Metachromatic Cells Research Articles

Mast cells and MMP‐9 in the lamina propria during eruption of rat molars: quantitative and immunohistochemical evaluation

During the active tooth eruption process, structural changes in the lamina propria are necessary to provide extracellular matrix remodelling and for the establishment of the eruptive pathway. A large number of resident cells, recruited cells and proteases have been demonstrated in the eruptive process, but the participation of MMP-9 and mast cells has not yet been demonstrated. In this study, we set out to evaluate the intensity of MMP-9 immunoexpression, the frequency of mast cells and the correlation between the incidence of mast cells and bone resorption in different phases of tooth eruption. Fragments of maxilla containing first molars, obtained from 9-, 11-, 13- and 16-day-old rats, were fixed in 4% formaldehyde, decalcified and embedded in paraffin. Sagittal sections were stained with Masson's trichrome or submitted to the tartrate-resistant acid phosphatase method for quantification of osteoclasts. Sections stained by 1% toluidine blue were used for quantification of metachromatic mast cells mm(-2) of lamina propria. The expression of MMP-9 in the lamina propria was evaluated by immunohistochemistry. In the 9-day-old rats, the lamina propria contained few mast cells and occasional osteoclasts were found in the bone surface overlying the occlusal portion of the tooth germs. Otherwise, a significant increase in the number of mast cells was observed in the intra-osseous phase of tooth eruption (11-day-old rats), period in which numerous TRAP-positive osteoclasts were found in the bone surface. MMP-9 immunolabelling was detected in fibroblasts, mast cells and macrophage-like cells of the lamina propria in all ages studied. However, an enhanced immunolabelling was evident in the advanced phase of tooth eruption (16-day-old rats). During the intra-osseous phase, the parallel between the high frequency of both mast cells and osteoclasts suggests that mast cells could exert a paracrine function on the osteoclasts and then stimulate bone resorption. The immunoexpression of MMP-9 in different cells of lamina propria, including mast cells, indicates that this enzyme participates in the degradation of ECM, mainly during late phase of mucosal penetration. Thus mast cells and MMP-9 are involved in the complex process of degradation of the eruptive pathway extracellular matrix.

Thymosin beta 4 expression in normal skin, colon mucosa and in tumor infiltrating mast cells.

Mast cells (MCs) are metachromatic cells that originate from multipotential hemopoietic stem cells in the bone marrow. Two distinct populations of MCs have been characterized: mucosal MCs are tryptase-positive while mast cells in skin contain tryptase and chymase. We now show that a sub-population of MCs is highly immunoreactive for thymosin β4, as revealed by immunohistochemical analyses of normal skin, normal colon mucosa and salivary gland tumors. Four consecutive serial sections from each case were immunostained for thymosin β4 (Tβ4), chymase, tryptase and stained for toluidine blue. In skin biopsies, MCs showed a comparable immunoreactivity for Tβ4, chymase and tryptase. In normal colon mucosa the vast majority of mucosal MCs expressed a strong cytoplasmic immunoreactivity for tryptase and for Tβ4, in the absence of chymase reactivity. A robust expression of Tβ4 was detected in tumor-infiltrating and peritumoral mast cells in salivary gland tumors and breast ductal infiltrating carcinomas. Tumorinfiltrating MCs also showed a strong immunoreactivity for chymase and tryptase. In this paper, we first demonstrate that normal dermal and mucosal mast cells exhibit strong expression of thymosin β4, which could be considered a new marker for the identification of mast cells in skin biopsies as well as in human tumors. The possible relationship between the degree of Tβ4 expression in tumor-infiltrating mast cells and tumor behaviour warrants further consideration in future investigations.

An 18-year Follow-up of Allergy Development Related to Nasal Metachromatic Cell Findings During Infancy

The ability to predict the development of allergic diseases in infants is important. Predictive biomarkers are wanted to improve the risk evaluation in addition to known heredity of allergy. Biomarkers taken during infancy need to be evaluated through longitudinal studies into adulthood. The objective of this study was to analyse the occurrence of metachromatic cells in the nasal mucosa during infancy (MC(infancy)) and evaluate the cells as predictive biomarkers of allergy development. Previously, MC(infancy) occurrences were analysed in 64 infants with and without allergy heredity, and related to allergy development at 18 months and 6 years of age. In this third follow-up at 18 years of age, current allergy symptoms were analysed. MC(infancy) findings were related to the cumulative number of allergic subjects. The predictive values of MC(infancy) and known heredity were compared. The cumulative number of subjects with allergy was 46, probable allergy 5, and no allergy 13. Detected MC(infancy) predicted allergy with high accuracy (31/33), but negative MC(infancy) findings did not exclude the risk (15/31). In the group of allergic subjects positive MC(infancy) were found in 31/46 (67%), positive heredity in 37/46 (80%) and one/both factors positive in 43/46 (93%). Detection of MC(infancy) could precede the debut of allergy symptoms by many years. Detected MC(infancy) predicted allergy development, but absence of MC(infancy) did not exclude the risk, and therefore this biomarker was not found to be adequate. There is a further need to find biomarkers with high ability to both predict and exclude the risk.

