Megakaryocyte biology and platelet production

INTRODUCTION — This topic reviews the biology of megakaryocytes and the mechanisms of platelet production in the bone marrow.

Separate topics discuss the biology of hematopoietic stem cells (HSCs), platelets, thrombopoietin (TPO), and clinical implications:

Basic science

●HSCs – (See "Overview of hematopoietic stem cells".)

●Platelet biology – (See "Platelet biology and mechanism of anti-platelet drugs".)

●TPO biology – (See "Biology and physiology of thrombopoietin".)

Clinical

●Inherited platelet function disorders – (See "Inherited platelet function disorders (IPFDs)".)

●Platelet function tests – (See "Platelet function testing".)

●Transfusion – (See "Platelet transfusion: Indications, ordering, and associated risks".)

●TPO receptor agonists (TPO-RAs) – (See "Clinical applications of thrombopoietic growth factors".)

HISTORICAL BACKGROUND — The megakaryocyte was discovered to be the hematopoietic cell that produces platelets over 100 years ago.

●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 to this day.

●Wright also demonstrated that changes in platelet number were associated with changes only in megakaryocytes [2].

Since these seminal observations, much has become known about megakaryocytes and how they produce platelets [3,4].

CHARACTERISTICS

Size and ploidy

●Size – Megakaryocytes are the largest cell in the bone marrow (picture 2). They have an average diameter of 20 to 25 microns and a volume of 4700 ± 100 fL [5]. By comparison, red blood cells (RBCs) have a diameter of 7 to 8 microns and a volume of 85 to 100 fL [5]. Large megakaryocytes have diameters of 50 to 60 microns and volumes of 65,000 to 100,000 fL.

●Ploidy – Mature megakaryocytes are polyploid, contain two (4N) to 32 (64N) times the normal diploid DNA content; the mean value is 16N in humans [6,7]. The DNA is contained within one highly lobulated nuclear envelope, in which each lobule represents one diploid amount of DNA (2N). This is the result of a process referred to as endomitosis [8]. (See "Basic genetics concepts: Chromosomes and cell division", section on 'Mitosis'.)

Few other cells are normally polyploid. Few other cells are normally polyploid. Placental trophoblasts invading the endometrium and myometrium may undergo polyploidization to create cells of 4 to 64N; this is felt to prevent their replication [9,10]. Occasional hepatocytes and macrophages have 4N or 8N DNA content contained in multiple separate nuclei.

●Relationship between size and ploidy – Megakaryocyte polyploidy appears to result in functional gene amplification, perhaps to increase protein synthesis in parallel with megakaryocyte enlargement [11,12].

In general, increased ploidy is associated with increased megakaryocyte size. However, given the time needed for megakaryocyte cytoplasm to mature, not all small megakaryocytes are of low ploidy.

Membrane — Megakaryocyte plasma membrane is similar to that of the platelet.

One of the first signs of differentiation along the megakaryocyte lineage is the appearance of membrane glycoprotein (GP) receptors including integrin alpha_IIbbeta₃ (previously called GPIIbIIIa), which becomes active on activated platelets and binds to fibrinogen, and later by GPIb and other collagen receptors [13,14]. (See "Platelet biology and mechanism of anti-platelet drugs", section on 'Collagen receptors (GPIa/IIa and GPVI)' and "Platelet biology and mechanism of anti-platelet drugs", section on 'Integrin alphaIIbbeta3 (GPIIb/IIIa) binding to fibrinogen, activation, and platelet aggregation'.)

The surface membrane of the mature megakaryocyte is deeply invaginated and highly redundant, creating a demarcation membrane system that divides the cytoplasm into platelet-sized subunits (picture 3). This enormous amplification of the surface membrane is thought to provide sufficient membrane to form platelets [15-18].

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 taken up from the plasma by both megakaryocytes and platelets.

●Alpha granules – The alpha granule body is made early in megakaryocyte development before the demarcation membrane system. Alpha granules contain numerous platelet proteins and growth factors.

