An immunomorphometric study on megakaryocyte precursor cells in bone marrow tissue from patients with chronic myeloid leukemia (CML)

Characteristic changes in platelet-large cell ratio, lactate dehydrogenase and C-reactive protein in thrombocytosis-related diseases

We examined the clinical usefulness of 3 parameters of routine laboratory tests [platelet-large cell ratio (P-LCR), lactate dehydrogenase (LDH) and C-reactive protein (CRP)] in 84 patients with thrombocytosis-related diseases (reactive thrombocytosis, chronic myeloid leukemia, essential thrombocythemia and polycythemia vera). These thrombocytosis-related diseases were characterized using the 3 parameters P-LCR, LDH and CRP as follows: high P-LCR and high LDH in chronic myeloid leukemia; high CRP in reactive thrombocytosis; slightly high P-LCR and high LDH in essential thrombocythemia and polycythemia vera. For essential thrombocythemia and polycythemia vera, levels of P-LCR and CRP were nearly identical, but the LDH level in essential thrombocythemia was significantly higher than in polycythemia vera. These characteristics of P-LCR, LDH and CRP may be useful for simple and very rough differentiation of the thrombocytosis-related disease mentioned above.

Dynamics of CD34+ progenitor cells following allogeneic bone marrow transplantation in Ph1+CML--an immunohistochemical study on 113 patients with sequential trephine biopsies

A retrospective immunohistological and morphometric study was performed on bone marrow trephine biopsies derived from 113 patients with Philadelphia chromosome-positive chronic myeloid leukemia (Ph(1+)-CML) before and at standardized intervals following allogeneic transplantation (BMT) with full unmanipulated marrow specimens. The purpose of this investigation was to quantify CD34+ progenitor cells and to determine their dynamics during the post-transplant period. Moreover, we tried to correlate their number with corresponding changes in the amount of nucleated erythroid precursors and megakaryocytes including pro- and megakaryoblasts and the fiber content. Monitoring the quantity of precursors after BMT revealed a very rapid recovery in comparison to a control group. However, a more detailed evaluation showed that at day 22 +/- 6 a higher number of progenitor cells was significantly associated with an earlier independence for platelet transfusion and also with a more pronounced growth of erythro- and megakaryopoiesis including their precursor cells. Furthermore, a slight increase in the density of the fibrous matrix (reticulin fibers) was present in these patients that were characterized by a more favorable engraftment. The latter feature sheds some light on the complex pathomechanisms of homing and differentiation of progenitors. In confirmation with in vitro findings, this phenomenon is dependent on proper anchoring sites to the fibrous bone marrow stroma. Finally, the size of a full BM graft exerted a distinctive influence on the number of CD34+ precursors in the early post-transplant period. In conclusion, the present study has validated a number of BM features by focusing on the CD34+ progenitor cells and associated hematopoietic reconstitution including reticulin fibers and precursor cells of the erythroid and megakaryocyte lineage, which are not readily evaluable by fluorescence-activated cell sorting (FACS) analysis.

Therapy-related changes of CD34+ progenitor cells in chronic myeloid leukemia: a morphometric study on sequential trephine biopsies

