What Do Mast Cells Look Like Under a Microscope?

Mast cells are immune cells from the bone marrow found throughout the body’s connective tissues. They are often positioned near blood vessels, nerves, and surfaces that interface with the external environment, like the skin, lungs, and digestive tract. These cells play a role in inflammatory and allergic responses by releasing potent chemical mediators. To understand their function, scientists use microscopy for detailed observation of their structure and activity.

What Mast Cells Reveal Under the Lens

Under a light microscope, mast cells have a distinct appearance. They are round to oval-shaped, though their form can become elongated or irregular depending on their location. Human mast cells measure from 8 to 20 micrometers in diameter, making them larger than some immune cells like lymphocytes but smaller than others. Their shape can be influenced by their surroundings; for instance, mast cells near bone surfaces may appear more flattened.

The defining feature of a mast cell is its cytoplasm, which is densely packed with large granules. These granules, numbering up to 200 per cell, are secretory vesicles that store chemical mediators like histamine and heparin. The high number of these granules can obscure other internal cell structures, including the nucleus.

When visible, the nucleus is a single, round structure located centrally or slightly off-center. The appearance of the cell and its granules can differ based on the tissue type and whether the cell is in a resting or activated state. The visibility of these features is heavily dependent on the specific preparation and staining techniques used in the laboratory.

Bringing Mast Cells into Focus: Preparation and Staining

Observing mast cells requires specific laboratory procedures to make them visible. The process begins with obtaining a tissue sample from a biopsy or a smear of bodily fluid. In their natural state, the details of mast cells are difficult to see, so scientists use specialized stains to highlight their components, particularly the granules.

A common and effective stain for this purpose is Toluidine Blue. This alkaline dye has a property known as metachromasia when it interacts with mast cells. Due to the acidic heparin within the granules, the dye changes color; while Toluidine Blue is itself blue, it stains the mast cell granules a distinct reddish-purple or violet. This color change makes the mast cells stand out against the blue-stained background of other tissues.

Another frequently used method is the May-Grunwald Giemsa (MGG) stain. This technique also highlights mast cell granules, staining them a pinkish-purple or purple, while the nucleus appears blue. While Toluidine Blue is often faster, some studies suggest MGG can be superior for overall identification. The effectiveness of these stains depends on their chemical ability to bind to the acidic compounds within the granules.

Decoding Mast Cell Images: Clues to Health and Disease

Microscopic examination of mast cells provides information about a person’s health. A primary assessment is identifying and counting the cells in a tissue sample. An unusually high number of mast cells in the skin or bone marrow can be an indicator of a mast cell disorder, such as mastocytosis.

Beyond quantity, the microscope reveals mast cell activity. A process called degranulation, where the cell releases the contents of its granules, is a part of allergic reactions and inflammation. Microscopically, degranulation can be seen as granules appearing outside the cell, or the cell itself may look like it is emptying with a disrupted membrane.

The morphology, or physical form, of the mast cells can also be a clue to disease. In some mast cell cancers, the cells may take on an abnormal, spindle-shaped appearance instead of their round form. The presence of dense clusters of more than 15 mast cells is a diagnostic criterion for systemic mastocytosis.