Desmin

Desmin is a protein found inside cells that form muscle tissue. It helps provide structure and support within muscle cells, allowing them to contract and function properly. Desmin is part of a group of proteins called intermediate filaments, which form a supportive framework in the cell. It is especially important in smooth, skeletal, and cardiac (heart) muscle.

Because desmin is found mainly in muscle cells, pathologists use it as a marker to identify tumours that show muscle differentiation. This includes tumours that develop from muscle or contain muscle-like cells. Testing for desmin can help confirm whether a tumour came from muscle tissue or contains muscle components.

What types of normal cells and tissues express desmin?

Desmin is normally found in the cytoplasm (the area outside the nucleus) of many cells in muscle tissues, including:

• Smooth muscle cells (found in the intestines, uterus, and blood vessels).

• Skeletal muscle cells.

• Cardiac (heart) muscle cells.

• Myoepithelial cells in the breast and salivary glands (in some cases).

• Mesothelial cells (lining of the chest and abdomen).

• Stellate cells in the liver.

• Endometrial and fallopian tube stromal cells.

In these cells, desmin helps maintain the structure of muscle fibers and supports their ability to contract and function.

How do pathologists test for desmin?

Pathologists test for desmin using a technique called immunohistochemistry (IHC). This method uses antibodies that attach to the desmin protein in a tissue sample. If desmin is present, it appears as a cytoplasmic stain when viewed under the microscope. In some tumours, desmin shows a specific pattern called perinuclear dot staining, where the protein collects around the nucleus.

A result is considered positive if staining is seen, and negative if no staining is present. The pattern and strength of the staining help pathologists determine whether a tumour contains muscle-like cells or originated from muscle tissue.

What types of tumours express desmin?

Desmin is commonly expressed in tumours that arise from or resemble muscle cells. These include:

Muscle tumours:

• Leiomyoma (benign smooth muscle tumour)

• Leiomyosarcoma (cancerous smooth muscle tumour)

• Rhabdomyoma (benign skeletal muscle tumour)

• Rhabdomyosarcoma (cancerous skeletal muscle tumour, including embryonal, alveolar, and pleomorphic types)

Tumours with muscle-like or myofibroblastic components:

• Aggressive angiomyxoma

• Myofibroblastoma

• Inflammatory myofibroblastic tumour

• Low-grade myofibroblastic sarcoma

• Angiomatoid fibrous histiocytoma

Other tumours:

• Desmoplastic small round cell tumour (shows a distinctive perinuclear desmin staining pattern)

• Endometrial stromal sarcoma

• PEComas (perivascular epithelioid cell tumours)

• Clear cell sarcoma (subset)

• Fibroepithelial stromal polyp

• Lipoblastoma (immature mesenchymal spindle cells)

Because desmin is also present in a few non-muscle-related tumours, it is often used alongside other markers such as SMA, myogenin, or MyoD1 to confirm the diagnosis.

Why is desmin important in a pathology report?

Desmin is important because it helps pathologists identify whether a tumour is showing signs of muscle differentiation. This is especially helpful when trying to distinguish:

• Muscle tumours from tumours of other soft tissues.

• Rhabdomyosarcoma from tumours that look similar under the microscope but are not muscle-related.

• Smooth muscle tumours from gastrointestinal stromal tumours (GIST), which are usually desmin-negative.

• Myofibroblastic tumours from other fibroblastic or fibrous growths.

Knowing whether a tumour expresses desmin helps guide the correct diagnosis, which in turn informs treatment and prognosis.

Questions to ask your doctor

• What does the desmin result mean in my pathology report?

• Does this result confirm that my tumour has muscle features?

• Were any other markers tested alongside desmin, like SMA or myogenin?

• How does desmin staining affect my diagnosis or treatment plan?

• Is the tumour benign or cancerous based on this result?