In chronic myeloid leukemia (CML), it has been assumed that the number of CD34(+) progenitor cells (PGCs) provides useful diagnostic and prognostic information regarding the evolution of accelerated phase and blastic crisis. However, until now no information is available about changes of this peculiar precursor cell population during therapy or possible associations with the other bone marrow constituents. For this reason, a retrospective clinicopathological study was performed on 83 patients with CML including 209 sequential bone marrow biopsies (intervals ranging between 6 and 143 months) and immunohistological staining of CD34(+) cells (QBEND10), megakaryocyte precursors (CD61), and erythropoiesis (Ret 40f). According to treatment modalities, three different groups of patients could be distinguished that received either monotherapy by interferon-alpha2b (IFN-alpha2b) or hydroxyurea (HU) and a combination of both. In comparison with a control group, morphometry revealed a significant increase in the quantity of CD34(+) PGCs per hematopoiesis (cellularity) in the CML bone marrow before treatment. Independently of treatment modalities and presentation of clinical findings nonresponding patients were generally characterized by a higher amount of progenitors in the initial biopsy specimens. Furthermore, calculation of the CD34(+) cell growth index showed a significant and rapid progression in nonresponding patients and in those developing an accelerated or blastic phase during therapy. This feature was prominently expressed following IFN treatment and related to a failing regeneration of nucleated erythroid precursors. In patients with a myelofibrotic bone marrow at onset no differences in the number of CD34(+) PGCs were recognizable in the pretreatment biopsies. This finding contrasted a significant and gradual change in progenitor cell frequency under treatment and evolving myelofibrosis. Opposed to HU therapy, the latter feature was explicitly detectable in the IFN group. In conclusion, the incidence of CD34(+) PGCs in the CML bone marrow reflects therapeutic efficacy. By demonstrating a significant relationship between fiber content and quantity of CD34(+) cells during treatment, experimental findings concerning the complex functional interactions between the fibrous stroma compartment and progenitor cell differentiation and proliferation are elucidated.

Megakaryopoiesis and myelofibrosis in chronic myeloid leukemia after allogeneic bone marrow transplantation: an immunohistochemical study of 127 patients

An immunohistochemical and morphometric study was performed on 363 trephine biopsies of the bone marrow derived from 127 patients with chronic myeloid leukemia at standardized end points before and after allogeneic bone marrow transplantation (BMT). The purpose of this investigation was to evaluate features of CD61+ megakaryopoiesis related to successful engraftment. Further, we tried to elucidate possible associations of this lineage, including precursor cells, with the platelet count and reticulin fibrosis during the pretransplant and, specifically, post-transplant periods. A significant correlation was recognizable between the quantity of CD61+ megakaryocytes and the platelet values before BMT and also after completed hematopoietic recovery. In the very early post-transplant period, which is associated with severe thrombocytopenia, patchy regeneration of disarranged hematopoiesis occurred, including dysplastic megakaryocytes. According to planimetric measurements after BMT, the atypical micromegakaryocytes characteristic for chronic myeloid leukemia disappeared, and the engrafted donor bone marrow revealed a prevalence of normal-size cells of this lineage. On the other hand, normalization of megakaryocyte size was absent in sequential examinations of the few patients with a leukemic relapse who had a predominance of atypical dwarf forms comparable with chronic myeloid leukemia. Before BMT occurred, reticulin fiber density was significantly correlated with the number of CD61+ megakaryocytes and its precursor cell population. In 34 patients with myelofibrosis that occurred after myelo-ablative therapy and BMT, an initial regression was followed by an insidious recurrence of fibers concentrated in the areas of regenerating hematopoiesis. This postgraft reappearance of reticulin fibrosis was significantly associated with the quantity of megakaryocytes. Regarding engraftment parameters, pretransplant presence of (reticulin) myelofibrosis exerted a distinctive impact because of a delayed hematopoietic reconstitution according to standard clinical criteria. In line with this finding, slowed engraftment was also significantly related with higher pretransplant megakaryocyte and platelet counts.

Effects of chemotherapy (busulfan-hydroxyurea) and interferon-alfa on bone marrow morphologic features in chronic myelogenous leukemia. Histochemical and morphometric study on sequential trephine biopsy specimens with special emphasis on dynamic features