Involvement of CD44 in mast cell proliferation during terminal differentiation

By using the recently established culture system that reproduces the terminal differentiation process of connective tissue-type mast cells, we found significant transcriptional induction of CD44. As CD44 is a primary receptor for hyaluronan (HA), which is one of the major extracellular matrix components, we investigated the role of CD44 in cutaneous mast cells. When co-cultured with fibroblasts, mouse bone marrow-derived cultured mast cells (BMMCs) were found to form clusters in an HA-dependent manner. As compared with BMMCs derived from the wild-type mice, those from the CD44−/− mice exhibited impaired growth during the co-cultured period. Furthermore, in the peritoneal cavities and ear tissues, mature mast cells were fewer in number in the CD44−/− mice than in the wild-type mice. We investigated roles of CD44 in mast cell proliferation by reconstituting BMMCs into the tissues of mast cell-deficient, KitW/KitW-v mice, and found that the number of metachromatic cells upon acidic toluidine blue staining in the tissues transplanted with CD44−/− BMMCs was not significantly changed for 10 weeks, whereas that in the tissues transplanted with the CD44+/+ BMMCs was significantly increased. These results suggest that CD44 plays a crucial role in the regulation of the cutaneous mast cell number.

Lectin histochemistry of epidermal glandular cells in the earthworm Lumbricus terrestris (Annelida Oligochaeta).

Carbohydrate residues were localized in the glandular cells of the epidermis of Lumbricus terrestris by lectin histochemistry. The following biotinylated lectins were used: ConA, PNA, WGA, UEA-I. Each lectin has a specific binding pattern in the epidermal glandular cells. The ConA binding is evident in the orthochromatic mucous cells; PNA in the metachromatic mucous cells; WGA in the neuroendocrine-like cells; UEA-I in the cuticle. The epidermal glandular cells possess specific sites for the different lectins in relation to their functional characteristics. Therefore, these sugar residues indicate different behaviours of the cells in epidermal functions related to ion transport, receptor-secretory processes and defence.

Mast Cells in the Salivary Glands and Tongue of the Ferret: Demonstration and Some Histochemical Observations

Previous investigations failed to demonstrate mast cells in the alimentary tract and extraparietal glands of the ferret. It was decided therefore to test this and assess factors that may be of influence. Major salivary glands and tongues of mature ferrets, which had been fixed in formalin-calcium, were examined by means of light microscopical histochemistry. Staining of paraffin sections with techniques depending on basic dyes or esterolytic activity was carried out for conventional times with and without previous oxidation, hot acid hydrolysis, and trypsin and beta-glucuronidase digestion. Aldehyde fuchsin and high iron diamine consistently revealed the presence of few mast cells in interstitial stroma of salivary glands and lingual musculature, and in the lamina propria of lingual mucosa. Alcian blue at 0.5 M MgCl2 and safranin produced less consistent results, and even fewer metachromatic mast cells were detected. No staining of mast cells was obtained with the technique for naphthol AS-D chloroacetate esterase. Pretreatment did not increase the numbers and/or staining reactions of mast cells. The results refute the previous misconception and suggest that ferret is a species with a low incidence of mast cells largely expressing a connective-tissue phenotype.

