Platelet precursors display bipolar behavior

In this issue, Thon et al. (2010. J. Cell Biol. doi: 10.1083/jcb.201006102) demonstrate that newly released platelets exhibit bipolar behavior, shifting back and forth between round cells and multibodied proplatelets (Thon et al., 2010). The authors define this intermediate as a preplatelet and, in doing so, shed new insight into the terminal steps of platelet maturation.     

Blood platelets are assembled principally at the ends of proplatelet processes produced by differentiated megakaryocytes

Megakaryocytes release mature platelets in a complex process. Platelets are known to be released from intermediate structures, designated proplatelets, which are long, tubelike extensions of the megakaryocyte cytoplasm. We have resolved the ultrastructure of the megakaryocyte cytoskeleton at specific stages of proplatelet morphogenesis and correlated these structures with cytoplasmic remodeling events defined by video microscopy. Platelet production begins with the extension of large pseudopodia that use unique cortical bundles of microtubules to elongate and form thin proplatelet processes with bulbous ends; these contain a peripheral bundle of microtubules that loops upon itself and forms a teardrop-shaped structure. Contrary to prior observations and assumptions, time-lapse microscopy reveals proplatelet processes to be extremely dynamic structures that interconvert reversibly between spread and tubular forms. Microtubule coils similar to those observed in blood platelets are detected only at the ends of proplatelets and not within the platelet-sized beads found along the length of proplatelet extensions. Growth and extension of proplatelet processes is associated with repeated bending and bifurcation, which results in considerable amplification of free ends. These aspects are inhibited by cytochalasin B and, therefore, are dependent on actin. We propose that mature platelets are assembled de novo and released only at the ends of proplatelets, and that the complex bending and branching observed during proplatelet morphogenesis represents an elegant mechanism to increase the numbers of proplatelet ends.     

RhoA Is Essential for Maintaining Normal Megakaryocyte Ploidy and Platelet Generation

RhoA plays a multifaceted role in platelet biology. During platelet development, RhoA has been proposed to regulate endomitosis, proplatelet formation, and platelet release, in addition to having a role in platelet activation. These processes were previously studied using pharmacological inhibitors in vitro, which have potential drawbacks, such as non-specific inhibition or incomplete disruption of the intended target proteins. Therefore, we developed a conditional knockout mouse model utilizing the CRE-LOX strategy to ablate RhoA, specifically in megakaryocytes and in platelets to determine its role in platelet development. We demonstrated that deleting RhoA in megakaryocytes in vivo resulted in significant macrothrombocytopenia. RhoA-null megakaryocytes were larger, had higher mean ploidy, and exhibited stiff membranes with micropipette aspiration. However, in contrast to the results observed in experiments relying upon pharmacologic inhibitors, we did not observe any defects in proplatelet formation in megakaryocytes lacking RhoA. Infused RhoA-null megakaryocytes rapidly released platelets, but platelet levels rapidly plummeted within several hours. Our evidence supports the hypothesis that changes in membrane rheology caused infused RhoA-null megakaryocytes to prematurely release aberrant platelets that were unstable. These platelets were cleared quickly from circulation, which led to the macrothrombocytopenia. These observations demonstrate that RhoA is critical for maintaining normal megakaryocyte development and the production of normal platelets.     

Hirudin and heparin enable efficient megakaryocyte differentiation of mouse bone marrow progenitors

