INTRODUCTION — The megakaryocyte is the hematopoietic cell that produces platelets. Evidence for this relationship was first provided in 1906 by James Homer Wright, who demonstrated that circulating platelets and a giant bone marrow cell now known to be the megakaryocyte shared common tinctorial properties when subjected to a modified Romanowsky stain (picture 1) [1]. Wright went on to show that megakaryocytes sent out pseudopodia into the bone marrow sinusoids from which platelets appeared to be shed [2]. This model of how megakaryocytes produce platelets remains with us to this day.
Wright also demonstrated in normal and abnormal human physiology that changes in platelet number were associated with changes only in the megakaryocytes [2]. Since these seminal observations, much has become known about megakaryocytes and how they produce platelets [3,4]. The characteristics of megakaryocytes, how they regulate platelet production, and their role in pathologic processes will be described here.
CHARACTERISTICS — In lower vertebrate species such as fish and birds, all the circulating blood cells, including the erythrocytes and the platelets (called thrombocytes), are nucleated and are produced by diploid bone precursor cells [5]. However, in higher vertebrates, platelets are produced by a different mechanism whose evolutionary advantage is unclear. Enucleate platelets are generated from bone marrow megakaryocytes that have a number of unique properties.
In humans, megakaryocytes normally account for approximately 0.05 to 0.1 percent of all nucleated bone marrow cells. Their number increases as the demand for platelets rises. In contrast to the erythrocyte, which has a diameter of 7 to 8 microns and a volume of 85 to 100 fL, megakaryocytes have an average diameter of 20 to 25 microns and a volume of 4700 ± 100 fL (picture 2) [6]. Some of the largest megakaryocytes have diameters of 50 to 60 microns and volumes of 65,000 to 100,000 fL.
Each megakaryocyte produces a total of 1000 to 3000 platelets. Although it has long been assumed that larger megakaryocytes make more platelets, this has never been conclusively demonstrated.
Mature megakaryocytes are invariably polyploid and contain from two (4N) to 32 (64N) times the normal diploid amount of DNA; the mean value is 16N in humans [7,8]. This polyploidy appears to result in functional gene amplification, perhaps to increase protein synthesis in parallel with megakaryocyte enlargement [9,10]. Few other cells are normally polyploid (eg, occasional hepatocytes and macrophages have 4N or 8N DNA content).
Unlike the polyploid hepatocytes and macrophages, whose DNA is contained in multiple separate nuclei, the DNA in the megakaryocytes is contained within one highly lobulated nuclear envelope in which each lobule represents one diploid amount (2N) of DNA. This is the result of a process referred to as endomitosis [11].
Surface membrane — The megakaryocyte surface membrane is similar to that of the platelet. One of the first signs of differentiation along the megakaryocyte lineage is the appearance on the surface of the megakaryocyte of the platelet glycoprotein (GP) receptor GPIIb/IIIa and later by GPIb and collagen receptors [12,13].
The megakaryocyte has a demarcation membrane system in which the surface membrane of the mature megakaryocyte is deeply invaginated and highly redundant (picture 3). This enormous amplification of the surface membrane is thought to be the means by which an adequate amount of platelet surface membrane is created and subsequently used to form platelets [14-17].
Specific platelet granules — Since platelets undergo little protein synthesis, their cytoplasmic characteristics are mostly determined by the megakaryocytes from which they arise. The membrane bodies of the alpha and dense platelet granules and some of their contents are made in megakaryocytes. However, some of the granule contents are actually taken up from the plasma by both megakaryocytes and platelets.
●Alpha granules of platelets contain numerous platelet proteins and growth factors. The granule body itself is made early in megakaryocyte development before the demarcation membrane system. Some of the granule contents such as platelet-derived growth factor, transforming growth factor-beta, platelet factor-4, and von Willebrand factor are synthesized in the megakaryocyte and transported to the alpha granules [18]. However, other proteins enter the alpha granules of both megakaryocytes and platelets from the plasma via IIb/IIIa receptor-mediated endocytosis (eg, fibrinogen) [19-21] or pinocytosis (eg, albumin and IgG) [22].
