The relation of thrombokinetics to bone marrow megakaryocytes and to the size of the spleen in polycythaemia vera

Bone marrow megakaryocytes and platelet kinetics in systemic lupus erythematosus. With special reference to corticosteroid and azathioprine therapy

The megakaryocyte number and mean megakaryocyte area were determined in histological sections of sternal bone marrow from 26 patients with systemic lupus erythematosus (SLE). Also 20 platelet survival studies were carried out in these patients. The results were analyzed with respect to corticosteroid (CS) and CS + azathioprine (AT) therapy. The mean bone marrow megakaryocyte number as highest in untreated SLE patients, slightly lower in patients receiving CSs and lowest in those receiving CSs + AT. The difference was, however, not significant. The mean megakaryocyte areas were smallest in untreated SLE patients, slightly larger in those treated with CSs and significantly (p less than 0.05) larger in patients who received CSs + AT than in untreated patients. Platelet production rate was normal in all 3 groups of SLE patients. The results suggest that CS and AT therapy in SLE intervenes with the bone marrow megakaryopoiesis without affecting the production rate of platelets.

The red cell mass, plasma erythropoietin and spleen size in apparent polycythaemia

It has been shown previously that measurement of the spleen size and plasma erythropoietin (EPO) concentration are valuable adjuncts in the diagnostic work-up of patients with polycythaemia vera. The aim of the present work was to evaluate their value in the assessment of apparent polycythaemia (AP). Therefore, over a 24-month period we routinely performed bone marrow biopsies, measurement of red cell mass (RCM) and plasma volume (PV), spleen size determination by gamma camera scintigraphy and determination of the plasma EPO concentration in consecutive patients referred to us because of elevated values for packed cell volume (>0.48 in females and >0.51 in males). After having excluded patients with clonal and secondary polycythaemias we were left with 38 patients (27 males and 11 females) with AP. In all of them the measured RCM was within normal range, i.e. <36 ml/kg for males and <32 ml/kg for females. The subjects were characterized by moderate increase in RCM and a concomitant moderate decrease in PV. Thus, as an average the measured RCM exceeded the predicted values by 14% in males and by 12% in females; conversely, as compared to the predicted values the average measured value for PV was reduced by 17% in males and by 8% in females. The average RCM for males was 29+/-3 ml/kg; the corresponding figure for females was 23+/-4 ml/kg. It was shown that 86% of the subjects had plasma EPO concentrations within the control range; the remaining had values slightly above or below the control range. The mean posterior spleen scan area was 57+/-16 cm2 and mean left lateral area 57+/-17 cm2; the reference value for spleen scan area (for both projections) is 57+/-12 cm2. Of the patients 35/38 (92%) had a spleen scan area within the mean+2SD for controls and 38 subjects (100%) had values within the mean+3SD. It is concluded that measurement of plasma EPO and a careful assessment of the spleen size should always be considered in the evaluation of patients with elevated values for venous packed cell volume.

Bleeding time and platelet function in essential thrombocythemia and other myeloproliferative syndromes

Bleeding time (BT) and platelet function tests have been widely used in patients with essential thrombocythemia (ET), with the aim to support diagnosis and to identify laboratory predictors of haemorrhagic and thrombotic complications. BT is significantly prolonged in 7-19% of ET patients and several functional abnormalities have been observed in platelet structure, biochemistry and survival. However, the attempt to relate these in vivo and in vitro platelet dysfunctions with diagnosis or clinical sequelae has been generally disappointing. Therefore, BT and platelet function tests are currently not recommended in the initial evaluation or during the follow-up of patients with ET, unless in the setting of a clinical or biological study. A noteworthy exception is represented by a subset of patients characterized by very high platelet count (> 1500 x 10(9)/L) and bleeding symptoms, who can have an acquired von Willebrand disease. In these cases, prolonged BT and abnormal multimeric pattern of von Willebrand factor are useful for diagnosing and monitoring this acquired hemorrhagic disease. BT and platelet function tests should be included in the baseline evaluation of ET patients enrolled in prospective clinical trials aiming assess their predictive role on clinical end-points.