Hematopoietic progenitors from murine fetal liver efficiently differentiate in culture into proplatelet-producing megakaryocytes and have proved valuable to study platelet biogenesis. In contrast, megakaryocyte maturation is far less efficient in cultured bone marrow progenitors, which hampers studies in adult animals. It is shown here that addition of hirudin to media containing thrombopoietin and serum yielded a proportion of proplatelet-forming megakaryocytes similar to that in fetal liver cultures (approximately 50%) with well developed extensions and increased the release of platelet particles in the media. The effect of hirudin was maximal at 100 U/ml, and was more pronounced when it was added in the early stages of differentiation. Hirugen, which targets the thrombin anion binding exosite I, and argatroban, a selective active site blocker, also promoted proplatelet formation albeit less efficiently than hirudin. Heparin, an indirect thrombin blocker, and OTR1500, a stable heparin-like synthetic glycosaminoglycan generated proplatelets at levels comparable to hirudin. Heparin with low affinity for antithrombin was equally as effective as standard heparin, which indicates antithrombin independent effects. Use of hirudin and heparin compounds should lead to improved culture conditions and facilitate studies of platelet biogenesis in adult mice.     

Localization of megakaryocytes in the bone marrow

In the bone marrow, megakaryocytes are located in the extravascular space, applied to the abluminal surface of endothelium. In this position, they send cytoplasmic projections into the lumen. Some of these projections are organelle free and may serve to anchor the cell to the endothelium. They could also serve to monitor the circulation and to receive information as to the requirement of the body for platelet formation. Megakaryocytes also send organelle containing projections into the lumen. This may be an early step in the migration of these cells into the lumen or, alternatively, part of the proplatelet formation. These proplatelets are 2.5 x 120 microns elongated structures that penetrate the lumen and can each subsequently make 1000 platelets. Each megakaryocyte can make six to eight proplatelets. In the perisinal position, megakaryocytes may subserve an adventitial function as well; many blood cells can then take a transmegakaryocytic route to reach the endothelium and enter the circulation.     

IL-1α induces thrombopoiesis through megakaryocyte rupture in response to acute platelet needs

Intravital visualization of thrombopoiesis revealed that formation of proplatelets, which are cytoplasmic protrusions in bone marrow megakaryocytes (MKs), is dominant in the steady state. However, it was unclear whether this is the only path to platelet biogenesis. We have identified an alternative MK rupture, which entails rapid cytoplasmic fragmentation and release of much larger numbers of platelets, primarily into blood vessels, which is morphologically and temporally different than typical FasL-induced apoptosis. Serum levels of the inflammatory cytokine IL-1α were acutely elevated after platelet loss or administration of an inflammatory stimulus to mice, whereas the MK-regulator thrombopoietin (TPO) was not elevated. Moreover, IL-1α administration rapidly induced MK rupture–dependent thrombopoiesis and increased platelet counts. IL-1α–IL-1R1 signaling activated caspase-3, which reduced plasma membrane stability and appeared to inhibit regulated tubulin expression and proplatelet formation, and ultimately led to MK rupture. Collectively, it appears the balance between TPO and IL-1α determines the MK cellular programming for thrombopoiesis in response to acute and chronic platelet needs.     

Actin Inhibition Increases Megakaryocyte Proplatelet Formation through an Apoptosis-Dependent Mechanism

Background

Megakaryocytes assemble and release platelets through the extension of proplatelet processes, which are cytoplasmic extensions that extrude from the megakaryocyte and form platelets at their tips. Proplatelet formation and platelet release are complex processes that require a combination of structural rearrangements. While the signals that trigger the initiation of proplatelet formation process are not completely understood, it has been shown that inhibition of cytoskeletal signaling in mature megakaryocytes induces proplatelet formation. Megakaryocyte apoptosis may also be involved in initiation of proplatelet extension, although this is controversial. This study inquires whether the proplatelet production induced by cytoskeletal signaling inhibition is dependent on activation of apoptosis.

Methods

Megakaryocytes derived from human umbilical cord blood CD34+ cells were treated with the actin polymerization inhibitor latrunculin and their ploidy and proplatelet formation were quantitated. Apoptosis activation was analyzed by flow cytometry and luminescence assays. Caspase activity was inhibited by two compounds, ZVAD and QVD. Expression levels of pro-survival and pro-apoptosis genes were measured by quantitative RT-PCR. Protein levels of Bcl-XL, Bax and Bak were measured by western blot. Cell ultrastructure was analyzed by electron microscopy.