•Megakaryocytes synthesize these proteins and transport them to the alpha granules [19]:

-Platelet-derived growth factor (PDGF)

-Transforming growth factor-beta (TGF-beta)

-Platelet factor-4 (PF4)

-von Willebrand factor

•Megakaryocytes and platelets take up these proteins from the circulation:

-Fibrinogen undergoes receptor-mediated endocytosis by the integrin alpha_IIbbeta₃ receptor [20-22].

-Albumin and immunoglobulin G (IgG) undergo pinocytosis [23].

●Dense granules – Dense granules are physically dense and, because they contain serotonin and calcium, they are also electron dense in transmission electron microscopy.

The dense granule membrane bodies are made in megakaryocytes, but do not take up serotonin and calcium until platelets are released into the circulation [24,25]. Most of the body's circulating serotonin is taken up by platelets.

A possible connection between selective serotonin reuptake inhibitors and bleeding is discussed separately. (See "Selective serotonin reuptake inhibitors: Pharmacology, administration, and side effects", section on 'Bleeding'.)

Platelet granule disorders are also discussed separately. (See "Inherited platelet function disorders (IPFDs)", section on 'Mechanisms and gene variants'.)

Microscopic appearance — Megakaryocytes normally account for approximately 0.05 to 0.1 percent of nucleated bone marrow cells. Their number increases as the demand for platelets rises.

Three stages of megakaryocytes may be identified [26]:

●Mature megakaryocytes are large and polyploid. These cells are readily identified by light microscopy (picture 1 and picture 2). Platelet shedding can occasionally be seen.

●Intermediate megakaryocytes have large polyploid nuclei with 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 GP IIb/IIIa, are small, diploid, lymphocyte-like megakaryocyte progenitor cells that have not yet undergone endomitosis [27].

ORIGIN AND DIFFERENTIATION

Stem and progenitor cells

●HSCs – Megakaryocytes are derived from multipotent hematopoietic stem cells (HSCs) (figure 1). (See "Overview of hematopoietic stem cells", section on 'Hematopoietic stem cells'.)

●MEPs – HSCs give rise to early bilineage progenitor cells, referred to as megakaryocyte-erythroid progenitors (MEPs), that can subsequently differentiate along the erythroid or megakaryocyte lineage.

A specific assay for MEPs is lacking, but their existence is supported by several findings:

•Shared genetic regulation – Expression of thymidine kinase in specific cell types allows selective killing upon administration of ganciclovir, which is converted to a toxic metabolite. In transgenic mice expressing the gene for thymidine kinase under the control of a platelet-specific promoter (ITGA2B, which encodes platelet glycoprotein [GP] IIb), administration of ganciclovir eradicated production of both platelets and erythrocytes [28]. This observation suggests that the GPIIb promoter was transcriptionally active in the MEP and possibly in a more potent progenitor cell [29,30].

•Shared maturation – Early erythroid and megakaryocyte differentiation share molecular, cellular, and pathological features.

-The figure illustrates shared transcription factors (figure 2) [31].

-Regulatory hematopoietic cytokines for megakaryocytes (thrombopoietin [TPO]) and erythrocytes (erythropoietin) share 50 percent sequence similarity [32]. (See 'TPO role in maintaining platelet count' below and "Regulation of erythropoiesis", section on 'Erythropoietin'.)

•Lineage specification – The microRNA miR-150 determines lineage specification for this common MEP, driving differentiation towards megakaryocytes and away from erythroid cells [33]. The transcription factor Myb is a critical target of miR-150 [34].

TPO increases miR-150, which in turn binds to the four miR-150 binding sites in the MYB 3' untranslated region and decreases Myb mRNA and protein [35]. Reduction in Myb expression subsequently leads to increased megakaryocyte differentiation and reduced erythroid differentiation.

Absence of miR-150 promotes erythroid differentiation.

●Meg-CFC – Differentiation ultimately produces a committed precursor cell only able to produce megakaryocytes, called the megakaryocyte colony-forming cell (Meg-CFC).