●Dense granules of platelets are physically dense and, because they contain serotonin and calcium, are also electron dense when seen with the transmission electron microscope. The dense granule membrane bodies are made in megakaryocytes but do not acquire their content of serotonin and calcium until platelets are released into the circulation where they take up calcium and most of the body's circulating serotonin [23,24].
Microscopic appearance — The large, mature, polyploid megakaryocytes are readily identified by light microscopy (picture 1 and picture 2). On occasion, they can be seen to be shedding platelets.
In addition to the mature megakaryocytes, three other stages of megakaryocytes may be identified [25]. Intermediate megakaryocytes have large polyploid nuclei but a modest amount of cytoplasm; they can be distinguished from immature megakaryocytes which have scant cytoplasm and large, basophilic nuclei. Also present, but identified only by special stains for von Willebrand factor or glycoprotein (GP) IIb/IIIa, are small, diploid, lymphocyte-like megakaryocyte progenitor cells that have not yet undergone endomitosis [26].
ORIGIN — Like all other bone marrow cells, megakaryocytes are derived from the multipotent stem cell (figure 1). The stem cell gives rise to early bilineage progenitors (megakaryocyte-erythroid progenitors, MEP) that are subsequently able to undergo erythrocyte or megakaryocyte differentiation. Although there is no specific assay for this common megakaryocyte-erythroid progenitor cell, its existence is supported by several lines of evidence.
●In mice in which the thymidine kinase gene was expressed under the control of the GPIIb promoter, administration of ganciclovir (which is converted to the toxic monophosphate by thymidine kinase) resulted in eradication of both megakaryocyte and erythroid progenitors [27]. This observation suggests that the GPIIb promoter was transcriptionally active in this bipotential progenitor cell [28] and possibly in a more potent progenitor cell [29].
There is a close relationship between early erythroid and megakaryocyte differentiation at the molecular, cellular, and pathological levels. As an example, common transcription factors exist in these two lineages [30] and their regulatory hematopoietic cytokines, erythropoietin and thrombopoietin, are 50 percent similar (figure 2) [31]. (See "Overview of hematopoietic stem cells".)
It has been demonstrated that a microRNA, miR-150, is important in the lineage specification that occurs to this common megakaryocyte-erythroid progenitor cell. When present, miR-150 drives differentiation towards megakaryocytes and away from erythroid cells [32]. The transcription factor Myb is a critical target of miR-150 [33]. Thrombopoietin increases miR-150, which in turn binds to the 4 miR-150 binding sites on the c-Myb 3'UTR and decreases Myb mRNA and protein. Reduction in Myb expression subsequently leads to increased megakaryocyte differentiation and reduced erythroid differentiation. When miR-150 is absent, erythroid differentiation occurs [34].
Ultimately the differentiation process produces a precursor cell committed only to megakaryocyte differentiation called the megakaryocyte colony-forming cell (Meg-CFC), which can be assayed by cell culture methods. This cell expresses some megakaryocyte properties such as the presence of glycoprotein (GP) IIb/IIIa [35,36] and undergoes mitosis [37]. It is stimulated by interleukin (IL-3) and thrombopoietin [38,39].
Eventually the Meg-CFC stops mitosis and enters endomitosis in which DNA replication continues, but neither the nucleus nor the cell undergoes division (cytokinesis). Polyploid precursors with scant cytoplasm are produced (figure 1). It was initially assumed that endomitosis was simply the absence of mitosis after each round of DNA replication. However, studies in mice showed that megakaryocytes enter mitosis and progress through normal prophase, prometaphase, metaphase, and up to anaphase A, but not to anaphase B, telophase, or cytokinesis [40]. After anaphase, the nuclear membrane is reassembled about the sister chromatids as a single nucleus skipping telophase and cytokinesis; the cells then enter the next round of DNA replication.