Results

Actin inhibition resulted in increased ploidy and increased proplatelet formation in cultured umbilical cord blood-derived megakaryocytes. Actin inhibition activated apoptosis in the cultured cells. The effects of actin inhibition on proplatelet formation were blocked by caspase inhibition. Increased expression of both pro-apoptotic and pro-survival genes was observed. Pro-survival protein (Bcl-x_L) levels were increased compared to levels of pro-apoptotic proteins Bak and Bax. Despite apoptosis being activated, the megakaryocytes underwent minimal ultrastructural changes during actin inhibition.

Conclusions

We report a correlation between increased proplatelet formation and activation of apoptosis, and that the increase in proplatelet formation in response to actin inhibition is caspase dependent. These findings support a role for apoptosis in proplatelet formation in this model.     

Mechanisms of platelet production

The precise mechanism by which platelets are formed from megakaryocytes (MK) remains unclear, despite numerous studies which have been performed during this century. Models have been proposed that attempt to account for platelet formation from disruption of elongated processes of MK cytoplasm, designated proplatelets, or by fragmentation of MK cytoplasm. MK demarcation membranes are hypothesized by some investigators to delineate platelet territories in the MK cytoplasm, and by others to act as a membrane reservoir for MK process formation. Platelet production has been variously speculated to occur primarily in the bone marrow or lung. Each theory or model has attempted to elucidate the phenomenon of size heterogeneity of circulating platelets and the changes that occur under conditions of altered thrombopoiesis. In this article, we have analyzed and compared the characteristics of previously proposed models for platelet production and suggested additional techniques for future studies of thrombopoiesis.     

Use of[75Se]-methionine as a tracer of thrombocytopoiesis: II-kinetics in normal rats and in platelet disorders in man: a new approach

We have shown in the rat and in man that it is possible to determine the life span of autologous platelets and to quantify the production of these circulating cells by injecting[⁷⁵Se]-methionine and analysing the overall platelet radioactivity curves obtained: platelets and their precursors are labelled $\text{in vivo}$ by adsorption of circulating plasma proteins onto their membrane, and in the marrow, principally during synthesis of thrombosthenin. This last phenomenon takes place 2 to 3 days before platelet release into the rat blood circulation and 5 to 6 days in the case of man. When there is a greatly increased production, the maturation time is slightly decreased.     

Effect of gravity change on the production of thrombopoietic growth factors.

It is reported that the stay in the space develops anemia, thrombocytopenia, and altered function and structure of red blood cell. The mechanism of these abnormalities was not clarified yet. The cloning of the thrombopoietin (TPO), followed by the analysis of TPO and c-mpl (its cellular receptor) knockout mice confirmed its role as the primary regulator of thrombopoiesis. TPO has been shown to stimulate both megakaryocyte colony growth from marrow progenitor cells and the maturation of immature megakaryocyte to form functional platelet. This process includes the massive cytoskeletal rearrangement, such as proplatelet formation and fragmentation of proplatelet. In this study we have focused on the production of thrombopoietic growth factors in mice those were exposed to gravity change by parabolic flight (PF).     