Meg-CFCs have the following properties:

•Can be assayed by cell culture methods

•Express integrin alpha_IIbbeta₃ (previously called GPIIbIIIa), the receptor for fibrinogen [36,37]

•Can undergo mitosis [38]

•Are stimulated by interleukin (IL)-3 and TPO [39,40].

●Immature megakaryocytes

•Endomitotic growth – Eventually Meg-CFCs stop mitosis and enter endomitosis, in which DNA replication and chromosome segregation occur without nuclear division or cytokinesis, producing large polyploid cells (figure 1). (See 'Size and ploidy' above.)

It was initially assumed that endomitosis was simply the absence of mitosis after each round of DNA replication. However, studies in mice showed that the cells enter mitosis and progress through normal prophase, prometaphase, metaphase, and up to anaphase A, but not to anaphase B, telophase, or cytokinesis [41]. After anaphase, the nuclear membrane is reassembled around the sister chromatids as a single nucleus; the cells then enter the next round of DNA replication. (See "Basic genetics concepts: Chromosomes and cell division", section on 'Somatic cell division'.)

Endomitosis is associated with increased cyclin D3 and reduced cyclin B1 and cyclin B1-dependent Cdc2 kinase [42,43]. These changes may facilitate aborted mitosis and reentry into S phase without cytokinesis [44].

•Cytoplasmic maturation – On completion of several rounds of endomitosis, immature megakaryocytes develop a mature cytoplasm, become morphologically identifiable megakaryocytes, and eventually release platelets.

It takes approximately five to seven days to produce platelets from Meg-CFCs.

Factors promoting differentiation — The mechanisms by which MEPs become committed to the megakaryocyte lineage and Meg-CFCs mature to become megakaryocytes are beginning to be unraveled.

GATA1 and NF-E2 transcription factors — GATA1 and NF-E2 (nuclear factor erythroid derived 2) are intrinsic lineage-specific transcription factors that help establish cell-specific phenotypes. They interact with a large number of other factors including JAK and STAT.

●GATA1 – GATA1 is zinc-finger transcription factor expressed in many hematopoietic cell types, including progenitor cells and precursor cells of erythroid, megakaryocyte, eosinophil, and mast cell lineages [45]. GATA transcription factors bind to the DNA sequence G-A-T-A. (See "Regulation of erythropoiesis", section on 'GATA1'.)

GATA1 gene knockout in mice is embryonically lethal due to severe anemia [45]. In a mouse model in which a portion of the GATA1 promoter was disrupted to selectively eliminate expression in megakaryocytes but not erythroid progenitor cells, the platelet count was reduced to 15 percent of normal and small, abnormal megakaryocytes were seen, with multilobulated nuclei, scant cytoplasm, few demarcation membranes, no platelet "territories," and few platelet granules [31,46,47].

Numerous genes are downregulated by the loss of GATA1 in mice, including JAK2, which mediates TPO signaling, and STAT1, which mediates interferon-gamma signaling. GATA-1 may promote megakaryopoiesis in part via activation of interferon-gamma/STAT1 signaling [48]. This may explain the mechanism by which inflammatory disorders can increase platelet counts. (See 'Cancer and inflammation' below.)

●NF-E2 – NF-E2 is a heterodimeric basic leucine zipper transcription factor composed of a p18 subunit that is widely expressed and a p45 subunit that is only expressed in erythroid, megakaryocyte, and mast cells.

NEF2 gene knockout in mice is embryonically lethal due to profound thrombocytopenia associated with a high rate of early hemorrhagic death [49,50]. The NFE2 gene encodes the p45 subunit.

NEF2 null mice have adequate numbers of large, abnormal megakaryocytes with hyperlobulated nuclei, rare granules, adequate amounts of demarcation membranes, but no platelet "territories," and they never produce proplatelets [51]. Proplatelets are elongated strands of megakaryocyte cytoplasm that are larger than normal platelets and later fragment into a number of platelets [52,53]. Thus, NF-E2 appears to affect megakaryocyte cytoplasmic differentiation and platelet production at a somewhat later step than GATA-1 [54].