The myeloproliferative disorders. An historical appraisal and personal experiences

According to strict morphological, biochemical and cytogenetic criteria Philadelphia chromosome positive essential thrombocythemia and chronic granulocytic leukemia constitute a separate malignant and individual disease entity, whereas Philadelphia chromosome negative essential thrombocythemia, polycythemia vera and agnogenic or megakaryocytic myeloid metaplasia form a chronic proliferation of three hematopoietic cell lines. Histopathology from bone marrow biopsies permits the characterization and diagnostic differention of the various myeloproliferative disorders and appears to be a main and specific diagostic criterion for polycythemia vera and essential thrombocythemia. Hemorrhagic thrombocythemia is a clinical syndrome of recurrent spontaneous mucocutaneous and secondary hemorrhages often preceded by thromboses, extremely high platelet counts, pseudohyperkalemia, increased bone marrow cellularity and frequently splenomegaly. The diagnostic criteria of essential thrombocythemia with paradoxical occurrence of thrombotic events and hemorrhagic manifestations are a platelet count in excess of 1000 x 10(9)/L and increased bone marrow cellularity in the majority of the cases. Erythromelalgia and other microcirculatory ischemic or thrombotic events or accidents in essential thrombocythemia and polycythemia vera already occur at platelet counts in excess of the upper limit of normal. First line treatment options in essential thrombocythemia and polycythemia vera are control of platelet function with low-dose aspirin and reductive control of platelet count and erythrocytes by bloodletting, interferon and busulfan or hydroxyurea monochemotherapy.

Flow cytometric analysis of megakaryocyte ploidy in chronic myeloproliferative disorders and reactive thrombocytosis

Megakaryocyte (MK) ploidy patterns were analysed by flow cytometry in 29 newly diagnosed and previously untreated patients with chronic myeloproliferative disorders (MPD) and concomitant thrombocytosis, in 9 patients with reactive thrombocytosis (RT) and in 12 healthy individuals. Unfractionated bone marrow from routine aspirates was used. MKs were identified with a fluorescein labelled monoclonal antibody specific for glycoprotein IIIa (GPIIIa) and DNA was stained with propidium iodide. For the 12 healthy volunteers the mean modal ploidy number was 16 N; the 9 patients with RT displayed an identical MK ploidy pattern. The frequency of MKs with a ploidy > or = 32 N was 45% among the patients with essential thrombocythaemia (ET) compared to 32% among the healthy volunteers (p < 0.001). MKs with ploidy number > or = 64 N, comprising approximately 13% of the total number of MKs, was a characteristic finding in the patients with ET. Similar findings were present in 8 patients with polycythaemia vera (PV). In patients with PV 34% and 6% of the MKs displayed ploidies > or = 32 N and > or = 64 N, respectively. In contrast, a distinct shift towards lower ploidy number, with 63% of MKs < or = 8 N, was found among the 4 patients with chronic myeloid leukaemia (CML). The present results indicate that by using flow cytometric analysis of MK ploidy distribution in patients with thrombocytosis, those with a reactive cause are likely to be discriminated from patients with myeloproliferative thrombocytosis, i.e. PV and ET on one hand and CML on the other hand. The distinction between ET and PV, however, has to be made on other grounds.

Assessment of spleen size using gamma camera scintigraphy in newly diagnosed patients with essential thrombocythaemia and polycythaemia vera

By using gamma camera imaging the spleen size was assessed in 18 consecutive patients with essential thrombocythaemia (ET) and in 18 consecutive patients with polycythaemia vera (PV). All ET and PV patients were newly diagnosed and had not received any myelosuppressive therapy prior to study. The spleen areas in both posterior and left lateral projections were determined. Eighteen consecutive patients with idiopathic thrombocytopenic purpura (ITP) served as a control group since by definition they do not present with splenic enlargement; in these latter subjects the mean posterior and left lateral splenic areas were almost identical (48 +/- 15 and 47 +/- 17 cm2, respectively). In comparison with this control group patients with ET and PC had significantly larger spleens. In both ET and in PV patients the left lateral spleen scan area exceeded the posterior one. Patients with PV had larger splenic areas in both projections than did patients with ET, but the differences were not statistically significant. Compared to the ITP patients it was found that at least 50% of the ET patients and at least 61% of the PV patients at diagnosis presented with splenomegaly.