Megakaryocyte rupture for acute platelet needs

Circulating platelets were thought to arise solely from the protrusion and fragmentation of megakaryocyte cytoplasm. Now, Nishimura et al. (2015. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201410052) show that platelet release from megakaryocytes can be induced by interleukin-1α (IL-1α) via a new rupture mechanism, which yields higher platelet numbers, occurs independently of the key regulator of megakaryopoiesis thrombopoietin, and may occur during situations of acute platelet need.Platelets, small anucleate cells that circulate in the blood stream, are essential for normal hemostasis but also play major roles in inflammation, immunity, wound healing, tumor metastasis, and the development and maintenance of lymph vessels (Leslie, 2010). Hence, reduced platelet numbers and/or impaired platelet function, as found in the context of numerous pathologies or upon pharmacological intervention, may have a negative impact on a large variety of physiological processes and under certain circumstances can become life threatening (Sachs and Nieswandt, 2007).Platelets are continuously produced by fragmentation of the cytoplasm of their giant polyploid precursors in the bone marrow, the megakaryocytes. Recent studies using intravital two-photon microscopy of the bone marrow confirmed the formation of long protrusions of megakaryocytes termed proplatelets in vivo, which extend into bone marrow sinusoids where larger cytoplasmic fragments, so-called preplatelets, are shed and further mature within the circulation ultimately giving rise to platelets (Junt et al., 2007; Zhang et al., 2012; Bender et al., 2014). Calculations of platelet consumption and production in humans and mice suggested that platelet production via proplatelet formation is sufficient to maintain platelet count in normal physiology (Kaufman et al., 1965; Junt et al., 2007). However, this mechanism may not be efficient enough to produce sufficient platelet numbers under conditions of increased platelet consumption, such as inflammation/infection, immune thrombocytopenia, or traumatic blood loss. In this issue, Nishimura et al. have now identified an interleukin-1α (IL-1α)–induced rupture-type mechanism for platelet production that yields ∼20-fold higher numbers of released platelet particles as compared with the classical mechanism of proplatelet formation during the same period of time (Fig. 1). This work provides for the first time an explanation of how megakaryocytes can maintain platelet mass equilibrium and quickly restore platelet numbers under pathological conditions associated with increased platelet turnover. Even though the platelets released by megakaryocyte rupture were mildly enlarged in size, they were functionally indistinguishable from proplatelet-derived platelets.Open in a separate windowFigure 1.Platelet production in normal physiology and upon acute platelet needs. In normal physiology (left), platelets are continuously produced by megakaryocytes via the classical process of proplatelet formation. Under these conditions, thrombopoietin (Thpo) drives megakaryopoiesis by signaling through its receptor c-Mpl, but Thpo is dispensable for proplatelet formation, which is a cell-autonomous process and presumably regulated by the vascular niche. Inhibition of Caspase-3 and a well-organized orchestration of microtubule dynamics (green) are prerequisites for proper proplatelet formation and protrusion into bone marrow sinusoids, where preplatelets are released and further mature within the circulation. Proplatelet formation is a rather slow process with low yields of platelets per period of time but is sufficient to compensate for the continuous loss of aged platelets. Under conditions of increased platelet loss or consumption (right), e.g., as a result of excessive blood loss or in the setting of infection/inflammation, this mechanism might not be sufficient to ensure appropriate platelet supply. Under these conditions, interleukin-1α (IL-1α) levels increase rapidly and trigger rupture-type platelet formation via its receptor IL-1R1 on megakaryocytes. IL-1α signaling leads to a deregulated expression and organization of β1-tubulin (green) as well as to the activation of Caspase-3, which in turn leads to a reduction of