Thrombopoietin (role in megakaryocyte maturation) — Thrombopoietin (TPO) has a major effect on almost all steps of megakaryocyte differentiation and maturation.

It has the following effects on Meg-CFCs [55,56]:

●Promotes growth

●Dramatically increases the rate of endomitosis

●Inhibits apoptosis

●Stimulates maturation

An increase in megakaryocyte ploidy is seen at even the lowest amounts of TPO and is one of the most prominent TPO effects. (See 'Size and ploidy' above.)

However, the final stage of platelet release occurs independently of TPO and in one model system was actually inhibited by large amounts of TPO [57]. (See 'Steps in platelet formation' below.)

TPO also promotes expansion of multipotent hematopoietic stem cells and precursor cells from other hematopoietic cell lineages. (See "Biology and physiology of thrombopoietin".)

Other cytokines — 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. IL-6 also stimulates megakaryocyte growth, but its major in vivo effect is to increase hepatocyte production of TPO and thereby elevate the platelet count [58].

PLATELET PRODUCTION

Steps in platelet formation — Megakaryocytes normally account for 0.05 to 0.1 percent of all nucleated bone marrow cells. Each megakaryocyte produces a total of 1000 to 3000 platelets.

Historical observations suggested that platelets are shed by megakaryocytes, but electron microscopy images show the megakaryocyte cytoplasm to be divided by the demarcation membrane system into future platelet "territories" that facilitate the budding off of platelets in a highly organized process (picture 3) [59].

A modern model of platelet formation outlines the following steps, which are illustrated in the figure (figure 3):

●Extension of pseudopodia – 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 [56,60]. Studies in animals have suggested that chemokine-mediated interaction of megakaryocyte progenitor cells with the bone marrow vascular niche allows them to relocate to a microenvironment that is permissive for maturation and thrombopoiesis [61]. SDF1 may induce megakaryocytes to migrate to the bone marrow sinusoid where proplatelet formation occurs. Platelet granules track into the elongating pseudopodia.

Megakaryocytes use the highly redundant demarcation membrane system to send out pseudopodia into the bone marrow sinusoids [15,16,52]. The bone marrow sinusoids are lined by very thin endothelial cells that are tightly bound to each other and may even overlap [52]. The megakaryocyte pseudopodia pass through, not between, the endothelial cells, which may in turn play some role in regulating the process [52].

●Budding of platelets and proplatelets – Platelets and proplatelets then bud off, possibly as a result of localized caspase activation, as illustrated in the electron micrograph (picture 4) [62]. The numbers of proplatelet processes in the sinusoids can be increased by thrombopoietic stimuli [28]. Images from a time lapse video are presented in the figure (picture 5).

Formation of these processes depends on cytoskeletal reorganization that produces pseudopods by evagination of the demarcation membrane system [63]. Platelet granules then track into the elongating pseudopodia, and the proplatelet fragments are released.

Transcription factors such as NF-E2 and GATA1 appear to play an essential role in proplatelet process formation [47,51,64]. The transcription factor nuclear factor (NF)-kB is necessary for platelet shedding from megakaryocytes [65].

●Migration to the lungs – Proplatelets (or megakaryocytes) are released from the bone marrow and may travel to the lungs, where they are transformed into platelets [66-68].

Megakaryocytes are commonly found in the lungs. Early studies based on the distribution of platelet sizes suggested that megakaryocytes or large fragments of megakaryocytes passing through the lungs might be cleaved into platelets, but the extent of pulmonary platelet production could not be measured directly [67,69]. A 2017 study in living mice suggested that approximately one-half of normal platelet production occurs in the lungs [70]. This was subsequently brought into question by a 2024 study that showed that lung megakaryocytes accounted for minimal platelet production [71].

Additional findings include [28,66,68]:

•Demonstration that megakaryocytes can cross the bone marrow endothelial cell barrier.

•Detection of megakaryocytes and megakaryocyte nuclei in the circulation and in the pulmonary vessels.

•Observations in mice of green fluorescent protein (GFP)-labeled megakaryocytes migrating to the lungs, extending proplatelet processes, and shedding platelets [70].