Genes with known roles in regulating the cell cycle such as the cyclin-dependent kinases (CDKs) and the cyclins have been studied. Although the results are incomplete, endomitosis is associated with increased cyclin D3 [41] and reduced levels of cyclin B1 and cyclin B1-dependent Cdc2 kinase [42]. These events may allow the megakaryocyte to abort mitosis and reenter a phase of DNA replication without cytokinesis [43].
Upon the completion of endomitosis, the immature megakaryocytes develop a mature cytoplasm, become morphologically identifiable, and eventually release platelets. Overall, it takes approximately five to seven days to progress from the megakaryocyte colony-forming cell (Meg-CFC) to the platelet.
Thrombopoietin — Thrombopoietin has a major effect on almost all steps of megakaryocyte differentiation and maturation. It promotes the growth of Meg-CFC, dramatically increases the rate of endomitosis, inhibits apoptosis [44,45], and stimulates megakaryocyte maturation. An increase in megakaryocyte ploidy is seen at even the lowest amounts of thrombopoietin and is one of the most prominent effects of thrombopoietin. Other cytokines such as IL-3 and IL-11 can also promote Meg-CFC growth and megakaryocyte maturation but have little effect on endomitosis; their actions may not be important in normal physiology. However, the final stage of platelet release is not dependent on thrombopoietin and in one model system was actually inhibited by large amounts of thrombopoietin [46].
DIFFERENTIATION — The molecular mechanisms by which cells become committed to the megakaryocyte lineage are just starting to be unraveled. Unlike thrombopoietin, which appears to prevent apoptosis and stimulate growth of cells already committed to megakaryocyte differentiation, intrinsic lineage-specific transcription factors are expressed in cells and then establish cell-specific phenotypes. The transcription factors GATA-1 and NF-E2 are important for megakaryocyte development. What lineage-specific genes are controlled by these factors remains unknown.
GATA-1 transcription factor — GATA-1 is zinc-finger transcription factor which is expressed in erythroid, megakaryocyte, eosinophil, and mast cells. Elimination of the entire GATA-1 gene results in embryonic death due to severe anemia [47].
Since GATA-1 is found in the megakaryocyte and GATA-1 binding sites are found on many megakaryocyte and platelet genes, it was anticipated that GATA-1 played a role in megakaryocyte proliferation and development [48]. When a unique portion of the GATA-1 promoter was disrupted, GATA-1 expression was eliminated in megakaryocytes but not in erythrocytes. These non-anemic animals had a platelet count 15 percent of normal and an increased number of small, abnormal megakaryocytes with multilobulated nuclei, scant cytoplasm, few demarcation membranes, no platelet "territories," and few platelet granules [49]. These findings suggest an early defect in cytoplasmic maturation, with a subsequent fall in platelet production [30]. (See "Regulation of erythropoiesis", section on 'GATA1'.)
Numerous genes are downregulated by the loss of GATA-1 in mice, including JAK2 and STAT-1, which mediate thrombopoietin and interferon-gamma (IF-gamma) signaling pathways, respectively. It has been suggested that GATA-1 promotes megakaryopoiesis in part via activation of IF-gamma/STAT1 signaling [50], possibly explaining the mechanism by which various inflammatory disorders can cause elevated platelet counts.
NF-E2 transcription factor — NF-E2 (nuclear factor erythroid derived 2) is a heterodimeric basic leucine zipper transcription factor that is composed of a widely expressed p18 subunit and a p45 subunit present only in erythroid, megakaryocyte, and mast cells. When NF-E2 is disrupted, mice develop a mild anemia and a profound thrombocytopenia associated with a high rate of early hemorrhagic death [51,52]. These mice have adequate numbers of large, abnormal megakaryocytes with hyperlobulated nuclei, rare granules, adequate amounts of demarcation membranes, but no platelet "territories." In addition, they never produce proplatelets [53]. Proplatelets are elongated strands of megakaryocyte cytoplasm that are larger than normal platelets and later fragment into a number of platelets [54,55]. Thus, NF-E2 appears to affect megakaryocyte cytoplasmic differentiation and platelet production at a somewhat later step than GATA-1 [56].