megakaryocyte membrane stiffness. Together, these processes lead to the formation of multiple membrane blebs that are predominantly released into bone marrow sinusoids to quickly replenish platelet numbers.The IL-1α procytokine is expressed in virtually all nonhematopoietic cells, but also in platelets, and is involved in inflammatory processes, modulation of immune responses, and hematopoiesis. IL-1α is released from damaged endothelial cells and activated platelets, where it triggers the recruitment of immune cells (Rider et al., 2013). As the work from Nishimura et al. (2015) indicates, this cytokine may also stimulate thrombopoiesis and rupture-type platelet release from megakaryocytes to compensate for platelet loss and restore platelet mass equilibrium. This could explain why supplementing cancer patients experiencing chemotherapy-induced thrombocytopenia with IL-1α accelerated platelet count recovery (Gordon and Hoffman, 1992; Smith et al., 1993). These findings are of particular importance when considering the development of IL-1α inhibitors to dampen inflammatory processes.The technical optimization of the temporal and spatial resolution of two-photon intravital microscopy in combination with an elegant series of experiments using a broad variety of knockout mouse models allowed Nishimura et al. (2015) to observe and characterize this alternative mechanism of platelet formation. The mechanism strongly resembles key features of FasL-induced apoptosis, including activation of Caspase-3, disorganization of the cytoskeleton, and membrane blebbing. However, in stark contrast to typical FasL-induced apoptosis, rupture-type platelet formation is relatively quick (within an hour vs. >80 min) and results in the release of a large number of phosphatidylserine-negative particles. These particles carry an increased content of β1-tubulin, which is reminiscent of disorganized α- and β-tubulin expression, and has not been described for apoptotic cells (Fig. 1). The increased formation of membrane blebs was accompanied by a reduction in megakaryocyte membrane stiffness that could be reverted by caspase inhibitors. The activation of Caspase-3 represents a central step in rupture-type platelet release, as Caspase-3–deficient megakaryocytes could not use this alternative pathway for platelet production. Future studies will be required to determine how IL-1α modulates megakaryocyte membrane stiffness and to identify the mechanisms that distinguish rupture-type platelet release from typical FasL-induced apoptosis.Nishimura et al. (2015) find that rupture-type platelet production occurs independently of thrombopoietin (Thpo), the key driver of thrombopoiesis, as rupture-type platelet production constituted the major source of circulating platelets in Thpo-deficient mice. This finding is in line with a study by Ng et al. (2014) showing that megakaryocyte-specific Thpo receptor (c-Mpl)–deficient mice presented a marked thrombocytosis despite the lack of Thpo stimulation during terminal thrombopoiesis. Unfortunately, IL-1α levels or the presence of rupture-type platelet biogenesis have not yet been assessed in c-Mpl–deficient mice or in patients suffering from congenital amegakaryocytic thrombocytopenia. In addition, it would be of particular interest to assess the contribution of IL-1α–induced rupture-type platelet release in human patients and also in mouse models reproducing inherited or idiopathic platelet disorders, such as the Wiskott–Aldrich syndrome, Gray-platelet syndrome, or immune thrombocytopenia.In conclusion, the novel rupture-type platelet release mechanism identified by Nishimura et al. (2015) will help to answer the long-standing question of how circulating platelet numbers are quickly restored under conditions of increased platelet consumption or loss. Furthermore, this finding may lead to the development of new drugs to modulate platelet turnover in humans, but we also need to carefully reconsider previous experimental data on megakaryopoiesis/platelet production to include the possible contribution of IL-1α–induced platelet release. Overall, the identification of a new mechanism of platelet production has advanced our understanding of platelet production and will certainly stimulate new research in the field of megakaryocyte biology.     