Cutis laxa acquisita associated with cutaneous mastocytosis

A female child, a product of a nonconsanguineous parentage and normal pregnancy, was seen first at the age of 4 years. She had suffered from recurrent episodes of multiple, erythematous, confluent plaques and a few bullous lesions all over the body since infancy. The child was irritable but had no history of flushing, dyspnea, wheezing, or frequent diarrhea. Darier's sign was strongly positive and systemic examination did not reveal any abnormality. Routine laboratory investigations were normal and a skin biopsy specimen revealed an unremarkable epidermis and dense proliferation of round to spindle‐shaped mast cells which stained positive for metachromatic granules (Fig. 1). With the diagnosis of urticaria pigmentosa, she was managed with antihistaminic drugs and the parents were advised to handle the child gently. She did not report for further follow‐up.Photomicrograph of skin histology showing metachromatic granules (toluidine blue, ×40)imageRecently, the child, now 8 years of age, was brought again with finely wrinkled, lax, and redundant skin, especially of the face, giving her a senile appearance (Fig. 2). Systemic examination did not show any abnormality. History revealed that the child had been asymptomatic during the last 4 years even without treatment, but developed slowly progressive lax and pendulous skin. A repeat biopsy from the lax facial skin revealed sparse and fragmented elastic fibres, normal collagen, and a perivascular and intradermal cellular infiltrate composed of metachromatic mast cells. Her serum α1‐antitrypsin levels were 190.0 mg/dL (normal, 93–224 mg/dL) and a pulmonary function test showed no abnormality. Treatment with antihistaminic drugs was started again and she was advised to have a cosmetic facial uplift at a later stage.Patient with urticaria pigmentosa seen at 8 years of age showing cutis laxa and a senile appearanceimage

408. Safety of Lysosomal Enzymes Over-Expression in HSC for Gene Therapy of Storage Disorders

In the past years we worked extensively on the development of HSC gene therapy for two fatal demyelinating lysosomal storage disorders, Metachromatic (MLD) and Globoid Cell (GLD) leukodystrophies. We recently demonstrated that transplantation of lentiviral vector-transduced Hematopoietic Stem Cells (HSC) prevented and corrected functional and pathological manifestations of MLD in the mouse model (Biffi A. et al., JCI 2004; Biffi A. et al, submitted for publication). Moreover, we proved the critical role of Arylsulfatase A (ARSA) over-expression to attain therapeutic efficacy.

Histochemical and ultrastructural analysis of the epidermal gland cells of Branchiomma luctuosum (Polychaeta, Sabellidae)

Abstract. This histochemical and ultrastructural study describes the epidermal gland cells of a tubicolous polychaete, Branchiomma luctuosum. The histochemistry was carried out using standard techniques and FITC‐labelled lectins. Four types of secretory cells were identified in two categories: orthochromatic cells (Type 1) and metachromatic cells (Types 2, 3, and 4). The secretory product of the Type‐1 orthochromatic cells contains neutral glycoproteins with Galβ1,3GalNAc residues. Metachromatic cells produce acidic, mainly sulfated, glycoconjugates with Galβ1,3GalNAc residues (Type 2) or glucosidic and/or mannosidic residues (Types 3 and 4). In sialylated chains, terminal sialic acid is bound to the penultimate GalNAc and Galβ1,3GalNAc residues. The complex composition of the mucus produced by epidermal gland cells of B. luctuosum may be correlated with its different functions. Ultrastructural studies of the epidermal gland cells showed differing morphology, and the presence in the gland cells of Types 3 and 4 of a funnel‐shaped structure for the extrusion of the secretory material.

Histological study on mast cells in conjunctiva of NC/Nga mice.

To investigate the pathological findings in conjunctiva of NC/Nga mice, which develop atopic dermatitis spontaneously, we focused our study on the density of mast cells as determined by histological methods. NC/Nga mice were divided into five groups; the 4W-group comprised four 4-week-old mice without treatment, the 10W-group comprised four 10-week-old mice without treatment, the 16W-group comprised four 16-week-old mice without treatment, the Dx group comprised three 16-week-old mice undergoing topical 0.1% dexamethasone (Dx) ointment treatment, and the FK506 group comprised three 16-week-old mice undergoing topical 0.1% FK506 ointment treatment. For the histological examination, the lids and eyeballs of the mice were removed and fixed with Carnoy's solution and thin sections were made. The Carnoy-fixed specimens were stained with toluidine blue or H&E and examined histologically. Toluidine blue-stained tissue sections were examined for the density per square millimeter of mast cells, which were identified as metachromatic cells in the conjunctival tissue and lid cutaneous tissue by light microscopy. Mast cell density in the conjunctiva was 8.6 +/- 8.2 cells/mm(2) in the 4W-group, 29.2 +/- 22.0 cells/mm(2) in the 10W-group, 41.0 +/- 21.1 cells/mm(2) in the 16W-group, 22.7 +/- 17.1 cells/mm(2) in the Dx group, and 33.6 +/- 27.7 cells/mm(2) in the FK506 group. Mast cell density increased significantly with age among the 4W-, 10W-, and 16W-groups ( P < 0.001). The mast cell density of lid cutaneous tissue was higher than the conjunctival mast cell density. Mast cell density was significantly higher in the 16W-group than in the Dx group, but not significantly higher than in the FK506 group. An increase in mast cell density with age indicates that NC/Nga mice develop atopic keratoconjunctivitis-like signs. Dexamethasone ointment has a suppressive effect on the increase in mast cell density.