PLATELET FORMATION — Although historically one of the first observations was the apparent shedding of platelets by megakaryocytes, the mechanism by which platelets are produced is becoming clearer. Early electron microscopy images showed the megakaryocyte cytoplasm to be divided by the demarcation membrane system into future platelet "territories" (picture 3) [57]. However, there was little evidence that megakaryocytes simply fractured into separate platelets in the bone marrow. Two different mechanisms have been proposed that have subsequently been merged into one complete model of platelet formation:
●Megakaryocytes use the highly redundant demarcation membrane system to send out pseudopodia into the bone marrow sinusoids [14,15,54]. Platelets and proplatelets then bud off, possibly as a result of localized caspase activation (picture 4) [58]. The bone marrow sinusoids are lined by very thin endothelial cells that are tightly bound to each other and may even overlap [54]. The megakaryocyte pseudopodia pass through, not between, the endothelial cells, which may in turn play some role in regulating the process [54].
This is the mechanism initially suggested and most available data confirm many elements of this model:
•Mice recovering from severe thrombocytopenia have increased numbers of proplatelet processes in the sinusoids [27].
•Mice lacking the transcription factor NF-E2 have severe thrombocytopenia that is probably related to the inability of the megakaryocytes to form proplatelet processes [53,59]. A similar defect occurs in mice with selective loss of the transcription factor GATA-1 [49].
•In vitro and in vivo studies have demonstrated the molecular mechanisms that result in proplatelet formation from megakaryocytes [60]. Cytoskeletal reorganization produces pseudopods by evagination of the demarcation membrane system; platelet granules then track into the elongating pseudopodia, and the proplatelet fragments are released.
Chemoattractants such as stromal-derived factor 1 (SDF1; also called CXCL12) induce metalloproteinase production by megakaryocytes that is necessary for transendothelial migration and platelet production [45,61]. Studies in animals have suggested that, while thrombopoietin (TPO) supports progenitor cell expansion, chemokine-mediated interaction of these progenitor cells with the bone marrow vascular niche allows them to relocate to a microenvironment that is permissive for megakaryocyte maturation and thrombopoiesis [62]. SDF1 may induce megakaryocytes to migrate to the bone marrow sinusoid where proplatelet formation occurs.
●Megakaryocytes or proplatelets are released from the bone marrow and travel to the lung, where they are transformed into platelets [63-65]. This model is a further elaboration of the preceding model in which the locus of the production of single platelets is not in the bone marrow but in the lungs. The log-normal distribution of platelet sizes has long suggested that megakaryocytes or large fragments of megakaryocytes are cleaved in the circulation into platelets, but the extent of pulmonary platelet production could not be directly measured by early methods [64,66]. Evidence for this mechanism includes the demonstration that megakaryocytes can cross the bone marrow endothelial cell barrier and detection of megakaryocytes and megakaryocyte nuclei in the circulation and in the pulmonary vessels [27,63,65]. Although initially thought to be errant megakaryocytes that escaped from the marrow and became trapped in the lungs, this now appears to be a major route of cell trafficking.
In a 2017 report that used Cre-Lox recombination to label megakaryocytes with green fluorescent protein (GFP) and track their movements in living mice, the megakaryocytes were observed to migrate to the lungs, where they extended proplatelet processes and shed platelets [67]. Direct measurements in vivo suggested that approximately half of normal platelet production occurs in the lungs. The lung was also demonstrated to be a hematopoietic organ containing multipotent hematopoietic stem cells. The clinical implications of these findings are only starting to be realized.