Polycythemia vera

Polycythemia vera (PV) is one of the myeloproliferative diseases, and, as such, is an example of clonal hematopoiesis. The progeny of a single, abnormal, hematopoietic stem cell gain a growth advantage over their normal counterparts resulting in overproduction of red cells generally accompanied by overproduction of granulocytes and platelets as well. There are a variety of nonspecific symptoms at onset related to the increased red cell mass and hematocrit accompanied by the more specific manifestations of pruritus, erythromelalgia, and hepatic, portal, and mesenteric vein thrombosis. Splenomegaly and hypertension are common. The laboratory hallmark is an increased red cell mass. There is also often an increase in white cell count, platelet count, and leukocyte alkaline phosphatase along with other findings reflecting the increased rate of turnover of hematopoietic cells. The bone marrow biopsy generally displays hypercellularity involving all three cell lines and absent iron stores. The diagnosis of PV depends on excluding spurious polycythemia in which there is a high hematocrit but a normal red cell mass and secondary polycythemia in which there is an increased red cell mass in response to tissue hypoxia or the inappropriate production of erythropoietin, generally by a tumor. In addition, one should try to establish the diagnosis in a positive fashion by a combination of studies of the blood and bone marrow. Phlebotomy and occasionally plateletpheresis should be used as acute therapy. Chronic therapy is guided by the knowledge that patients treated with phlebotomy alone have an increased rate of thrombotic complications particularly in older patients and those with previous thrombotic disease. Myelosuppressive therapy can reduce the incidence of these complications, but is commonly associated with an increased incidence of second malignancies, particularly acute leukemia. At present, hydroxyurea is the myelosuppressive agent of choice. Antiplatelet agents have a limited role except in the palliation of the syndrome of erythromelalgia. Median survival is approximately 10 years. As implied above, the causes of morbidity and mortality vary with the mode of chronic therapy which has been employed, leukemia being more common after myelosuppressive therapy and thrombotic complications being more common after therapy with phlebotomy alone. Ten percent to 50% of patients move into a spent phase followed by postpolycythemic myeloid metaplasia, irrespective of previous therapy employed. Eventually, the major problems may be cytopenias and massive splenomegaly.

Pro-megakaryoblasts in bone marrow tissue from patients with primary (idiopathic) osteo-myelofibrosis (agnogenic myeloid metaplasia). An immunomorphometric study on trephine biopsies

An immunomorphometric study was performed on bone marrow biopsies from 40 patients with primary osteomyelofibrosis--OMF, (agnogenic myeloid metaplasia) by employment of a monoclonal antibody against glycoprotein IIIa (Y2/51) to determine the number of pro-megakaryoblasts. Specimens from 15 individuals without any hematological disorder served as controls. With reference to the pertinent literature on megakaryocyte precursors and following a pilot study on corresponding smears, in tissue sections pro-megakaryoblasts were characterized by a size of 42.1 +/- 2.6 microns 2 (diameter 7.5 +/- 0.3 microns). In comparison with controls, in OMF no relevant increase in the number of pro-megakaryoblasts per square and cubic millimeter bone marrow was evaluable. The relative frequency of these precursors was significantly reduced due to an increase in the total amount of conspicuously large and abnormal megakaryocytes. Statistical analysis failed to reveal any correlations between counts for pro-megakaryoblasts or the total number of Y2/51--positive megakaryocytic elements with the density of argyrophilic fibers (determined by morphometry) or the platelet values. Our findings imply that in OMF the marked increase in circulating progenitor cells of the megakaryocyte lineage may be generated by extramedullary, probably splenic hematopoiesis. Moreover, the evolution of medullary fibrosis is thought to be associated with the striking predominance of large atypical, possibly overaged and hyperpolyploid megakaryocytes and not with an increase in precursor cells.

DNA content and nuclear size of megakaryocytes in thrombocythaemia

Total nuclear DNA content and nuclear size of megakaryocytes were studied in biopsies of the iliac bone marrow of individuals with normal or increased platelet counts. The DNA content was determined using Feulgen cytophotometry of bone marrow smears and the nuclear area by morphometric analysis of megakaryocytes of bone marrow sections. The mean DNA content and the mean nuclear area were both significantly larger in megakaryocytes of patients with thrombocytosis as a result of myeloproliferative disease than in patients with secondary thrombocytosis as well as in two control groups of individuals with normal platelets counts, one comprising healthy volunteers, the other with various non-haematological disorders. There was a statistically significant correlation between the DNA content and nuclear area of the megakaryocytes (r = 0.92) in the entire group of bone marrows studied.