In vitro expansion of human basophils by interleukin-3 from granulocyte colony-stimulating factor-mobilized peripheral blood stem cells.

A number of studies support the belief that human basophils play an important role in allergic inflammation. The exact mechanism of basophil activation at the site of allergic inflammation, however, has not been well understood, mainly due to their low number in blood and difficulty in obtaining a sufficient number of highly purified basophils for investigation. The purpose of this study is to expand human basophils in vitro with high yield and purity by culturing peripheral blood stem cells (PBSCs). We collected PBSC-rich mononuclear cells containing CD34+ cells (0.15-4.9%) by leukapheresis from patients with malignant lymphoma and lung cancer during haematopoietic recovery after chemotherapy plus granulocyte colony-stimulating factor-induced mobilization. PBSC-rich mononuclear cells were cultured in the presence of IL-3. When PBSC-rich mononuclear cells containing more than 1% of CD34+ cells were cultured, 20.0-83.3% of the cells, mostly with a yield of >10%, were metachromatic cells after 3 weeks of culture. These cells resembled mature peripheral blood basophils morphologically when examined by light and electron microscopy. Flow cytometric analysis showed that they expressed both FcepsilonRI and FcgammaRII. FcepsilonRI cross-linking resulted in intracellular calcium mobilization, histamine release and synthesis of cysteinyl leukotrienes. The intracellular histamine content and the release of these chemical mediators triggered by anti-IgE antibodies were comparable to those of peripheral blood basophils. These findings suggest that PBSC-derived basophils expanded in vitro are morphologically and functionally mature and will be a useful tool for the analysis of basophil functions.

The effects of an anti-CD11a mAb, efalizumab, on allergen-induced airway responses and airway inflammation in subjects with atopic asthma

Background: Efalizumab is a humanized IgG 1 mAb against the lymphocyte function antigen-1 (LFA-1) alpha chain, CD11a. Blocking of LFA-1/intercellular adhesion molecule interactions could inhibit asthmatic inflammation by blocking adhesion and activation of LFA-1-positive leukocytes. Objective: A randomized, double-blinded, placebo-controlled, parallel group, multicenter study investigated the effects of efalizumab on allergen-induced airway responsiveness and airway inflammation. Methods: Thirty-five nonsmoking subjects with mild allergic asthma were randomized to receive efalizumab (n = 24) or placebo (n = 11) in 8 weekly subcutaneous doses (0.7 mg/kg conditioning dose followed by 7 weekly doses of 2.0 mg/kg). Allergen challenges were performed at screening and after 4 and 8 weeks of treatment. Samples of sputum (n = 18 subjects) and blood (n = 35 subjects) were collected the day before challenges, and sputum was collected again at 7 and 24 hours after each challenge. Nonparametric tests were used to compare allergen-induced differences between efalizumab and placebo groups. Results: Subjects receiving efalizumab developed headache (48%) and flu syndrome (28%) compared to subjects receiving placebo (0%). After 8 weeks of efalizumab, the maximum late percent fall in FEV 1 (late asthmatic response) was inhibited by 50%, but neither the late response nor the late area under the curve was statistically different than placebo ( P = .098 and .062, respectively). Efalizumab had no effect on the maximum early percent fall in FEV 1 (early asthmatic response) or early area under the curve compared to placebo ( P > .59). Efalizu-mab significantly reduced the postallergen increase in sputum EG2-positive cells and metachromatic cells ( P < .05). No other comparisons were statistically different. Conclusions: Blocking of LFA-1/intercellular adhesion module interactions by efalizumab inhibits the development of allergen-induced cellular inflammatory responses measured in induced sputum and might attenuate the late asthmatic response. Larger studies are needed to confirm this. (J Allergy Clin Immunol 2003;112:331-8.)

Correlation of bone marrow angiogenesis and mast cells with tryptase activity in myelodysplastic syndromes.

Bone marrow samples from 30 patients with myelodysplastic syndromes (MDS) grouped according to the International Prognostic Scoring System for MDS were investigated for counts of microvessels, total metachromatic mast cells (MC) and MC expressing tryptase, an angiogenesis-inducing molecule. Counts were higher in patients with a poor prognosis. The observation of a high correlation between microvessel counts and both total metachromatic and tryptase-reactive MC in all samples suggests that angiogenesis in MDS increases with their progression and that MC may intervene in the angiogenic response in MDS through tryptase contained in their secretory granules.

