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We have examined the ultrastructural characteristics of peroxidase activity in human bone marrow mast cells. These studies were performed in three patients with systemic mast cell disease, and in another six patients showing bone marrow mast cell hyperplasia. Endogenous peroxidase activity was localized in the perinuclear cisternae and strands of endoplasmic reticulum, but never in the granules. We have also demonstrated the "in vivo" existence of exogenous peroxidase activity in two of the three cases of systemic mast cell disease. The peroxidase internalization involved its binding to the plasma membrane, followed by its incorporation into the cell by a general endocytic process comprising the uptake of dispersed peroxidase-positive material mainly by phagocytosis of granular structures containing peroxidase. The exogenous peroxidase appeared in non-membrane bound granules, vacuoles or aggregates, but we have never seen the enzyme linked to the mast cell granules.

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In a case of systemic mast cell disease with moderate bone marrow involvement, we studied the sensitivity of mast cell cationic dye-binding to formaldehyde fixation, as well as the mast cell ultrastructure, so as to determine whether these cells possess characteristics of mucosal mast cells. This histochemical method has proved to be one of the few which reveal heterogeneity of the mast cells in human intestine. Even though rat bone marrow mast cells have been shown to belong to the mucosal mast cell compartment, we have found no difference in mast cell counts in samples fixed either in formaldehyde or Carnoy's fixative. The ultrastructure did not show major differences with cutaneous mast cells. A few cells presented two nuclei, suggesting mitotic division of mature mast cells in bone marrow.

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We have studied peroxidase activity in human cutaneous and adenoidal mast cells using different methods, in order to determine the optimal technical conditions for its demonstration. In 1.25% glutaraldehyde-fixed cells, no peroxidase activity was seen. On the contrary, in tannic acid-aldehyde-fixed cells or in unfixed cells peroxidase activity was revealed independently of the DAB concentration or the incubation time in DAB medium. The reaction product was localized in perinuclear cisternae and endoplasmic reticulum. Granules were always unreactive with all techniques employed. Golgi apparatus was generally negative and only occasional cells exhibited one or two positive peripheral cisternae. This activity appears sensitive to fixation by glutaraldehyde and is inhibited by 3-amino-1,2,4-triazole (AMT) and by lack of H2O2 or DAB in the incubation medium, but not by potassium cyanide, sodium azide, or sodium pyruvate, at the concentrations used. The peroxidase activity described in this report is an endogenous peroxidase and is not related to uptake of exogenous peroxidase by mast cells. It can therefore be considered as an ultracytochemical marker of human mast cells.

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Although the hematopoietic origin of mast cells is very probable, the cell from which they originate is still a matter of speculation. The description of "transitional basophil/mast cells" in myeloproliferative disorders has suggested a common origin for basophils and mast cells. In a case of mast cell transformation of chronic granulocytic leukemia, the authors have studied the morphology and peroxidase activity by three classical technics, of circulating mast cells and transitional "basophil/mast cells." These results were compared with those of blood and bone marrow basophils and those of cutaneous mast cells. In both mast cells and "transitional basophil/mast cells," peroxidase activity was revealed in the nuclear envelope, endoplasmic reticulum, and granules. This activity was detected in unfixed cells and in tannic acid-aldehyde-fixed cells but not in 1.25% glutaraldehyde-fixed cells, where the staining appeared only in the granules. The comparison of this activity with that of normal basophils and mast cells suggests that the proliferating cells in this case possess at the same time the peroxidase activity of basophils and mast cells.

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The toxic oil syndrome is a new multisystemic disease, linked to the consumption of denatured rapeseed oil, which occurred in Spain in 1981. The hematological symptoms, present in about all the patients since the beginning of this disease have been dominated by leukocytosis and eosinophilia. The ultrastructural study of circulating blood cells has shown the presence of multiple abnormal lipid vacuoles in the eosinophil series and less frequently in neutrophils, monocytes and lymphocytes, associated to a high phagocytic activity. Bone marrow cells contrasted by the absence of lipid vacuoles in any hemopoietic series, while phagocytic activity and cytoplasmic processes were abundant. Even though patients entered the chronic phase of the disease, the hematological symptoms have spontaneously corrected themselves.

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An intense and reproducible peroxidase staining in the cutaneous mast cells of two patients with systemic mast cell disease and urticaria pigmentosa is demonstrated at the ultrastructural level. This enzyme activity was demonstrated by use of a cytochemical technique employing 3,3'- diaminobenzicine (DAB) as an oxidizable substrate, after fixation by a tannic acid-aldehyde mixture. Enzyme activity was localized in the perinuclear cisterna and strands of endoplasmic reticulum. Granules appeared unreactive. This peroxidase activity appears sensitive to fixation by aldehydes; it is inhibited by 3-amino-1,2,4-triazole (AMT) and by lack of H2O2 or DAB in the incubation medium. These characteristics are fundamentally different from the peroxidase activity of basophils, and the demonstration of this enzyme is therefore not a further argument for a common ontogenetic origin of both cells. On the other hand, the cytochemical characteristics of this enzyme are very similar to those of platelet peroxidase (P-PO), which has been connected to the synthesis by platelets of prostaglandins. Since the mast cell is known to generate prostaglandins, the relationship between the enzyme described and prostaglandin synthesis by mast cells is discussed.

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