Many questions regarding the nuances of the production of platelets from megakaryocytes have not been clearly answered. Incomplete data suggest that platelets are not shed from megakaryocytes with ploidy less than 8N and that larger megakaryocytes make more platelets than smaller ones [68]. In general, there is a relationship between increased ploidy and increased megakaryocyte size but, given the time needed for megakaryocyte cytoplasm to mature, not all small megakaryocytes are of low ploidy.
REGULATION OF PLATELET PRODUCTION — Over the past 50 years, a number of principles of the regulation of platelet production have emerged from clinical studies [69,70].
●The platelet count in any individual remains constant throughout life unless perturbed by physiologic (eg, pregnancy) or pathologic (eg, myelodysplasia) processes [70].
●There is a large variation in the platelet count among normal individuals, which ranges approximately three-fold, from 150,000 to 450,000/microL [71]. This is different from the erythrocyte count, which shows much less variability.
●There is an inverse relation between the normal platelet count and the normal mean platelet volume (MPV) [72], resulting in a roughly constant circulating platelet mass [73]. This inverse relationship extends to other species and is an example of "phylogenic canalization" [74]. As an example, mice have a normal platelet count of 1,200,000/microL and an MPV of 2.1 fL whereas porcupines have a normal platelet count of 30,000/microL and an MPV of 105 fL.
●The body "defends" the total mass of platelets, not the platelet count. Approximately one-third of the total platelet mass is normally sequestered in an exchangeable splenic pool [75]. In animals [76,77] or humans with enlarged spleens [75], the platelet count decreases proportionally to the increase in the size of the spleen but the total body mass of platelets remains normal and unchanged.
●The bone marrow megakaryocytes respond to changes in the demand for platelets by altering their number, size, and ploidy. In animals made thrombocytopenic by the injection of antibody to platelets, bone marrow megakaryocytes increase their number, size (picture 5), and ploidy (figure 3) [8,78,79]; in animals made thrombocytotic by platelet transfusion, the opposite changes occur [8,79].
Thrombopoietin — Thrombopoietin (TPO) is the major hematopoietic growth factor responsible for regulating the circulating platelet mass. TPO also promotes expansion of precursor cells from other hematopoietic cell lineages, and of multipotent hematopoietic stem cells. (See "Biology and physiology of thrombopoietin".)
TPO is produced at a relatively constant rate by the liver and enters the circulation where most is removed by avid TPO (c-mpl) receptors on normal platelets and possibly some by bone marrow megakaryocytes. The residual amount of TPO (50 to 150 pg/mL) provides basal stimulation of megakaryocytes and a basal rate of platelet production. Studies in mice have challenged assumptions about whether hepatic TPO production is constitutive (unless affected by liver disease). The Ashwell-Morell receptor (AMR) on murine hepatocytes can bind platelets that have lost sialic acids on their surface. Once bound by the AMR, a JAK-STAT signaling pathway is activated that increases hepatic TPO mRNA and TPO production in this model system [80]. This finding might explain a component of the thrombocytosis seen in JAK2 mutated myeloproliferative neoplasms. Whether this plays any role in normal human physiology is unclear. Subsequent studies called this analysis into question by showing that it is hepatic Kupffer cells, not hepatocytes, that clear senescent platelets from the circulation [81].
Since it is the total number of circulating platelet c-mpl receptors that determines the clearance of TPO, the normal circulating platelet mass, not the platelet count, remains relatively constant.
●When the platelet production rate decreases, as during thrombocytopenia following chemotherapy, the platelet mass and the amount of c-mpl receptors decreases, clearance of TPO falls, TPO concentrations rise, and megakaryocyte growth is stimulated.
●With experimental transfusion of platelets to levels above normal, the total platelet mass and amount of c-mpl receptors rise, TPO clearance is increased, TPO concentrations fall, and megakaryocyte growth decreases.
When TPO or its receptor have been "knocked-out" by homologous recombination in mice, the megakaryocyte and platelet mass are reduced to approximately 10 percent of normal, but the animals are healthy and do not spontaneously bleed (figure 4). The factors responsible for the residual platelet production in this setting are not well understood. (See "Biology and physiology of thrombopoietin", section on 'Effects on platelet production'.)
