Evidence for morphologic diversity of human mast cells. An ultrastructural study of mast cells from multiple body sites

The ultrastructural features of 502 mast cells, including 34,187 granules, were studied from seven human tissue sites (lung, skin, colon, stomach, small bowel, breast parenchyma, and axillary lymph nodes). Granule areas and substructure were detailed according to the microenvironment (lung alveoli, lung bronchi, bowel mucosa, bowel submucosa, breast skin, breast parenchyma, and axillary lymph nodes). Although completely closed, discrete scrolls were present in some mast cell granules at all tissue locations, they were present more frequently in granules from bowel mucosa and lung (scroll-rich morphology). In contrast, most of the mast cell granules from skin, breast parenchyma, axillary lymph nodes, and bowel submucosa were rimmed by incomplete scrolls forming parallel lamellae; centrally, amorphous granular material and/or grating/lattice-like structures occurred (scroll-poor morphology). In breast, the latter granules had a mean granule area almost twice that of all other sites. Individual mast cells having granules with both scroll-rich and scroll-poor features were common in all tissue sites and microenvironments. In fact, 7.8% of mast cells had granules showing at least one completely closed, discrete scroll and grating/lattice-like structures, sometimes within the same granule. These observations indicate that mixed forms occur and that there is considerable morphologic diversity between the predominant mast cell found in mucosa and lung (scroll-rich morphology) and the predominant mast cell found in skin, breast parenchyma, axillary lymph nodes, and bowel submucosa (scroll-poor morphology).