Although initially thought to be errant megakaryocytes that escaped from the marrow and became trapped in the lungs, this appears to be a major route of cell trafficking.

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.

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 6), and ploidy (figure 4) [7,72,73]. In animals made thrombocytotic by platelet transfusion, the opposite changes occur [7,73].

Many questions regarding the nuances of platelet production remain. 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 [74]. (See 'Size and ploidy' above.)

Megakaryocytes can migrate from the lungs back to the bone marrow, but the role of lung diseases in affecting platelet production or the role of platelet cytokines in contributing to lung disease are not well understood [70]. Platelets contain many cytokines with inflammatory and fibrotic characteristics, such as transforming growth factor beta (TGF-beta) and platelet-derived growth factor (PDGF).

The benefits of having platelets that are anucleate are also unexplained [75]. Platelets in lower vertebrate species such as fish and birds all contain a cell nucleus.

Control of platelet mass

●Stability of platelet mass – The platelet count is remarkably constant throughout the life of an individual but remarkably variable between individuals [76,77]. It is actually the total platelet mass, rather than the platelet count, that is being controlled, and platelet mass is very similar across individuals.

Approximately one-third of the total platelet mass is normally sequestered in an exchangeable splenic pool [78]. The platelet count represents the concentration of the remaining platelets in the circulation.

•Thrombocytopenia with normal platelet mass – Conditions that reduce the platelet count but not the total platelet mass include [77]:

-Pregnancy, in which the platelet count decreases as the plasma volume increases. (See "Thrombocytopenia in pregnancy", section on 'Gestational thrombocytopenia (GT)'.)

-Splenomegaly, in which the platelet count decreases proportionally to the increase in spleen size [78].

•Altered platelet mass – Conditions that alter the total platelet mass include:

-Bone marrow disorders in which platelet production is impaired, such as myelodysplastic syndromes (MDS), or increased, such as myeloproliferative neoplasms. (See 'Myelodysplastic syndromes' below and 'Myeloproliferative neoplasms' below.)

-Severe liver disease, in which thrombopoietin (TPO) production is reduced. (See "Hemostatic abnormalities in patients with liver disease", section on 'Thrombocytopenia and platelet dysfunction'.)

●Between-individual variation – The reference range for platelet count is large (from 150,000 to 450,000/microL), an approximately three-fold range [79]. This contrasts with the white blood cell (WBC) and red blood cell (RBC) counts, which show much less variability. However, the platelet volume (or platelet mass) across individuals has a much narrower range.

●Platelet count versus platelet volume – Despite the large difference in normal platelet count between individuals, there is an inverse relation between the platelet count and the mean platelet volume (MPV) [80]; this results in a roughly constant circulating platelet mass for all individuals [80-82].

This inverse relationship extends to other species and is an example of "phylogenic canalization" [83]. 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.

TPO role in maintaining platelet count — TPO is the major hematopoietic growth factor responsible for regulating the circulating platelet mass. It is produced at a relatively constant rate by the liver and enters the circulation where most is cleared by avid TPO receptors (c-mpl) 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.

●When platelet production decreases, as during thrombocytopenia following chemotherapy, the platelet mass and the amount of c-mpl receptors decrease; clearance of TPO falls; TPO concentrations rise; and megakaryocyte growth is stimulated [84]. (See 'Chemotherapy' below.)

●With experimental transfusion of platelets to supranormal levels, the total platelet mass and amount of c-mpl receptors rise; TPO clearance is increased; TPO concentrations fall, and megakaryocyte growth decreases (figure 4) [72].

Since the total number of circulating platelet c-mpl receptors determines the clearance rate of TPO, the normal circulating platelet mass, not the platelet count, remains relatively constant. (See 'Control of platelet mass' above.)

Platelet lifespan — Normal platelets undergo apoptosis, limiting their lifespan to approximately seven days [85-88]. (See "Platelet biology and mechanism of anti-platelet drugs".)

MEGAKARYOCYTES IN DISEASE — Several disorders produce characteristic changes in megakaryocytes [5].