Cytokines — Increases in platelet count may be seen during acute infection or inflammatory states; the mechanism may include interleukin 6 (IL-6) and IL-11 signaling. Interleukin 11 stimulates megakaryocyte growth and platelet production independent of TPO signaling.
MEGAKARYOCYTES IN DISEASE — As initially described in part by Wright, characteristic changes in megakaryocytes are associated with several disease processes. Most of these clinical conditions have been evaluated by extensive bone marrow and platelet kinetic studies [6].
Chemotherapy — The thrombocytopenia that follows most chemotherapy is due to a reduced number of megakaryocytes, perhaps due to drug-induced apoptosis of stem cells and/or megakaryocyte progenitors [82]. In this setting, hepatic thrombopoietin (TPO) mRNA is constant (despite a dramatic decrease in desialylated platelets), but TPO levels rise due to reduced platelet clearance. The elevated plasma concentration of TPO increases the average ploidy of the remaining megakaryocytes in an effort to increase platelet production [83]. Platelet kinetic studies have also demonstrated some element of ineffective platelet production (ineffective thrombopoiesis) in this setting.
However, some chemotherapy drugs (eg, bortezomib) have little or no effect on bone marrow stem cells or megakaryocyte progenitor cells but produce thrombocytopenia by inhibiting NF-kB, which is necessary for platelet shedding from megakaryocytes [84].
Normal platelets undergo apoptosis, which regulates their lifespan [85-88]. While etoposide is probably the only available chemotherapy drug to have this effect, several drugs in development (eg, ABT-737) inhibit the anti-apoptotic factors Bcl-2 and Bcl-x(L) and produce rapid and profound thrombocytopenia by causing platelets to undergo apoptosis [87].
Pernicious anemia — In severe pernicious anemia, the low platelet count (figure 5) is associated with a marked increase in the number of megakaryocytes (figure 6) but diminished ploidy, resulting in an expanded megakaryocyte mass (figure 7) but reduced platelet production per megakaryocyte (figure 8). The ineffective platelet production from the megakaryocytes is comparable to the ineffective erythrocyte production also seen in this disorder. (See "Causes and pathophysiology of vitamin B12 and folate deficiencies".)
Congestive splenomegaly — Congestive splenomegaly can be associated with thrombocytopenia by different mechanisms, depending on the underlying cause.
●In patients with congestive splenomegaly and normal hepatic function (eg, post-infectious splenomegaly), the thrombocytopenia is primarily due to redistribution of the normal circulating mass of platelets to the spleen, without major alterations in megakaryocyte production or platelet survival (figure 5) [75,89]. (See "Diagnostic approach to the adult with unexplained thrombocytopenia", section on 'Pathophysiology'.)
●In patients with splenomegaly due to liver disease, fewer platelets are produced because hepatic production of thrombopoietin is decreased (figure 6 and figure 7) [90]; platelet redistribution to the spleen may also occur. (See "Biology and physiology of thrombopoietin".)
Immune thrombocytopenia (ITP) — In animal models and in humans, chronic immune thrombocytopenia (ITP) is characterized by an increase in the number, size and ploidy of bone marrow megakaryocytes (figure 6).
Early platelet kinetic studies suggested that these morphologic findings were associated with a six- to eightfold increase in the rate of platelet production [6,91,92] and a shortened platelet survival time (figure 5 and figure 8). However, subsequent platelet kinetic studies have failed to demonstrate a significant increase in platelet production in ITP and suggested that the morphologic findings might be related to ineffective platelet production [93-95]. Electron microscopic examination of bone marrow from ITP patients shows that many megakaryocytes are undergoing apoptosis, as illustrated in the picture (picture 6) [96].