Myelomastocytic leukemia: myeloid neoplasm characterized by partial differentiation of mast cell-lineage cells.

A novel subtype of myeloid leukemia exhibiting a partial differentiation of mast cell-lineage cells is described. The disease is characterized by an increase in myeloblasts as well as an increase in immature (blast-like) metachromatic cells (>10% in bone marrow or blood smears). Metachromatic cells express KIT (CD117) and tryptase, but lack basophil-related antigens. In contrast to mast cell leukemia/systemic mastocytosis, metachromatic cells do not express CD2 or CD25, do not form multifocal dense aggregates in the bone marrow, and do not exhibit transforming mutations at codon 816 of c-kit. In the few patients recorded so far, a complex karyotype without recurring anomaly was found. The prognosis appears to be grave, although complete remission in response to chemotherapy has been described.

Mast cells are present in epithelial layers of different tissues of the mollusc Anomalocardia brasiliana. In situ characterization of heparin and a correlation of heparin and histamine concentration.

Heparin, other sulphated glycosaminoglycans and histamine were extracted from various dissected organs of Anomalocardia brasiliana, a mollusc from the South Atlantic, and quantified. A good correlation between heparin and histamine content was found in the labial palp, intestine, ctenidium, mantle and foot tissues. The tissue location of metachromatic cells, putatively containing heparin, was identified histologically with Alcian Blue, Toluidine Blue, Masson trichrome, Haematoxylin-Eosin and PAS. Except for the foot, cells containing metachromatic granules were found in the epithelium surfaces of all the organs analysed. An in situ identification of heparin using nitrous acid and heparinase degradation has established unequivocally the presence of this compound in the metachromatic cells. The location of 'mast-like' cells at the epithelium surface of mollusc tissues exposed to the environment are very similar to the distribution of mammalian and other vertebrate mast cells and gives support to the suggestion for a role of mast cells in defense mechanisms.

Immunohistochemical localization of nNOS in the skin and nerve fibers of the earthworm Lumbricus terrestris L. (Annelida Oligochaeta)

The epidermis of the earthworm Lumbricus terrestris is a multifunctional tissue. It is composed of supporting, mucous, neuroendocrine-like, sensory and basal cells. NO is considered to be a molecule that regulates numerous functional activities (also in non-neuronal cells) in vertebrates. In the earthworm epidermis, we found neuronal NO synthase immunopositivity in orthochromatic and metachromatic mucous cells, neuroendocrine-like cells and in epidermal and subepidermal nerve fibers and striated muscle fibers. It is suggested that NO has a multitude of biological actions, affecting functional activities of the epidermis such as tissue homeostasis, control of secretion, proliferation, respiration, defense, water-salt balance, as well as regulation of tonus in vascular and striated muscles.

Histochemical and ultrastructural study of the chicken salivary palatine glands

Salivary glands are a good model to investigate the relationship between cell secretion and glandular structure. Most studies of this organ deal with mammals, but we are interested in a morphofunctional characterization of these glands in poultry in relation with particular feeding habits. For this purpose, conventional and lectin histochemical methods as well as ultrastructural methods have been applied to the chicken lateral and medial palatine salivary glands. It was found that periodic acid-Schiff (PAS)-positive, alcianophilic, and metachromatic or orthochromatic cells were present with a more homogeneous distribution pattern in lateral glands than in medial palatine glands. Lectin staining depended on the lectin type that was applied, but also on the glandular part both in lateral and medial glands. Ultrastructural studies showed cytoplasmic membranous structures with a scattered granular or filamentous content depending on the secretory cell. In conclusion, morphofunctional characteristics of salivary glands of chicken suggest that their products are involved in lubrication and humidification of food ingested, and probably in protection of the oral surface, as has been previously described for other animals showing similar histochemical staining patterns.