We performed a retrospective clinicopathologic study on sequential biopsy specimens from 90 patients with Philadelphia chromosome-positive chronic myelogenous leukemia to study therapy-specific effects of busulfan (28 patients), hydroxyurea (32 patients), and interferon-alfa (IFN-alfa; 30 patients). Bone marrow specimens were evaluated by morphometry after silver impregnation and staining with monoclonal antibodies to identify reticulin fibers, nucleated erythroid precursors, megakaryocytes, and macrophages. To compute dynamics of histopathology implicating corresponding changes in time, relevant indices were calculated. Quantification of megakaryocytopoiesis and its precursor cell population showed a significant increase in the IFN-alfa and busulfan groups compared with the hydroxyurea group. These changes were associated with a development of myelofibrosis during therapy. Although a significant increase in fiber density was detectable in the busulfan group, the progression index proved to be twice as high after IFN-alfa therapy. In contrast, a considerable number of patients displayed a regression of myelofibrosis after hydroxyurea treatment. The general association of the megakaryocyte lineage with myelofibrosis was in line with experimental findings. The mature macrophage population and its activated subfraction revealed a marked proliferation (IFN-alfa group) during treatment. Growth and activation of macrophages may be compatible with their putative function during erythrocytopoietic regeneration and with stimulation of their phagocytic properties.

Diagnosis and classification of erythrocytoses and thrombocytoses

An erythrocytosis describes an increased peripheral blood packed cell volume (PCV) and is deemed to be absolute or apparent depending on whether or not the measured red cell mass (RCM) is above the reference range. This reference range must be related to the individual's height and weight to avoid erroneous interpretations using ml/kg total body weight expressions in obesity. Absolute erythrocytoses are divided into primary, where the erythropoietic compartment is intrinsically abnormal, secondary, where the erythropoietic compartment is normal but is responding to external pathological events leading to an increased erythropoietin drive, and idiopathic, where neither a primary nor a secondary erythrocytosis can be established. Both primary and secondary erythrocytoses have congenital and acquired forms. The only form of primary acquired erythrocytosis that has been defined is the clonal myeloproliferative disorder, polycythaemia vera (PV). Modified diagnostic markers for PV are proposed. Thrombocytoses can be classified into primary, where megakaryopoiesis is intrinsically abnormal, secondary, where megakaryopoiesis is normal but increased platelet production is a reaction to some other unrelated pathology, and finally idiopathic. This latter new group would be used for patients not satisfying the criteria for primary or secondary thrombocytoses, if these were more precise and rigidly used than currently is the case. While theoretically congenital and acquired forms of primary and secondary thrombocytoses might exist, only one cause of secondary congenital thrombocytosis has been established, and primary congenital thrombocytosis has not yet been precisely defined. Primary (essential) thrombocythaemia (PT) is one of the forms of primary acquired thrombocytoses. The diagnostic criteria of PT traditionally involve the exclusion of secondary thrombocytoses and other myeloproliferative disorders but marrow histology could hold a key positive diagnostic role if objective histological features of PT were agreed.

Comparative morphometric study of immunohistochemical versus conventional staining for the evaluation of megakaryocytopoiesis in normal and pathological bone marrow biopsies

Identification of megakaryocytes by immunohistochemistry may be superior to hematoxylin-eosin (HE) stain method for assessing megakaryocyte size and number in clinical specimens; however, a side-by-side comparison of the two methods has not been reported. In the present study, comparative morphometry using both methods was performed on marrow biopsies of normal individuals, and of patients with myelodysplastic syndrome, chronic myeloid leukemia and immune thrombocytopenia. Morphometric results in the present study showed that precise megakaryocyte size can be calculated in normal and pathologic bone marrow sections by using HE stain if one employs stereological corrections. In contrast, megakaryocyte numbers can be more precisely detected by immunohistochemistry than by HE stain, particularly in myelodysplastic syndrome and chronic myeloid leukemia. Differentiation disturbances and ineffective megakaryocytopoiesis in myelodysplastic syndrome were demonstrated by immunomorphometric analyses.