The figures illustrate the effects of these disorders on megakaryocyte number and size (figure 5) and platelet count and platelet survival (figure 6).

Conditions with thrombocytopenia

Chemotherapy — Chemotherapy can alter megakaryocyte function and platelet production in several ways [89]:

●Megakaryocyte apoptosis – Thrombocytopenia following most cytotoxic chemotherapy is due to a reduced number of megakaryocytes, perhaps due to drug-induced apoptosis of stem cells and/or megakaryocyte progenitor cells [90]. Platelet kinetic studies have also demonstrated ineffective thrombopoiesis with cytotoxic chemotherapy.

Hepatic thrombopoietin (TPO) production is constant, and when the platelet count decreases, clearance of TPO by platelets is reduced, resulting in a net increase in TPO levels. This increases the average ploidy of the remaining megakaryocytes, increasing platelet production [91]. (See 'Platelet production' above.)

●Inhibited platelet production – The proteasome inhibitor bortezomib causes thrombocytopenia by inhibiting nuclear factor (NF)-kB, which is necessary for platelet shedding from megakaryocytes [65].

●Platelet apoptosis – Etoposide causes platelets to undergo apoptosis. Several drugs in development (eg, ABT-737) inhibit the anti-apoptotic factors Bcl-2 and Bcl-xL and produce rapid and profound thrombocytopenia by causing platelets apoptosis [87,89].

Vitamin B12 deficiency — Vitamin B12 is a cofactor in DNA synthesis pathways. (See "Causes and pathophysiology of vitamin B12 and folate deficiencies", section on 'DNA synthesis, RNA synthesis, DNA methylation'.)

In vitamin B12 deficiency, there is a marked increase in megakaryocyte number but decreased ploidy, likely due to decreased DNA replication in megakaryocyte progenitor cells. (See 'Stem and progenitor cells' above.)

This leads to an expanded megakaryocyte mass (figure 7) but reduced platelet production per megakaryocyte (figure 8).

Similar effects occur in the white blood cell (WBC) and red blood cell (RBC) lineages. (See "Causes and pathophysiology of vitamin B12 and folate deficiencies".)

Congestive splenomegaly — Congestive splenomegaly can be associated with thrombocytopenia by different mechanisms, depending on whether liver function is also abnormal.

●Normal liver function – With congestive splenomegaly and normal liver function, such as in mononucleosis or other postinfectious splenomegaly, 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 6) [78,92]. (See 'Control of platelet mass' above and "Splenomegaly and other splenic disorders in adults", section on 'Hypersplenism'.)

●Concomitant liver disease – In patients with splenomegaly due to liver disease, fewer platelets are produced because hepatic production of TPO is decreased (figure 5 and figure 7) [93]. Platelet redistribution to the spleen may also occur. (See "Biology and physiology of thrombopoietin".)

Immune thrombocytopenia (ITP) — ITP is caused by autoantibody-mediated platelet destruction, primarily by macrophages in the liver and spleen.

Reduced platelet numbers leads to an increase in TPO, which in turn increases the number, size, and ploidy of bone marrow megakaryocytes (figure 5).

However, antiplatelet antibodies and cytotoxic T cells target megakaryocytes and promote megakaryocyte apoptosis, thereby preventing platelet shedding [94-100]. (See 'Steps in platelet formation' above.)

The electron micrographs show megakaryocytes undergoing apoptosis in the bone marrows of patients with ITP (picture 7) [97]. (See "Immune thrombocytopenia (ITP) in adults: Clinical manifestations and diagnosis", section on 'Pathogenesis'.)

Myelodysplastic syndromes — Both thrombocytopenia and thrombocytosis may be seen in myelodysplastic syndromes (MDS), 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 8) [101-103]. (See "Clinical manifestations, diagnosis, and classification of myelodysplastic syndromes (MDS)", section on 'Cytopenias'.)

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 5 and figure 8) [5].

However, platelet production may be increased in the 5q- syndrome. (See "Cytogenetics, molecular genetics, and pathophysiology of myelodysplastic syndromes/neoplasms (MDS)", section on 'del(5q)'.)