Megakaryocyte precursors (pro- and megakaryoblasts) in bone marrow tissue from patients with reactive thrombocytosis, polycythemia vera and primary (essential) thrombocythemia. An immunomorphometric study

To determine the number of megakaryocyte precursors (pro- and megakaryoblasts), an immunomorphometric study was performed on paraffin-embedded trephine biopsies of the bone marrow using a monoclonal antibody against platelet glycoprotein IIIa. Eighteen control specimens from patients with no evidence of any hematological disorder and a normal platelet count were selected and assessed together with the same number of specimens from patients with reactive thrombocytosis, polycythemia vera rubra (P. vera) or primary (essential) thrombocythemia (PTH). A strikingly proportionate increase in early megakaryocytes occurred in all patients enrolled in this study, compared with the controls. Moreover, there were no significant correlations between counts for precursors or total megakaryocytes per square millimeter of bone marrow with the corresponding values for platelets. This indicates that despite an orderly increase in immature forms in the bone marrow, the number of platelets circulating in the blood is influenced by other additional factors, such as the expanded platelet pool in the enlarged spleen. The non-disproportionate expansion of megakaryocyte precursors extends previous findings on progenitor cells of this lineage in vitro, particularly in PTH. Histological evaluation of the bone marrow of patients with P. vera and PTH indicated that megakaryopoiesis proceeded to the production of appropriate mature forms with no obvious excess of very small or blastic elements.

Histomorphometry of bone marrow biopsies in chronic myeloproliferative disorders with associated thrombocytosis--features of significance for the diagnosis of primary (essential) thrombocythaemia

A histomorphometric analysis was performed on trephine biopsies of the bone marrow in 55 patients with chronic myeloproliferative disorders (CMPDs) and marked thrombocytosis (platelet count exceeding 600 x 10(9)/l). This study aimed at discriminating primary (essential) thrombocythaemia (PTH) from the various other subtypes of CMPDs presenting with thrombocytosis. Following the diagnostic requirements postulated by the Polycythemia-vera-Study-Group for PTH and polycythaemia vera rubra (P.vera) and the generally accepted criteria for the establishment of chronic myeloid leukaemia (CML) and agnogenic myeloid metaplasia (AMM), our cohort of 55 patients was divided into the following subgroups: CML (16 cases), P.vera (11 cases), AMM (13 cases) and finally PTH (15 cases). Histomorphometric measurements revealed that PTH was distinguishable from the other subtypes of CMPDs with respect to several histological variables: patients with PTH had a normal amount of neutrophilic granulo- and erythrocytopoiesis as well as a non-increased content of reticulin (argyrophilic) fibers in contrast to the findings in CML, P.vera and of course AMM. Moreover, sizes of megakaryocytes and their nuclei were significantly greater in PTH and internalization of haematopoietic cells (emperipolesis) was more frequently encountered in comparison with the other subtypes of CMPDs. Deviation of the circular perimeter of megakaryocyte shape was most prominently expressed in CML and AMM, and consequently generated an increased number of a-nuclear cytoplasmic fragments. In contrast to this feature aberration of the nuclei from a circular outline occurred in a less pronounced way in CML, but was excessive in P.vera, AMM and PTH. Our morphometric evaluation demonstrates that certain histological features may serve as a valuable aid in discriminating PTH from the other occasionally thrombocythaemic subtypes of CMPDs.

Density distribution and size of megakaryocytes in inflammatory reactions of the bone marrow (myelitis) and chronic myeloproliferative diseases

Morphometric evaluation was performed on semi-thin sections of core biopsies of the bone marrow and included 20 cases of each group of diseases besides control specimens. (i) Hyperergic myelitis of rheumatic origin. (ii) Chronic granulocytic leukaemia (CGL). (iii) Polycythaemia vera (P. vera). (iv) Chronic megakaryocytic-granulocytic myelosis (CMGM). (v) Myelofibrosis or osteomyelosclerosis (MF/OMS). The following classification of megakaryopoiesis was applied: normal megakaryocytes; giant forms; microforms; intussusceptions; cytoplasmic fragments; naked nuclei. The density distribution shows an increase of megakaryocyte number in those 5 different marrow disorders, ranging from about 13/mm2 in the normal sample up to 65 cells/mm2 in MF/OMS. Microforms are most frequently encountered in CGL, whereas giant megakaryocytes, intussusceptions and many cytoplasmic fragments characterize P. vera, CMGM and MF/OMS. Our measurements suggests 3 distinct categories of bone marrow lesions with corresponding alterations of the megakaryopoiesis: (i) myelitis and CGL; (ii) P. vera; (iii) CMGM and MF/OMS.

