What is smooth muscle actin (SMA)?


SMA stands for smooth muscle actin, a type of protein found inside certain cells in the body. It plays an important role in helping cells move and maintain their shape. There are different types of actin proteins, but alpha smooth muscle actin (α-SMA) is the type most commonly used in pathology testing. Pathologists use SMA as a marker to identify cells that show features of smooth muscle, myofibroblasts, or myoepithelial cells, all of which are important in diagnosing a variety of tumours and other conditions.

What types of normal cells and tissues express SMA?

SMA is normally found in the cytoplasm (the part of the cell outside the nucleus) of cells that are involved in contraction or structural support. These include:

  • Smooth muscle cells, such as those in the walls of blood vessels, intestines, and uterus.

  • Myoepithelial cells in the breast, salivary glands, and sweat glands.

  • Myofibroblasts, which help with tissue repair and scarring.

  • Pericytes, specialized cells that surround small blood vessels.

  • Other normal cells such as osteoblasts, chondrocytes, and some immune cells.

In these tissues, SMA helps the cells contract, stabilize tissue structure, and heal injuries.

How do pathologists test for SMA?

Pathologists use a technique called immunohistochemistry (IHC) to test for SMA. This test involves applying antibodies to a tissue sample that will bind specifically to SMA if it is present. Under the microscope, SMA appears as a cytoplasmic stain, meaning the colour change is seen in the body of the cell. A result is considered positive if SMA is detected and negative if there is no staining.

What types of tumours express SMA?

SMA is expressed in many types of tumours, especially those that contain smooth muscle, myofibroblasts, or myoepithelial cells.

Smooth muscle and myofibroblastic tumours:

  • Leiomyoma (benign smooth muscle tumour)

  • Leiomyosarcoma (cancerous smooth muscle tumour)

  • Nodular fasciitis and inflammatory myofibroblastic tumour

  • Myofibroma and myofibroblastic sarcoma

Glandular and myoepithelial tumours:

  • Adenoid cystic carcinoma

  • Myoepithelioma and epithelial-myoepithelial carcinoma

  • Pleomorphic adenoma of the salivary gland

  • Myoepithelial cells in breast papillary lesions and sclerosing conditions

Other tumours:

  • Glomus tumour

  • Endometrial stromal sarcoma

  • Desmoplastic melanoma (a variant of melanoma)

  • Some sarcomas, such as GIST (gastrointestinal stromal tumour), PEComas, and liposarcomas

SMA can also be helpful in identifying perineural invasion, where cancer spreads along nerves, and in assessing stromal response around tumours.

Why is SMA important in a pathology report?

SMA is important because it helps pathologists identify the types of cells in a tumour. For example:

  • Distinguishing tumour types: For example, SMA helps separate smooth muscle tumours from other soft tissue tumours and myoepithelial tumours from breast and salivary gland cancers.

  • Identifying invasion: SMA highlights myoepithelial cells, which are often lost when a tumour becomes invasive.

  • Evaluating tumour behaviour: Some cancers with SMA expression may behave more aggressively, such as dedifferentiated liposarcoma or sarcomatoid mesothelioma.

In many cases, SMA is used alongside other markers to get a complete picture of a tumour’s type, origin, and potential behaviour.

Questions to ask your doctor

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

  • Does the presence of SMA help confirm the type of tumour I have?

  • Is the tumour showing smooth muscle or myofibroblastic features?

  • Were other markers used to confirm the diagnosis?

  • Does SMA expression affect the treatment plan or prognosis?

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