Calyculin A retraction of mature megakaryocytes proplatelets from embryonic stem cells

Platelets are produced by megakaryocytes (MKs) through proplatelet formation (PPF), or cytoplasmic extensions, in vitro. Through the use of video-enhanced light microscopy, as well as localization of cytoskeletal proteins by confocal microscopy, the reaction of fully mature MK proplatelets, derived from murine embryonic stem cells, to various agents was studied. Calyculin A (protein phosphatase 1/2A inhibitor) treatment induced proplatelet retraction. In MKs with PPF, the expression of actin, myosin IIA, monophosphorylated myosin light chain (MLC-P1), and diphosphorylated myosin light chain (MLC-P2) was diffusely located. Following calyculin A treatment, actin was diffusely localized in retracted MKs and was expressed particularly in the periphery. MLC-P1 was also localized primarily in the periphery; however, MLC-P2 was expressed mostly in the inner area of proplatelets. Protein phosphatase inhibitors may result in increased hyperphosphorylation of localized MLC, which could alter the balance of actomyosin force in a cell, and therefore induce proplatelets retraction.     

Cytoskeletal regulation of platelet formation: Coordination of F-actin and microtubules

Platelets are small, anucleate blood cells which play an important role in haemostasis. Thrombocytopenia is a condition where the platelet count falls below 150 × 10⁹/l and patients suffering from severe forms of this condition can experience life-threatening bleeds requiring platelet transfusions. Platelets are produced from large progenitor cells called megakaryocytes which are found in the bone marrow. The process of megakaryocyte maturation and the formation of proplatelets are essential steps in the production of mature platelets and both depend heavily on the actin and microtubule cytoskeletons. Understanding these processes is important for the development of in vitro platelet production which will help to treat thrombocytopenia as well as produce model systems for studying platelet-associated disorders. This review will highlight some of the recent advances in our understanding of the role of the cytoskeleton in platelet production, especially the key molecules and signalling pathways that regulate actin and microtubule crosstalk.     

A lineage-restricted and divergent beta-tubulin isoform is essential for the biogenesis, structure and function of blood platelets

BACKGROUND: Mammalian megakaryocytes release blood platelets through a remarkable process of cytoplasmic fragmentation and de novo assembly of a marginal microtubule band. Cell-specific components of this process include the divergent beta-tubulin isoform beta1 that is expressed exclusively, and is the predominant isoform, in platelets and megakaryocytes. The functional significance of this restricted expression, and indeed of the surprisingly large repertoire of metazoan tubulin genes, is unclear. Fungal tubulin isoforms appear to be functionally redundant, and all mammalian beta-tubulins can assemble in a variety of microtubules, whereas selected fly and worm beta-tubulins are essential in spermatogenesis and neurogenesis. To address the essential role of beta1-tubulin in its natural context, we generated mice with targeted gene disruption. RESULTS: beta1-tubulin(-/-) mice have thrombocytopenia resulting from a defect in generating proplatelets, the immediate precursors of blood platelets. Circulating platelets lack the characteristic discoid shape and have defective marginal bands with reduced microtubule coilings. beta1-tubulin(-/-) mice also have a prolonged bleeding time, and their platelets show an attenuated response to thrombin. Two alternative tubulin isoforms, beta2 and beta5, are overexpressed, and the total beta-tubulin content of beta1-tubulin(-/-) megakaryocytes is normal. However, these isoforms assemble much less efficiently into platelet microtubules and are thus unable to compensate completely for the absence of beta1-tubulin. CONCLUSIONS: This is the first genetic study to address the essential functions of a mammalian tubulin isoform in vivo. The results establish a specialized role for beta1-tubulin in platelet synthesis, structure, and function.     

A uniform‐shear rate microfluidic bioreactor for real‐time study of proplatelet formation and rapidly‐released platelets

Platelet transfusions, with profound clinical importance in blood clotting and wound healing, are entirely derived from human volunteer donors. Hospitals rely on a steady supply of donations, but these methods are limited by a 5‐day shelf life, the potential risk of contamination, and differences in donor/recipient histocompatibility. These challenges invite the opportunity to generate platelets ex vivo. Although much progress has been made in generating large numbers of culture‐derived megakaryocytes (Mks, the precursor cells to platelets), stimulating a high percentage of Mks to undergo platelet release remains a major challenge. Recent studies have demonstrated the utility of shear forces to enhance platelet release from cultured Mks. In this study, we performed a computational fluid dynamics (CFD) analysis of several published platelet microbioreactor systems, and used the results to develop a new 7‐µm slit bioreactor—with well‐defined flow patterns and uniform shear profiles. This uniform‐shear‐rate bioreactor (USRB‐7µm) permits real‐time visualization of the proplatelet (proPLT) formation process and the rapid‐release of individual platelet‐like‐particles (PLPs), which has been observed in vivo, but not previously reported for platelet bioreactors. We showed that modulating shear forces and flow patterns had an immediate and significant impact on PLP generation. Surprisingly, using a single flow instead of dual flows led to an unexpected six‐fold increase in PLP production. By identifying particularly effective operating conditions within a physiologically relevant environment, this USRB‐7µm will be a useful tool for the study and analysis of proPLT/PLP formation that will further understanding of how to increase ex vivo platelet release. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 33:1614–1629, 2017     

Decreased platelet level in peripheral blood of mice by microgravity.

