The megakaryocyte platelet system and vascular disease

Abstract. Platelets form a heterogeneous population of cells produced from the uniquely large polyploid cell found in the bone marrow, the megakaryocyte. The platelet megakaryocyte axis forms a dynamic equilibrium varying in normal biology and in disease. Prolonged platelet destruction leads to the production of large platelets from large, high ploidy megakaryocytes. In vivo and ex vivo studies show that such platelets have more haemostatic potential than smaller less dense platelets. The evidence suggesting that prothrombotic changes in the megakaryocyte platelet axis precede coronary artery thrombosis and the importance of platelet reactivity in atherosclerosis will be reviewed.     

Aspects of platelet formation and release

The surface architecture and location of megakaryocytes in the extravascular compartment of mouse bone marrow are considered. The finger-like cytoplasmic processes that extend into sinuses pass through apertures, approximately 3.0 mμ in diameter, occurring singly or in groups. The cytoplasmic processes enlarge on first entering a sinus and form villi that seem to anchor them to the endothelium. The villi consist largely of microfilaments. The cytoplasmic processes extend, attenuate, and undergo irregular constriction along their lengths. The constrictions demarcate modicums of cytoplasm of platelet size one from another that are released when they break. Extravascular platelet release may also occur, and evidence is presented that points to the likelihood of the platelet's being engulfed and phagocytosed by macrophages before reaching the microcirculation.     

Quantitative detection of platelet aggregates in whole blood without fixation

Platelet counting detects lesser degrees of platelet aggregation than conventional aggregometry. In order to prevent progressive platelet aggregation or disaggregation after sampling it is customary to fix blood samples. However fixation may introduce other artefacts. We first compared stability of platelet counts in EDTA-, citrate- and r-hirudin-anticoagulated blood from healthy volunteers. Second, the stability of platelet counts in unfixed EDTA- and hirudin-anticoagulated blood was compared with glutaraldehyde-fixed blood in the same anticoagulants. Third, the effect of in vivo heparin administration on platelet counts in EDTA- and hirudin-anticoagulated blood was studied. Platelet counts within 2 h of collection were significantly higher in EDTA than in hirudin- or citrate-anticoagulated blood ( P =0.002 vs. hirudin and P =0.001 vs. citrate). Twenty-four hour counts in hirudin and EDTA were unchanged ( P =0.3 and P =0.2, respectively, vs. earlier counts). Counts in citrate increased significantly ( P =0.007; n =10). Platelet counts in fixed blood did not differ significantly from those in unfixed blood. Heparin administered for cardiopulmonary bypass reduced platelet counts in hirudin-anticoagulated blood from (mean +/-1 standard deviation) 180+/-45 to 162+/-30 x 10 9 l -1 ( P =0.01; n =14), without significantly lowering counts with EDTA-anticoagulation, consistent with increased platelet aggregation. Hirudin and EDTA provided stable platelet counts, suggesting that fixation is unecessary.     

Mature micromegakaryocytes: an unusual developmental pattern in term infants

We sought to gain perspective on platelet production in the fetus and the newborn by counting and characterizing megakaryocytes from available cord blood. Elutriation was used to isolate circulating megakaryocytes from umbilical arteries and veins obtained at scheduled caesarean sections of nine normal term fetuses. Megakaryocytes were identified by established criteria, their diameters measured, and maturation stages recorded. Large numbers of megakaryocytes, mostly mature, were found in both the umbilical arteries and veins, many times more than previously observed circulating in adult blood. In term infants more than a third of the mature megakaryocytes had unusually decreased nuclear lobation and were dwarf cells with diameters as small as 13 μm, which we considered to be micromegakaryocytes. The atypicality of these small but mature cells is seen as merely a leftward skewing in the development of megakaryocyte ploidies. We believe that in normal fetuses the extent of megakaryocyte ploidization and development is distinctive and probably regulated differently to the adult pattern.     

红细胞生成素对血小板体外生成的影响

本文研究了红细胞生成素(EPO)在体外对巨核细胞成熟及血小板生成的影响。两种纯度的EPO在一定浓度范围内,均能使血小板数呈线性增加,但巨核细胞数保持不变。EPO-Ⅰ可使2～8倍体(N)巨核细胞减少,16～64N细胞增多,同时Ⅰ期巨核细胞比例减少,Ⅳ期细胞比例增多。两种EPO以等单位或最大效应(增加血小板数的作用)相比,EPO-Ⅰ的作用强于EPO-Ⅱ。实验表明EPO使体外血小板数升高的作用是由于促进了巨核细胞DNA合成及胞浆成熟。     

The role of bone marrow microenvironment in platelet production and their implications for the treatment of thrombocytopenic diseases

Objectives: Impaired platelet production has been found to be an important pathological mechanism of thrombocytopenia in many diseases. Platelet generation is a complex process that mainly occurs in the bone marrow, and thus is closely regulated by the bone marrow microenvironment. This review attempts to summarize the most current knowledge referring the role of bone marrow microenvironment in the regulation of platelet production.

Methods: The effects of multiple microenvironment ingredients in regulating megakaryopoiesis and thrombocytopoiesis have been discussed. Abnormalities of these components in thrombocytopenic diseases are also described.

Discussions: Thrombocytopenia is a common clinical manifestation of a variety of diseases. The functional importance of platelets has driven the developments of a broad range of studies. Platelet generation mainly occurs within the bone marrow, where the cells, soluble factors, and extracellular matrix proteins collaboratively form a complex regulatory network, directing megakaryocytic proliferation and differentiation. Alteration in any part of the regulating network may result in defective platelet formation, and eventually lead to thrombocytopenia. A variety of thrombocytopenic diseases have been found to be related with the disregulated bone marrow microenvironment. Identification of the variations of these niche ingredients in certain diseases has facilitated the developments of multiple therapeutic regimes. Further studies that can combine these niche factors with their downstream regulatory factors will be beneficial for developing more effective therapies.

Conclusions: Further definition of the role of bone marrow microenvironment in platelet generation may deepen our understanding of the underlying mechanisms as well as provide new therapeutic targets for thrombocytopenic diseases.     

小分子肝素联用阿司匹林治疗心血管疾病的临床研究概况

低分子肝素(LMWH)是从标准肝素中分离出来的分子量较小片段。对于治疗不稳定型心绞痛及无Q波心肌梗死,LMWH已得到了充分的评价。临床经验常常将低分子肝素作为辅助疗法与溶栓剂治疗急性心肌梗死。阿司匹林能通过抑制血小板内血栓素a2(TXA2)的合成而起到抗凝作用,广泛应用于心绞痛、急性心肌梗死、脑卒中和短暂性脑缺血等血栓栓塞性疾病,其抗血栓治疗的疗效已为大量国际性多中心临床试验所肯定。利用小分子肝素与阿司匹林治疗如不稳定型心绞痛、预防冠状动脉支架亚急性血栓形成、急性脑梗死等在国内已有一定程度研究与应用,本文就近5年的研究成果作一综述。