Analysis of sputum taken from wheezy and asthmatic infants and children, with special reference to respiratory infections

Children who are destined to develop asthma are considered to be susceptible to a variety of respiratory pathogens. To elucidate respiratory inflammation among these children, we measured the levels of eosinophil cationic protein (ECP) and tryptase in sputum taken from three different groups of wheezy infants and young children: those with a first wheeze (n = 15); those with recurrent wheeze (n = 27); and those with recurrent wheeze with respiratory distress, namely asthma (n = 56). The numbers of eosinophils or metachromatic cells determined by microscopic analysis of sputum samples were also evaluated in combination with the ECP and tryptase levels. Although neither sputum ECP nor tryptase was a clear discriminative marker that differentiated the three different types of wheezy disease, ECP levels in sputum from the asthma group were significantly higher (2,269.2 +/- 6,216.8 ng/g) than those in the recurrent wheezy group (440.3 +/- 1,199.8 ng/g) or in the first-wheeze group (209.0 +/- 172.9 ng/g). A similar trend was observed with tryptase levels in sputum, but there were no significant differences among the three groups. Sputum taken from asthmatic children showed a marked accumulation of eosinophils. However, an accumulation of eosinophils in sputum (even in the presence of an elevated level of sputum ECP) was not identified in the asthmatic infants < 1 year of age. An accumlation of eosinophils in sputum was not evident until children became > 1 year old and thereafter the eosinophils rapidly increased in number until the children reached 5 years of age. It was noteworthy that sputa positive for pathogenic bacteria, taken from the 1- and 2-year-old asthmatic infants, had a tendency to show high levels of ECP but a reduced number of eosinophils. Along with the wheezy episodes induced by viral infection, primarily and occasionally in combination with secondary bacterial infection, eosinophil activation and infiltration may develop. These predestined immune reactions to various pathogens might be associated with triggering the onset of asthma.

Mast cells/basophils in the peripheral blood of allergic individuals who are HIV-1 susceptible due to their surface expression of CD4 and the chemokine receptors CCR3, CCR5, and CXCR4

A population of metachromatic cells with mast cell (MC) and basophil features was identified recently in the peripheral blood of patients with several allergic disorders. This study now shows that these metachromatic cells express on their surface the high-affinity IgE receptor (FcepsilonRI), CD4, and the chemokine receptors CCR3, CCR5, and CXCR4, but not the T-cell surface protein CD3 and the monocyte/macrophage surface protein CD68. This population of MCs/basophils can be maintained ex vivo for at least 2 weeks, and a comparable population of cells can be generated in vitro from nongranulated hematopoietic CD3(-)/CD4(+)/CD117(-) progenitors. Both populations of MCs/basophils are susceptible to an M-tropic strain of human immunodeficiency virus 1 (HIV-1). Finally, many patients with acquired immunodeficiency syndrome have HIV-1-infected MCs/basophils in their peripheral blood. Although it is well known that HIV-1 can infect CD4(+) T cells and monocytes, this finding is the first example of a human MC or basophil shown to be susceptible to the retrovirus. (Blood. 2001;97:3484-3490)

Myelomastocytic overlap syndromes: biology, criteria, and relationship to mastocytosis

Although mast cells (MC) appear to be myeloid cells, MC lineage involvement in myelogenous malignancies has been described only rarely. Based on clonal evolution, biology of afflicted cells, and disease criteria, three major groups of patients have been recognized: The first meets criteria for both diagnoses ‘systemic mastocytosis’ and ‘associated hematologic clonal non-mast cell lineage disease (AHNMD)’. In such patients, myeloproliferative (MPS) or myelodysplastic syndromes (MDS), or acute myeloid leukemia (AML) is diagnosed apart from mastocytosis. In a second group of patients, large numbers of very immature MC-lineage cells (metachromatically granulated blast-like cells) are detectable, but the criteria to diagnose mastocytosis are not met. These patients have advanced myeloid neoplasms (MDS or MPS with blast cell increase, or AML) and variably suffer from mediator-related symptoms (flush, GI-tract ulcer, diarrhoea, coagulopathy). In some cases, the disease mimics mast cell- or basophilic leukemia. In contrast to basophilic leukemia, however, the metachromatic cells are strongly KIT+ and tryptase+. In contrast to true mast cell leukemia (MCL), MC do not form multifocal dense infiltrates in the bone marrow. Also, MC lack CD2 and CD25, and the C-KIT mutation Asp-816-Val. We propose the term ‘myelomastocytic leukemia’ or ‘myelodysplastic mast cell syndrome’ for these cases. In a third group of patients, myeloid neoplasms (MDS, MPS, AML) show constitutive expression of MC-associated antigens (tryptase, histamine) or mastocytosis-related gene defects (mutated C-KIT) without significant increase in metachromatic cells or criteria of mastocytosis. Whether these neoplasms display aberrant gene expression (or gene defects) or represent ‘pre-pre-mast cell leukemias’, remains unknown.