A new understanding of ITP has emerged. ITP is a disorder of increased platelet destruction and inappropriately low platelet production [97,98]. ITP is characterized by antibody-mediated platelet destruction (primarily in the liver and spleen) and a compensatory increase in megakaryocyte number, size and ploidy. However, the increased platelet mass does not produce a significant increase in platelet production because anti-platelet antibody [94,96] and cytotoxic T cells [99] bind to the megakaryocytes and produce apoptosis, thereby preventing platelet shedding. (See "Immune thrombocytopenia (ITP) in adults: Clinical manifestations and diagnosis", section on 'Pathogenesis'.)
Reactive thrombocytosis — Reactive thrombocytosis occurs in association with iron deficiency, malignancy, and inflammatory states (table 1). It is associated with an increased number of megakaryocytes but with average ploidy less than normal (figure 6), an elevated megakaryocyte mass, and an enhanced rate of platelet production (figure 7 and figure 8).
The platelet count rises in about one-third patients with severe iron deficiency [100]. Thrombopoietin levels are not increased, and there have been no iron-responsive regulatory elements identified in the thrombopoietin-dependent signaling pathways. Although it has not yet been proven in humans, studies in mice that are iron deficient due to knockout of Tmprss6-/- gene suggest a mechanism for this increase in platelet production by which the megakaryocytic-erythroid progenitor (MEP) cells were more likely to be committed to megakaryocytic differentiation due to a switch in gene expression and reduced ERK signalling [101]. It was suggested that this switch of the MEP away from erythroid differentiation was a defense against making more red blood cells in an iron-deficient environment.
In inflammation and malignancy, the increase in platelet count is probably secondary to the expansion of megakaryocyte number due to inflammatory cytokines such as interleukin-6 (IL-6) [102-104]. Ploidy is reduced because of a lower plasma concentration of thrombopoietin resulting from increased clearance by the expanded platelet mass. The best studied example of this effect is in patients with ovarian cancer, who often have marked thrombocytosis associated with increased levels of IL-6 and TPO. Studies have shown that the ovarian tumors secrete IL-6, which in turn increases hepatic production of TPO [104].
A comprehensive approach to the evaluation of thrombocytosis is presented separately. (See "Approach to the patient with thrombocytosis".)
Essential thrombocythemia — In essential thrombocythemia (ET) and the related myeloproliferative disorders such as chronic myeloid leukemia and polycythemia vera (PV), there is a clonal proliferation of megakaryocytes that are of high ploidy and actively produce platelets [105]. Whether the normal regulatory mechanism via thrombopoietin and its receptor is functioning is unclear, although the finding of a specific mutation in JAK2 in about 50 percent of patients with ET and 95 percent of patients with PV is consistent with altered GATA-1-JAK/STAT signaling. Approximately 5 to 10 percent of patients with ET have mutations in the thrombopoietin receptor (MPL), and 67 to 82 percent of the remainder have multiple mutations in calreticulin (CALR) [106,107]. (See 'GATA-1 transcription factor' above and "Overview of the myeloproliferative neoplasms", section on 'Mutations in PV, ET, and PMF'.)
Platelet kinetic data suggest the proliferation is autonomous of thrombopoietin since at increased platelet mass (figure 5) there is a greatly increased megakaryocyte ploidy (figure 6) despite a normal or slightly elevated plasma thrombopoietin concentration [108,109]. The latter finding is explained by the demonstration that platelet, and possibly megakaryocyte, thrombopoietin receptors are markedly reduced via an uncertain mechanism [109,110], resulting in a net overall normal clearance of thrombopoietin at these elevated platelet counts.
Polycythemia vera — In addition to an elevated red blood cell mass, many patients with polycythemia vera also have thrombocytosis. The thrombocytosis may be present for years before the red blood cell mass becomes elevated. Proposed mechanisms of thrombocytosis are the same as for ET. (See "Clinical manifestations and diagnosis of polycythemia vera" and "Approach to the patient with thrombocytosis", section on 'Hematologic malignancies'.)