Clinicopathological impact of the interaction between megakaryocytes and myeloid stroma in chronic myeloproliferative disorders: a concise update

In this report an attempt has been made to discuss some of the issues pertinent to myelofibrosis complicating chronic myeloproliferative disorders (CMPDs) that are significantly associated with megakaryocyte function. In this context, biochemical, clinical and particularly morphological features were reviewed. Morphological findings based on elaborate techniques were in keeping with the assumption that in chronic myeloid leukemia (1) the number of CD61-positive megakaryocytes, and in particular their precursors were the parameters most closely associated with myelofibrosis (2) an increased content of reticulin fibers in follow-up biopsies significantly correlated with laboratory data indicative of a high tumor burden (anemia, peripheral blasts, hepatosplenomegaly) and thus a more advanced stage of the disease process (3) even a slight increase in reticulin, i.e. doubling of the normal fiber density was associated with a worse prognosis independent of therapeutic regimens given (4) Dynamics of myelofibrosis was significantly influenced by treatment. In this context, calculation of the myelofibrosis progression index (MPI) revealed a higher score following interferon therapy compared with busulfan. In addition, in idiopathic myelofibrosis (5) the evolution of myelofibrosis was unpredictable and according to the MPI, progression occurred at a relatively low rate (6) proliferation and dilatation of sinusoids accompanying intravascular hematopoiesis and collagen type IV deposits were predominant features in later (fibro-osteosclerotic) stages in the course of disease (7) transmural migration of megakaryocytes demonstrated by three dimensional reconstruction revealed a mole-like tunneling through the thickened sinusoidal wall. A very careful assessment of the numerous correlations between bone marrow features and laboratory data will allow clinicians and pathologists to gain a better insight into the mutual relationships between hematological and morphological findings in CMPDs.

Megakaryocytes and platelets in myeloproliferative disorders

Increased megakaryocyte (MK) proliferation in bone marrow is a feature common to the three Ph-negative myeloproliferative disorders (MPDs), i.e. essential thrombocythaemia (ET), polycythaemia vera (PV), and myelofibrosis with splenic myeloid metaplasia (MMM), and to chronic myelocytic leukaemia (CML). Enlarged MKs with multilobulated nuclei and cell clustering in close proximity are the hallmark of all the Ph negative MPDs. Clonality of haematopoietic cells, based on X chromosome inactivation, can now be studied in a majority of female patients in all nucleated cell fractions as well as in platelets. Cytofluorometric studies have demonstrated a shift towards higher ploidy classes in PV and ET MKs which may be useful in discriminating between both primary and reactive thrombocytosis and CML patients which show a significant shift to lower MK ploidy values. The role of MK proliferation on the evolution of myelofibrosis common to MPDs has been firmly established. Implication of platelet-derived growth factor (PDGF) in myelofibrosis has already been demonstrated. More recently transforming growth factor beta (TGF beta) synthesized and secreted by MK has been implicated in fibroblasts stimulation. A significant increase in circulating colony-forming units of MKs (CFU-MK) has been repeatedly observed in MPDs as well as a spontaneous MK colony formation in a majority of ET patients. Hypersensitivity to thrombopoietin (TPO) in relation to a functional defect of the TPO-MPL pathway may play a major role in spontaneous MK growth. There is no currently available test of platelet functions able to predict the risk of occurrence of thrombotic or haemorrhagic complications in MPD patients. However, the role of platelet activation in the pathogenesis of ischaemic erythromelalgia has been established and a correlation between presenting haemorrhagic manifestations and platelet counts in excess of 1000 x 10(9)/l has been found.

