Human MCTC type of mast cell granule: the uncommon occurrence of discrete scrolls associated with focal absence of chymase.

Two types of mast cells were previously defined based on neutral protease composition and ultrastructurally distinguished by granule morphology. The MCT cell contains tryptase with little, if any, chymase and was noted to have varying numbers of irregularly-shaped granules with discrete scrolls or particulate or beaded material. The MCTC cell contains both tryptase and chymase and was noted to have more regularly-shaped electron-dense granules with characteristic grating or lattice substructures. This study reports the use of electron microscopy and immunogold staining with antibodies against tryptase and chymase to demonstrate in mature unstimulated MCTC cells in situ, the focal occurrence of discrete or complete scrolls in peripheral regions of certain granules where chymase is deficient. these scrolls often appeared to be protruding from the granule. Granules containing discrete scrolls were observed in 10 of 340 mature MCTC cells, accounting for less than 1% of MCTC granules. Other granules in such cells as well as other regions of the granule under consideration, showed strong staining for both tryptase and chymase. These results strengthen the association of morphology with protease composition in human mast cell secretory granules, but weaken the use of morphology alone to identify the MCTC and MCT types of human mast cells. Whether the uncommon occurrence of focal absence of chymase in MCTC cells arises by chance or as a result of factors relating to mast cell development, interconversion, activation, or regranulation will require further clarification. In conclusion, the appearance of grating or lattice structures in mast cells indicates the presence of chymase and tryptase, characteristic of the MCTC phenotype, whereas multiple discrete scrolls in irregularly shaped granules suggests the MCT phenotype.