Caldesmon

Caldesmon is a protein found inside cells that helps control how smooth muscle contracts and relaxes. Smooth muscle is found in many parts of the body, including the walls of blood vessels, intestines, uterus, and airways. Caldesmon works by interacting with other proteins—such as actin, tropomyosin, and calmodulin—to regulate muscle function. By controlling when muscles tighten or relax, caldesmon helps prevent unwanted contractions and maintain normal tissue function.

What types of normal cells and tissues express caldesmon?

Caldesmon is normally found in the cytoplasm of smooth muscle cells. These cells are found in many different areas of the body, including:

• The digestive tract, where they help move food through the intestines.

• The walls of blood vessels, where they help regulate blood pressure and flow.

• The uterus, where they contract during menstruation and childbirth.

• The urinary bladder, helping to store and release urine.

• The respiratory system, where they help control airflow in the airways.

In pathology, caldesmon is used as a marker to identify these cells and the tumours that arise from them.

How do pathologists test for caldesmon?

Pathologists use a test called immunohistochemistry (IHC) to look for caldesmon in tissue samples. This test uses antibodies that attach to caldesmon if it is present. If caldesmon is found, it causes a visible cytoplasmic stain when viewed under a microscope. A positive result means that smooth muscle is likely present, while a negative result means it is not.

The strength and pattern of staining help pathologists determine whether a tumour shows smooth muscle differentiation. Because caldesmon is very specific to smooth muscle cells, it is often used to confirm the diagnosis when a tumour shows only subtle or partial muscle features.

What types of tumours express caldesmon?

Caldesmon is commonly expressed in tumours that come from or contain smooth muscle. These include:

• Leiomyoma – A benign (noncancerous) tumour made of smooth muscle, most often found in the uterus or gastrointestinal tract.

• Leiomyosarcoma – A cancerous tumour of smooth muscle that can occur in soft tissue or internal organs.

• Angioleiomyoma – A benign tumour made up of smooth muscle and blood vessels, usually found in the limbs.

• Glomus tumour – A rare tumour that arises from cells involved in blood vessel regulation, often found under the skin or in the nail bed.

• Myopericytoma – A tumour involving cells that wrap around small blood vessels, often seen in the skin and soft tissues.

• Sclerosing PEComa – A rare tumour with features of both smooth muscle and melanocytic cells, found in various soft tissues and organs.

• Gastrointestinal stromal tumour (GIST) – A tumour of the digestive tract that may sometimes show caldesmon expression, although this is less common.

• Inflammatory myofibroblastic tumour – A tumour with features of inflammation and muscle-like cells, which may show weak or focal caldesmon staining.

Caldesmon is especially useful for distinguishing true smooth muscle tumours, which are caldesmon-positive, from myofibroblastic tumours, which usually do not express caldesmon.

Why is caldesmon important in a pathology report?

Caldesmon is important because it helps pathologists determine whether a tumour is made up of smooth muscle cells. This information is critical for:

• Diagnosing the type of tumour – For example, caldesmon can help distinguish a smooth muscle tumour from an endometrial stromal tumour or a gastrointestinal stromal tumour.

• Assessing muscle differentiation – In tumours with mixed features, caldesmon can confirm whether smooth muscle is present.

• Choosing the right treatment – The type of tumour affects which treatments are most appropriate, and accurate diagnosis is the first step.

Because caldesmon is more specific than other markers like SMA, it is often used to confirm findings when other results are unclear.

Questions to ask your doctor

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

• Does this result confirm that the tumour has smooth muscle features?

• Were other muscle markers like SMA or desmin also tested?

• How does caldesmon affect my diagnosis and treatment plan?

• Are more tests needed to understand the tumour type?