Megakaryocytes carry the fused bcr-abl gene in chronic myeloid leukaemia: a fluorescence in situ hybridization analysis from bone marrow biopsies

Histological examination of bone marrow biopsies shows that about one-third of chronic myeloid leukaemia (CML) patients exhibit an increase of megakaryocytes. The megakaryocytic predominance may be so striking that differentiation from other chronic myeloproliferative disorders (CMPD) may be difficult in some CML patients. Megakaryocytes in CML are clonal as demonstrated by loss of glucose-6-phosphate dehydrogenase isoenzymes. The Ph translocation, fusing the abl and bcr genes on chromosomes 9 and 22, however, obviously occurs as a second step in tumour development. So far, the Ph translocation has not been assigned explicitly to megakaryocytes. The question is whether the megakaryocytic cell lineage could harbour the bcr/abl fusion in those CML cases with striking proliferation of megakaryocytes but lack this genetic defect in cases with normal or decreased megakaryocyte counts. We therefore performed triple-colour fluorescence in situ hybridization (FISH) for portions of the bcr and abl genes flanking the breakpoint in CML in paraffin sections of CML cases with normal and with increased numbers of megakaryocytes. This method allows identification of the bcr/abl fusion in single, morphologically intact cells, whereas conventional cytogenetics requires lysis and thus destruction of the cell. Among the 21 CML patients examined by FISH, 10 were informative for bcr and abl genes and displayed distinct hybridization signals within nuclei of bone marrow cells. Besides the granulopoietic cells, megakaryocytes of all those patients (4 without and 6 with varying grades of megakaryocytic increase) displayed bcr/abl fusion signals indicative of a Ph translocation. The lack of hybridization signals in the remaining 11 cases indicates that this technique is not of value diagnostically and should be reserved for scientific questions. Positive controls consisted of conventional chromosome preparations from bone marrow aspirates demonstrating the Ph chromosome in all patients examined, and negative controls of paraffin sections of bone marrow biopsies from non-CML patients. These showed no fusion signals in bone marrow cells, including megakaryocytes, using FISH. Our results demonstrate clearly that not only the transforming event but also the Ph translocation leading to the bcr/abl fusion happens prior to the differentiation of the pluripotent stem cell into different myeloid lineages. The megakaryocytic proliferation evident in some CML cases is probably a consequence of the disease progress.

Megakaryocyte markers in myeloproliferative disorders

Identification of megakaryocyte precursors with immunohistochemical methods in bone marrow trephine biopsy specimens (embedded in a plastic resin, Immuno-Bed) was performed from patients with blastic phase of chronic granulocytic leukaemia (five cases), from chronic megakaryocytic-granulocytic myelosis (four cases) and from acute megakaryoblastic leukaemia (11 cases). In megakaryoblasts of bone marrow biopsies immunohistochemical reactions using the ABC method and monoclonal antibodies against von Willebrand antigen and GpIIb/IIIa (CD41) were visible in various percentages depending on the maturation's degree of megakaryocyte precursors. The number of circulating blast cells determined by flow cytophotometry was nearly similar to those of observed in biopsies. The greatest bone marrow reticulin content could be detected in acute megakaryoblastic leukaemia cases. Despite the different clinicopathological entities, the presence of the same phenotype (megakaryoblasts) was associated with a short survival in these haematological malignancies (in CGL MKB phase 4.0, in CMGM MKB phase 4.2, and in AML M7 5.8 months, respectively).

Histological features of prognostic significance in CML--an immunohistochemical and morphometric study (multivariate regression analysis) on trephine biopsies of the bone marrow

