Intraepithelial migration of mucosal mast cells in hay fever: ultrastructural observations

Evidence has been presented suggesting that a migration of nasal mast cells from the mucosal connective tissue stroma into the epithelium is part of the mucosal response in birch pollen allergy. In a previous study, the identification of these intraepithelial cells as tissue mast cells rather than blood basophils was based on light microscopical morphology and histochemistry. We have now studied the ultrastructure of these cells in mucosal biopsies taken before and during the birch pollen season. Intraepithelial cells with basophil or metachromatic granules were only observed in biopsies taken during the season. Some of these cells had the ultrastructural appearance of tissue mast cells, including cytoplasmic lipid droplets and a granular substructure composed of multilamellar arrays and scrolls, serving to distinguish human mast cells from blood basophils. The ultrastructural traits of the remaining cells were heterogeneous, some reminiscent of human blood basophils, others of globule leucocytes of other species, but entirely typical blood basophils could not be identified. The results thus support our previous suggestion that a migration of mucosal mast cells from the connective tissue stroma into the epithelium is part of the human allergic mucosal response. It cannot be determined whether the ultrastructural heterogeneity of these cells is the result of an adaptation to the intraepithelial environment of one single mast cell type or to the existence of an ultrastructurally distinct mucosal mast cell.