Abnormalities of megakaryocytes in myelitis and chronic myeloproliferative diseases

A planimetric study of megakaryopoiesis in various chronic myeloproliferative diseases (CMPD) was performed and the results compared with those from controls and myelitis of rheumatic origin. Morphometric measurements included at least 200 megakaryocytes in each case observed in Giemsa-stained semithin sections of resin-embedded core biopsies. Twenty specimens were evaluated from the controls and inflammatory disorders and from each of the following CMPD: 1, chronic granulocytic leukaemia (CGL); 2, polycythaemia vera (P. vera); 3, chronic megakaryocytic-granulocytic myelosis without or with minimal increase in reticulin fibre content (CMGM); 4, myelofibrosis or osteomyelosclerosis (MF/OMS). Megakaryocytes were classified as follows: 1, normal megakaryocytes at all stages of maturation; 2, giant forms; 3, microforms; 4, intussusceptions; 5, a-nuclear cytoplasmic fragments; 6, naked nuclei or necrotic forms. The results of this study demonstrate obvious abnormalities of megakaryopoiesis in addition to the increase in absolute numbers of megakaryocytes per marrow area and their different sizes as reported earlier (Thiele et al. 1982). Aberrations are particularly conspicuous when pure granulocytic proliferation or neoplasia of CGL is compared with the so-called mixed cellularity of megakaryocytes and granulocytes in CMGM including MF/OMS. Abnormalities of the giant forms of megakaryocytes are especially evident and comprise irregular cellular and nuclear perimeters (as calculated by a modified shape factor) in the two latter entities (CMGM-MF/OMS). This remarkable feature is associated with a disorganization of nuclear development and/or a disproportionate nuclear-cytoplasmic ratio which has never been observed in CGL previously. In combination with this striking cellular anomaly, which is compatible with an extreme amoeboid shape of giant forms in CMGM and MF, intussuceptions and a-nuclear cytoplasmic fragments are frequently encountered. The final stage of megakaryopoiesis, i.e. naked nuclei, are increased in number in all CMPD, probably because of enhanced proliferation and platelet shedding. Naked nuclei are often small in CGL (as remnants of the frequent micromegakaryocytes) and large in P. vera and CMGM/MF (depending on the high incidence of giant megakaryocytes in these latter disorders).

The relation of platelet kinetics to bone marrow megakaryocytes in chronic granulocytic leukaemia

The relation of thrombokinetics to quantitative determinations of megakaryocytes (mgkc) in bone marrow sections was studied in 11 consecutive cases of untreated Ph1-positive chronic granulocytic leukaemia (CGL). The results were compared with controls and with previously obtained data in polycythaemia vera (PV), primary thrombocythaemia (PT) and in idiopathic thrombocytopenic purpura (ITP). Platelet survival was significantly reduced in CGL. Platelet production was 5.8 x normal and the mgkc number and volume/microliter bone marrow were significantly increased as compared to controls. The increase in mgkc volume was not in proportion to that of number due to a significant decrease of mgkc size. Platelet production was strongly related to mgkc number/mm2 and to the mgkc volume/microliter bone marrow. The platelet production rate in relation to a unit of mgkc volume/microliter bone marrow was, however, greater in CGL than in controls, PV, PT and ITP. The chief reason for this is most probably the greater expansion of the total bone marrow mass in CGL.

[Relationships between hemopoiesis and peripheral blood counts in untreated and 32P-treated patients with polycythaemia vera (author's transl)]

The composition of the hemopoiesis was determined in iliac crest biopsies of 51 patients with polycythaemia vera. There was a good correlation between thrombopoiesis and thrombocytes and, to a minor degree also between erythropoiesis and erythrocytes in untreated patients as well as between granulopoiesis and granulocytes in 32P-treated patients. In patients with normal blood counts there existed no correlation between the bone marrow and the blood counts within smaller limits of the cell count. Histomorphometric analysis shows no difference either between untreated and 32P-treated patients or between patients with and without splenomegaly. This is an argument against a significant intrasplenic hemopoiesis or an intrasplenic cell pooling, or destruction (hypersplenism), respectively. The blood sinusoids are hyperplastic and distended. With increasing hyperplasia of the hemopoiesis the sinusoids become relatively smaller. So changes in vascularisation may be of importance in the infrequent transitions into myelofibrosis and/or leukemia.