To evaluate the prognostic significance of clinical as well as histological disease features at the time of diagnosis, an immunohistochemical and morphometric study was performed on bone marrow trephine biopsies in 130 patients with Ph(1+)-CML. For identification of all cell elements of the megakaryocytopoiesis we used the monoclonal antibody CD61 (Y2/51) and for the macrophages, the recently characterized antibody PG-M1. Density of argyrophilic fibers was determined per fat cell-free marrow area. Based on a multivariate analysis-derived risk model, the reproducibility of the prognostic score described by Sokal and co-workers was tested, particularly with regard to histological variables. Additionally, we calculated the disease-specific loss in life expectancy. Our prognostic model (Cox model) consisted of the variables: age, spleen size, peripheral erythro-normoblasts, pseudo-Gaucher cells, and fiber density. To assess the validity of this new CML score, a receiver-operating curve (ROC) of sensitivity and specificity was constructed. The improved prognostic efficiency of this newly developed risk model in predicting death within 3 years after diagnosis of CML was demonstrated in comparison with generally accepted staging systems. Immunohistochemistry revealed that not the total number of macrophages, but only the subfraction of pseudo-Gaucher cells exerted a significant impact on survival. Furthermore, it was feasible to calculate the number of atypical micromegakaryocytes and pro- and megakaryoblasts. This abnormal and immature cell population showed a significant correlation with fiber density and prognosis. Finally, the practical value of the Hannover classification was tested. This histological classification enabled a discrimination between two groups with different survival patterns, i.e., granulocyte and/or megakaryocyte-rich subtypes versus subtypes with increase in reticulin and collagen fibers.

Nucleolar organizer regions of megakaryocytes in chronic myeloproliferative disorders

To study megakaryocyte activation, the argyrophilic staining method of nucleolar organizer regions (AgNOR) has been applied to decalcified bone marrow biopsies of 16 individuals with no haematopoietic disorders and 59 patients with chronic myeloproliferative disease. Of the 59 patients, 18 had chronic myeloid leukaemia (CML), 21 chronic megakaryocytic granulocytic myelosis (CMGM), 13 polycythaemia vera (PV) and 7 essential thrombocythaemia (ET). The AgNOR number of megakaryocytes in CML was significantly lower, and in CMGM, PV and ET significantly higher than in healthy individuals. The high number and the clusters of fine-grained AgNORs of megakaryocytes in CMGM, PV and ET are suggestive of active, proliferating cells. The AgNOR number of megakaryocytes and the platelet counts of the patients did not show a convincing correlation. In CMGM, PV and ET the pyknotic, heterochromatinized megakaryocytes with narrow rims of cytoplasm called bare (nude) nuclei, possessed few, large AgNOR granules. The AgNOR staining of bare nuclei and the roughly identical number of granules found in CMGM, PV and ET indicate a common, active mechanism of apoptosis.

Morphometric analysis of myelofibrosis in agnogenic myeloid metaplasia and chronic myelogenous leukemia

A morphometric analysis of bone marrow biopsy specimens from patients with myelofibrosis was made to determine the amount of lattice fiber and the number of megakaryocytes, to compare the degree of myelofibrosis in primary and secondary myelofibrosis, and to assess the relationship between the morphometric findings and other parameters. Eight patients with agnogenic myeloid metaplasia (AMM) and six with chronic myelogenous leukemia associated with frank myelofibrosis (CML-MF) were studied. When the main clinical, hematological, and laboratory features of both groups of patients were compared, the only significant difference was in the neutrophil alkaline phosphatase score. Morphometric study showed that the amount of lattice fiber and the number of megakaryocytes in AMM were not statistically different from those in CML-MF, and that neither the number of megakaryocytes nor the platelet count correlated with the amount of lattice fiber.

Megakaryocytes in chronic myeloproliferative disorders: numerical density correlated between different entities

A morphometric evaluation of number and grouping of megakaryocytes (MK) in five different groups of chronic myeloproliferative disorders (CMPD) was performed by counting 60 high power fields equaling approximately 14.28 mm2 of haematopoiesis in each case. Twenty-one up to 29 cases were evaluated for each of five categories of CMPD and one control group; a total of 132 cases of CMPD and 33 control cases were used. The mean number of MK per square millimetre was 15.54 +/- 1.53 in chronic myeloid leukaemia of common or granulocytic type (CML.CT), 69.91 +/- 5.85 in CML with megakaryocytic increase (CML.MI), 59.59 +/- 3.27 in polycythaemia vera (P. vera), 59.85 +/- 4.59 in primary thrombocythaemia (PTH), 67.58 +/- 4.11 in chronic megakaryocytic granulocytic myelosis (CMGM), and 19.7 +/- 3.07 in controls. The distinction between free or isolated MK, and between clustered or grouped MK corresponds to the total cell counts of MK in the various groups of CMPD. Clustering of MK was significantly higher in CMGM and PTH compared to other groups, but the difference between them was not statistically significant. Significant differences in the mean number of MK were obtained between controls and CML.CT on the one hand and all other groups of CMPD on the other. The results further support the histological sub-classification of CMPD according to the primary disorders of the Hannover classification (not advanced by sclerosis, fibrosis or excess of blasts, respectively).

