Mast Cells and Basophils

Mast cells: Human lung mast cells contain granules that stain metachromatically with toluidine blue or alcian blue. The metachromatic color is violet in contrast to the usual blue.

Mast cells: A section of bronchial wall from this patient was stained with toluidine blue. Note that the mast cells (arrows) are mostly around smooth muscle bundles. One lies just beneath the epithelial basement membrane. Many of the cells around the lower arrows are eosinophils. Mast cells are also found within the epithelial layer in patients with asthma.

Mast cells: At higher magnification, two mast cells (arrows) are seen along with several eosinophils with black granules in this stain. Note the dilated vessels with enlarged, reactive endothelial cells that have been activated by cytokines.

 

Mast cell: This electron micrograph of a portion of a mast cell shows part of its round nucleus and multiple specific granules of various sizes and densities. The granules appear fairly homogeneous at this magnification.

Mast cell: At higher magnification, the granules have ill-defined scrolls and lamellae characteristic of TC mast cells (see below).

Basophil: Electron micrograph of a basophil (A). The blood basophil differs from the mast cell in having a lobulated nucleus. Note that the specific granules have different sizes, densities, and shapes. In Wright-stained blood smears, the basophil has large, blue granules that cover the nucleus (B).

AB

The electron micrographs were provided by Dr. Dorothy Bainton. The photograph of the blood basophil was provided by Dr. Susan Atwater.

Mast cells: Mast cells are derived from CD34+ precursors in the bone marrow. Their development depends primarily on stem cell factor, which is produced in bone marrow and other tissues by fibroblasts and vascular endothelial cells. Mast cells in different sites are heterogeneous in their morphology, mediator content, and response to drugs and activators. Granules contain neutral proteases (tryptase, chymase), histamine, proteoglycans (heparin and chondroitin sulfate that regulate the release of granule contents), and VEGF/VPF, which may play a role in the neovascularization of the chronically inflamed airway wall [1-3]. They also produce many cytokines and arachidonic acid metabolites that cause inflammation [1]. Mast cells have been separated into two types according to their content of neutral proteases: those with tryptase--T mast cells, and those with tryptase and chymase--TC mast cells. T mast cells comprise most of those in the lung, but some TC mast cells are also present. T and TC mast cells can also be separated on the basis of the ultrastructure of the granules. T mast cells contain granules with discrete scrolls; TC mast cells have more homogeneous granules with ill-defined scrolls and lamellae [4].

Degranulation: Mast cells have receptors for IgE, and degranulation occurs when the IgE-sensitized cell encounters its specific allergen. During experimentally-induced degranulation, granules swell, channels form between granules and also between granules and the cell membrane. The granular contents are extruded to the exterior through the interconnected channels. The granular membranes thus develop continuity with the plasma membrane and create a convoluted cell surface that is gradually shed, and the cell becomes rounded and ready for another cycle [5].

Basophils: These cells also develop from CD34+ hematopoietic precursors in the bone marrow. Whereas mast cells are long-lived and occur in tissues, basophils live for only a few days in the blood, but they may be recruited to tissues during immune or inflammatory reactions [1].

References

1. Costa J, Weller P, Galli S. The cells of the allergic response. Mast cells, basophils, and eosinophils. JAMA 1997; 278:1815-1822.

2. Metcalfe D, Baram D, Mekori Y. Mast cells. Physiol Rev 1997; 77:1033-1079.

3. Grützkau A, Krüger-Krasagakes S, Baumeister H, Schwarz C, Kögel H, Welker P, Lippert U, et al. Synthesis, storage, and release of vascular endothelial growth factor/vascular permeability factor (VEGF/VPF) by human mast cells: implications for the biological significance of VEGF206. Molec Biol Cell 1998; 9:875-884.

4. Craig S, Schechter N, Schwartz L. Ultrastructural analysis of maturing human T and TC mast cells in situ. Lab Invest 1989; 60:147-157.

5. Dvorak A, Schulman E, Peters S, MacGlashan Jr D, Newball H, Schleimer R, Lichtenstein L. Immunoglobulin E-mediated degranulation of isolated human lung mast cells. Lab Invest 1985; 53:45-56.

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