It is reported that the stay in the space develop anemia, throbocytopenia, and altered function and structure of red blood cell. The mechanism of these abnormalities was not clarified yet. TPO has been shown to stimulate both megakaryocyte colony growth from marrow progenitor cells and the maturation of immature megakaryocyte to form functional platelet. This process include massive cytoskeletal rearrangement, such as proplatelet formation and fragmentation of proplatelet. Our previous reports (Fuse and Sato, 2001, Fuse et al, 2001) showed an inverse relationship between decreased platelet count and increased TPO concentrations in peripheral blood of mouse was induced by parabolic flight (PF). We have studied which gravity change during PF involved this phenomenon.     

The Blood Platelet as a Model for Regulating Blood Coagulation on Cell Surfaces and Its Consequences

Platelets actively participate in regulating thrombin production following physical or chemical injury to blood vessels. Injury to blood vessels initiates activation of the large numbers of platelets that appear in the subendothelium where they become exposed to tissue factor and to molecules adhesive for platelets and normally found in the extracellular matrix. The complex of plasma factor VIIa with extravascular tissue factor both initiates and localizes thrombin production on platelets and on extravascular cells. Thrombin production at these sites in turn enhances platelet activation and the subsequent hemostatic plug formation to minimize bleeding. Thrombin production and platelet activation also initiate the process of wound healing requiring thrombin-dependent cell activation and platelet-dependent formation of new blood vessels (angiogenesis). Activated platelets release from their storage granules several proteins and other factors that regulate local thrombin formation and the responses of blood vessel cells to injury to assure hemostasis and effective wound healing. Failure to localize and adequately regulate thrombin production and/or platelet activation can have pathological consequences, including the development and propagation of atherosclerosis and enhancement of tumor development. The primary basis for the pathological consequences of the failure to adequately regulate thrombin production is that the multi-functional thrombin activates several types of cells to initiate their mitogenesis. Mitogenesis precedes many of the undesirable consequences of poorly regulated thrombin production and platelet activation. In addition, activated platelets release a variety of products which influence the functions of several cell types to the extent that inadequate regulation of platelet activation (by excessive thrombin production) could contribute to the pathogenesis of acute and chronic arterial thrombosis and to tumor development. Activated platelets participate in tumor development by releasing several factors that positively (and negatively) regulate blood vessel formation.     

血小板生成刺激因子对巨核血小板系统体外生成的影响

本 文应用血小板生成液体培养体系及纯化的血小板生成刺激因子(TSF)研究了 TSF对巨核细胞成熟及血小板生成的作用。TSF 在0.5—2U/ml 浓度范围内能够刺激巨核细胞DNA 合成,胞浆成熟,胞体直径增加以及血小板直径增加,但对巨核细胞与血小板计数没有影响。实验表明 TSF 作为一种血小板生成素,通过促进巨核细胞分化成熟,以增加血小板体积的方式,促进血板小生成。     

The transmural passage of blood cells into myeloid sinusoids and the entry of platelets into the sinusoidal circulation; a scanning electron microscopic investigation.

Scanning electron microscopic observations of rat bone marrow reveal that the sinusoidal wall is continuous and has no permanent patent apertures allowing free communication between the extravascular and intravascular myeloid compartments. Blood cells migrate into the sinusoids by perforating the endothelial cell body. Platelets are derived from long intrasinusoidal "proplatelet" processes which originate from the cell body of extravascularly located megakaryocytes. Proplatelet processes frequently occur in clusters, with the probability that all processes in a cluster arise from a single megakaryocyte. The release of platelets into the circulation may be initiated by local constriction along these processes, at which places either individual platelets or larger segments of proplatelet cytoplasm are pinched off. The larger segments may subsequently undergo further fragmentation into individual platelets.