Dysmegakaryopoiesis in myelodysplastic syndromes (MDS): an immunomorphometric study of bone marrow trephine biopsy specimens

An immunohistochemical and morphometric analysis was performed on trephine biopsy specimens of the bone marrow in 40 patients (23 men and 17 women, mean age 62 years) with different subtypes of myelodysplastic syndromes (MDS) to determine dysmegakaryopoiesis, but particularly precursor cells--that is, pro- and megakaryoblasts. In 31 of the 40 patients the numbers of megakaryocytes were increased which was associated with a predominance of smaller cell forms (micromegakaryocytes). Compared with periodic acid Schiff, immunostaining with a formalin resistant monoclonal antibody against glycoprotein IIIa (Y2/51(CD61) showed a clinically important proportion of immature elements. These could be designated pro- and megakaryoblasts by taking morphometric measurements on smears and bone marrow sections. There was a relevant increase in the number of promegakaryoblasts in 32 patients, consistent with uncontrolled expansion of the precursor pool. Seventeen repeated bone marrow biopsy specimens taken after chemotherapy largely showed a decrease in the numbers of megakaryocytes including the precursor cell population. Moreover, morphometric evaluation disclosed that micromegakaryocytes in MDS differ significantly from those in chronic myeloid leukaemia (CML) due to distinctive nuclear features and a disturbed nuclear:cytoplasmic ratio. These changes generate a more pleomorphic or atypical appearance of this cell population in MDS, compared with micromegakaryocytes in CML. It is concluded that the disproportionate increase in megakaryocyte precursors and the grossly abnormal aspects of micromegakaryocytes in MDS are characteristics of the severe defect involving haematopoiesis in this disorder.

Morphologic and flow cytometric analysis of circulating megakaryoblasts in chronic myeloid leukaemia

The immunophenotype (a), ultrastructural features (b) and cell kinetics (c) of circulating megakaryoblasts have been studied in two cases of pure megakaryoblastic and one case of mixed (myeloblastic, megakaryoblastic) cell proliferation in chronic myeloid leukaemia (CML). (a) The blast cells showed early megakaryocyte differentiation antigen (HLA-DR), platelet specific GpIIIa (CD61) and GpIIb-IIIa (CD41) antigens in different percentages. (b) The megakaryoblasts were recognized by the presence of platelet GpIIIa (CD61) demonstrated by an immunoelectron microscopic method. The labelled cells were "lymphocyte-like" megakaryoblasts and cells with features of cytoplasmic maturation (demarcation membranes, alpha granules and vacuoles). (c) Cellular DNA content of the megakaryoblasts was measured by propidium iodide (PI) staining of cells expressing platelet GpIIIa (CD61). Flow cytometric (FC) DNA analysis revealed no aneuploidy and high ploidy (greater than 4N) cell population. In the two cases of pure megakaryoblastic proliferation a high percentage of the megakaryoblasts were in the S-phase, while the non-megakaryoblastic cell fraction showed no elevated S-phase compartment. It is concluded that in CML the circulating megakaryoblasts (1) have a nuclear maturation arrest and accumulation at the level of tetraploid DNA content, (2) surface antigen expression and cytoplasmic organelles show a tendency to mature and (3) in pure megakaryoblastic proliferation the myeloid cells are not in the cell compartment showing high proliferation.