Primary myelofibrosis — The typical finding in primary myelofibrosis (chronic idiopathic myelofibrosis) is an increase in bone marrow megakaryocytes with a large amount of fibrosis and occasional megakaryocyte dysplasia. The fibrotic response is due to a polyclonal proliferation of fibroblasts which has been attributed to the release of mesenchymal growth factors such as transforming growth factor-beta from the abnormal megakaryocytes [111]. Mutations in JAK2, MPL, and CALR are associated with primary myelofibrosis in the same percentages as in patients with ET. (See "Pathogenetic mechanisms in primary myelofibrosis".)
However, this may not be the entire mechanism. Overexpression of the thrombopoietin gene using adenoviral vectors in SCID mice (severe combined immune deficient) results in thrombocytosis, increased marrow megakaryocytes, fibrosis, and extramedullary hematopoiesis that mimics primary myelofibrosis [112]. However, similar overexpression of thrombopoietin in NOD-SCID mice (which have reduced monocyte and macrophage function in addition to the lymphocyte deficiency in SCID mice) produced similar thrombocytosis and megakaryocytosis but no fibrosis. These results imply that other monocyte/macrophage mediators may be involved in causing the fibrosis which does not result solely from thrombocytosis.
Myelodysplastic syndromes — Both thrombocytopenia and thrombocytosis may be seen in the myelodysplastic syndromes, findings that have been attributed to abnormal megakaryocytes. The morphologic picture consists of an increased number of small megakaryocytes of low ploidy, occasionally displaying a characteristic "pawn ball" nucleus with three lobes (picture 7) [113-115].
Platelet kinetic studies have demonstrated a greatly expanded megakaryocyte mass (increased number of megakaryocytes of low ploidy) and ineffective platelet production from the megakaryocytes (figure 6 and figure 8) [6]. However, in the 5q- syndrome, platelet production may be increased. (See "Clinical manifestations and diagnosis of myelodysplastic syndromes (MDS)", section on 'MDS with isolated del(5q)'.)
Pulmonary fibrosis and other lung syndromes — The identification of the lung as a major site of platelet production (see 'Platelet formation' above) suggests a potential role of platelet production and platelet destruction in conditions such as pulmonary fibrosis [67]. Platelets contain many cytokines with inflammatory and fibrotic characteristics (eg, transforming growth factor beta [TGF-b], platelet-derived growth factor [PDGF]).
SUMMARY — Megakaryocytes normally account for 0.05 to 0.1 percent of all nucleated bone marrow cells. They have an average diameter of 20 to 25 microns and a volume of 4700 ± 100 fL (picture 2). Each megakaryocyte is capable of producing 1000 to 3000 platelets. Mature megakaryocytes are invariably polyploid and contain from two (4N) to 32 (64N) times the normal diploid amount of DNA. (See 'Characteristics' above.)
●Origin and differentiation – Megakaryocytes are derived from the multipotent hematopoietic stem cell (figure 1), which gives rise to early progenitors able to undergo erythrocyte or megakaryocyte differentiation. A micro RNA miR-150 drives differentiation towards megakaryocytes. A number of factors affect this differentiation process, including thrombopoietin, GATA-1, and NF-E2. (See 'Origin' above and 'Differentiation' above.)
●Platelet formation and regulation – Cytoskeletal reorganization within the megakaryocyte produces pseudopodia by evagination of the demarcation membrane system. Platelet granules then track into the elongating pseudopodia, and proplatelet fragments are extruded into the circulation where they are transformed into mature platelets in both the bone marrow and the lungs. Thrombopoietin (TPO) produced by the liver enters the circulation where most is removed by TPO (c-mpl) receptors on normal platelets. Residual TPO provides basal stimulation of megakaryocytes and a basal rate of platelet production. (See 'Platelet formation' above and 'Regulation of platelet production' above.)
●Altered megakaryocyte function – A number of disorders and clinical settings affect megakaryocyte and platelet production. These are discussed in the text as well as in other sections of UpToDate dealing with the specific disorders mentioned. (See 'Megakaryocytes in disease' above.)
