, meaning the presence of the brown stain, or a negative result, its absence, carries different implications in different scenarios.

For example, a continuous p63-positive layer around a structure, such as in the prostate or breast, often indicates a benign or non-invasive condition. Conversely, the complete absence of this layer in a suspicious group of cells is a strong indicator of invasive cancer. In other cases, a strong and widespread positive result can help identify a tumor’s origin, such as confirming a diagnosis of squamous cell carcinoma or urothelial carcinoma.