Conditions with thrombocytosis — Evaluation of thrombocytosis is presented separately. (See "Approach to the patient with thrombocytosis".)

Reactive thrombocytosis — Reactive thrombocytosis occurs in association with iron deficiency, malignancy, and inflammatory states; the table summarizes these and other causes (table 1). It is associated with an increased number of megakaryocytes and megakaryocyte mass but reduced ploidy (figure 5).

Iron deficiency — The platelet count rises in approximately one-third of patients with severe iron deficiency [104]; this increase may be associated with an increased risk of thrombosis [104]. TPO levels are not increased, and no iron-responsive regulatory elements have been identified in TPO-dependent signaling pathways.

Studies in mice rendered iron deficient due to knockout of the TMPRSS6 gene suggest that megakaryocytic-erythroid progenitor (MEP) cells were more likely to be committed to megakaryocytic differentiation due to a switch in gene expression and reduced signalling via ERK (extracellular signal-related kinase) pathways [105]. It was suggested that this switch of the MEP away from erythroid differentiation was a defense against making more RBCs in an iron-deficient environment.

Cancer and inflammation — Increases in platelet count may be seen with cancer and during acute infection or inflammatory states. TPO may be increased or decreased depending on the setting.

●Increases in platelet count that can occur in cancer and inflammatory states are likely secondary to the expansion of megakaryocyte number due to inflammatory cytokines such as interleukin-6 (IL-6) and IL-11 [106-108]. IL-11 stimulates megakaryocyte growth and platelet production independent of TPO signaling. Ploidy is reduced due to reduced TPO, which occurs because the expanded platelet mass increases TPO clearance. (See 'TPO role in maintaining platelet count' above.)

●In ovarian cancer, there is often 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 [108].

Rebound thrombocytosis — The platelet count often increases above normal during recovery from an acute thrombocytopenic event caused by chemotherapy, infection, or alcohol-induced bone marrow suppression. The peak platelet count occurs approximately 14 days after the platelet nadir and is presumed to reflect the effect of increased circulating TPO during the thrombocytopenic period [109-111].

Redistributive thrombocytosis — The spleen sequesters an exchangeable pool of about one-third of the platelet mass [82]. After splenectomy there is a period of redistributive thrombocytosis with the platelet count ultimately returning to values above baseline [112].

Myeloproliferative neoplasms — In essential thrombocythemia (ET) and the related myeloproliferative neoplasms (MPNs), chronic myeloid leukemia (CML), polycythemia vera (PV), and primary myelofibrosis (PMF), clonal proliferation of megakaryocytes of high ploidy and active platelet production can lead to very high platelet counts [113]. (See "Overview of the myeloproliferative neoplasms".)

Acquired mutations in certain genes are responsible for increased megakaryopoiesis and platelet production:

●JAK2 – Mutations in JAK2, which encodes a Janus kinase, occur in approximately 60 to 65 percent of patients with ET and virtually 100 percent of patients with PV. These mutations make the early hematopoietic cells especially sensitive to growth factors and cytokines that act via altered GATA1-JAK/STAT signaling. (See "Overview of the myeloproliferative neoplasms", section on 'JAK2 mutations'.)

MPL – Mutations in MPL, which encodes the TPO receptor, are seen in approximately 5 to 10 percent of patients with ET [114,115]. (See "Overview of the myeloproliferative neoplasms", section on 'MPL mutations'.)

●CALR – Mutations in CALR, which encodes calreticulin, occur in approximately 70 percent of patients with ET or PMF who do not carry a mutation in either JAK2 or MPL [114,115]. (See "Overview of the myeloproliferative neoplasms", section on 'Calreticulin (CALR) mutations'.)

TPO levels are not appropriately downregulated by the high platelet mass in MPNs; in most cases TPO levels are normal or slightly elevated [116,117]. TPO receptors on platelets and possibly megakaryocyte are markedly reduced via an uncertain mechanism [117,118]. This results in a net overall normal clearance of TPO despite elevated platelet counts. Platelet kinetic data suggest the clonal proliferation is autonomous of TPO, since platelet mass is increased (figure 6) and megakaryocyte ploidy is high (figure 5).