Megakaryocyte quantifications in relation to thrombokinetics in primary thrombocythaemia and allied diseases

Megakaryocyte morphology was studied quantitatively in primary thrombocythaemia (PT) and in chronic myelogenous leukaemia (CML). The relation of thrombokinetics to megakaryocyte quantifications was evaluated in PT and compared to previously obtained results in polycythaemia vera (PV) and idiopathic thrombocytopenic purpura (ITP). Megakaryocyte area, number and volume per mul bone marrow were significantly higher in PT as compared to controls. The nuclear lobe number was significantly increased and the megakaryocytes were shifted towards more mature forms, suggesting a prolonged megakaryocyte generation time. In CML the megakaryocyte number and volume per mul bone marrow were also significantly above normal, but the megakaryocyte area, number of lobes and degree of megakaryocytic maturation were significantly below normal. Platelet production was in PT 6.2 times normal and proportional to the increase in megakaryocyte volume which was 6.8 times normal. In PV with major splenomegaly the mean platelet production rate was higher (9.5 times normal) although their peripheral platelet count was lower than in PT. This discrepancy is explained by the greatly enlarged splenic platelet pool in the PV patients. In ITP the mean platelet production rate was 2.2 to 3 times normal and was significantly lower than in PT and PV.

Splenomegaly, macrothombocytopenia and stomatocytosis in healthy Mediterranean subjects (splenomegaly in Mediterranean macrothrombocytopenia)

Spleen size, stomatocytosis, macrothrombocytopenia, haemoglobin level, white cell count, and abdominal pain episodes were assessed in a coded study of healthy Mediterranean immigrants to Australia. Spleen size was estimated from a length measurement, L, on a standardized plain abdominal radiograph and expressed both as spleen weight and as a spleen length index, L/square root BSA; the platelet count and size parameters were determined electronically and the presence of stomatocytes was evaluated in stained blood films. In relation to 16 Northern European control women 12 of 25 Mediterranean women had radiographic splenomegaly, 10 had macrothrombocytopenia, 9 had stomatocytosis, but none had episodes of abdominal pain. The median spleen weights of the two groups were estimated as 157 and 247 g with ranges from percentile 2.3 to 97.7 of 75 to 328 and 112 to 669 g. Within the Mediterranean group splenomegaly correlated with macrothrombocytopenia (P less than 0.001) but not with stomatocytosis, haemoglobin values or white cell counts. Thus, mild splenomegaly may be expected in Mediterranean macrothrombocytopenia, Mediterranean stomatocytosis appears unrelated, and all of these apparently benign anomalies may be incidental findings in patients from the Italian and Balkan peninsulas.

Platelet kinetics in idiopathic thrombocytopenic purpura (ITP) before and at different times after splenectomy

Platelet kinetics were studied in 18 patients with idiopathic thrombocytopenic purpura (ITP) before splenectomy, 1 month and 8 months after splenectomy. A gamma function was used for the calculation of platelet mean life span (MLS). Before splenectomy platelet MLS was greatly reduced and platelet production was 2.8 times normal. The peripheral platelet count and platelet recovery were significantly related to platelet MLS. One month after splenectomy platelet MLS increased appreciably while platelet production decreased but continued to be significantly above normal. These changes together with the removal of the splenic platelet pool resulted in a peripheral platelet count significantly higher than normal and a total platelet mass not differing from normal. Eight months after splenectomy platelet MLS increased slightly but was not significantly different from that found 1 month after splenectomy. Platelet production, however, decreased significantly and became normal. This resulted in a normal peripheral platelet count and a decreased total platelet mass which was significantly lower than in non-splenectomized control subjects. The results suggest that platelet production is regulated by a feed-back mechanism that is governed by the peripheral platelet count. The high platelet count encountered 1 month after splenectomy is due to a sluggishness of this regulatory mechanism.