Thrombocytosis in PV may be present for years before the RBC mass becomes elevated.

In PMF, bone marrow megakaryocytes are increased, 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 (TGF)-beta from the abnormal megakaryocytes [119]. However, studies using mouse models are able to uncouple increases in bone marrow megakaryocytes from fibrosis, suggesting that other mediators may be responsible for the bone marrow fibrosis [120].

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Immune thrombocytopenia (ITP) and other platelet disorders" and "Society guideline links: Myeloproliferative neoplasms".)

SUMMARY

●Megakaryocytes – Megakaryocytes are the largest cell in the bone marrow (picture 2), with average diameter 20 to 25 (up to 50 to 60) microns. They are polyploid, containing 2 to 32 times the normal diploid DNA content within a lobulated nucleus, generated by endomitosis. They produce alpha granules and dense granules; megakaryocytes and platelets, also endocytose granule constituents. (See 'Characteristics' above.)

●Megakaryocyte origins – Multipotent hematopoietic stem cells (HSCs) (figure 1) give rise to megakaryocyte-erythroid progenitor cells (MEPs) that can differentiate into megakaryocytes. Thrombopoietin (TPO) stimulates HSCs and acts on megakaryocyte colony-forming cells to promote growth, increase endomitosis, inhibit apoptosis, and stimulate maturation. (See 'Origin and differentiation' above.)

●Platelet production – Each megakaryocyte produces 1000 to 3000 platelets (figure 3). Megakaryocytes in bone marrow sinusoids extend pseudopods into endothelial cells using the demarcation membrane system (picture 4). Proplatelets bud off (picture 5) and migrate to the lungs, where they are transformed into platelets. (See 'Steps in platelet formation' above.)

●Platelet count regulation – The total platelet mass is kept constant by TPO. Megakaryocytes and platelets express TPO receptors that take up TPO, leaving residual TPO to stimulate megakaryocyte and platelet production. When the platelet count increases, TPO uptake is greater and platelet production declines. When the platelet count decreases, less TPO is taken up by platelets; circulating TPO levels rise; and platelet production is stimulated. Platelets live approximately seven days. Approximately one-third of the platelet mass is sequestered in the spleen. (See 'Control of platelet mass' above and 'TPO role in maintaining platelet count' above and 'Platelet lifespan' above.)

●Megakaryocytes in disease

•Thrombocytopenic conditions – (See 'Conditions with thrombocytopenia' above and "Diagnostic approach to thrombocytopenia in adults".)

-Cytotoxic chemotherapy can cause apoptosis of HSCs and/or MEPs; bortezomib can prevent platelet shedding from megakaryocytes, and etoposide can cause platelet apoptosis.

-Vitamin B12 deficiency decreases platelet production by reducing megakaryocyte ploidy.

-Splenomegaly redistributes the circulating platelet mass; concomitant liver disease decreases TPO production.

-In immune thrombocytopenia (ITP), autoantibodies and cytotoxic T cells promote platelet destruction by macrophages and cause megakaryocyte apoptosis.

-In myelodysplastic syndromes (MDS), megakaryocyte mass is increased but platelet production is ineffective.

•Thrombocytotic conditions – (See 'Conditions with thrombocytosis' above and "Approach to the patient with thrombocytosis".)

-Reactive thrombocytosis can be seen with iron deficiency, cancer, inflammation, and other conditions listed in the table (table 1).

-Rebound thrombocytosis can occur with recovery from bone marrow suppression by chemotherapy, infection, or alcohol. Redistributive thrombocytosis can occur following splenectomy.

-Clonal myeloproliferative neoplasms (MPNs), including essential thrombocythemia (ET), chronic myeloid leukemia (CML), polycythemia vera (PV), and primary myelofibrosis (PMF), can cause thrombocytosis by a TPO-independent mechanism driven by mutations in JAK2, CALR, and MPL. MPL encodes the TPO receptor.