Endothelium is essential for maintenance of health of the vessel wall and for the local regulation of vascular tone and structure and haemostasis. Regular physical exercise, which is known to promote a favourable cardiovascular state, may improve endothelial function via several mechanisms. Indeed, it augments blood flow and laminar shear stress, resulting in increased nitric oxide production and bioavailability. In this regard, the beneficial effects of training on endothelial function can be mediated in a number of ways, including synthesis of molecular mediators, changes in neurohormonal release and oxidant/antioxidant balance. On the other hand, physical exercise can also elicit systemic molecular pathways connected with angiogenesis and chronic anti-inflammatory action with consequent modification of the endothelial function. However, its benefit depends on the type and intensity of training performed. While strenuous exercise increases oxidative metabolism and produces a pro-oxidant environment, only regular moderate physical activity promotes an antioxidant state and preserves endothelial function. Thus, exercise may have a beneficial effect on the development of cardiovascular disease through preserving endothelial function.

We describe a new two-step culture method for mass production in vitro of erythroid cells from either CD34+ (10(5) cells/mL) or light-density (10(6) cells/mL) cells purified from the blood of normal donors and thalassemic patients. The method includes (i) culture of the cells in the presence of dexamethasone and estradiol (10(-6) M each) and (ii) the growth factors SCF (50 ng/mL), IL-3 (1 ng/mL), and EPO (1 U/mL). In their proliferative phase, these cultures generated approximately 1.2 x 10(7) erythroblasts for each milliliter of blood collected from normal donors or thalassemic patients. They were composed mostly (90%) of CD45(low)/glycophorin (GPA)(neg)/CD71(1ow) cells at day 7, 50-60\% of which became CD45(neg)/GPA+/CD71high by days 15-20. However, when cells from days 7 to 12 of the proliferative phase were transferred in differentiation medium containing EPO and insulin, they progressed to mature erythroblasts (g90% benzidine(pos) and CD45(neg)/GPA+/CD71medium) in 4 days. Because of the high number of erythroid cells that are generated from modest volumes of blood, this method will prove useful in donor-specific studies of erythroid differentiation.

Deletion of megakaryocytic-specific regulatory sequences of GATA-1 (Gata1 tm2Sho or GATA-1 low mutation) results in severe thrombocytopenia, because of defective thrombocytopoiesis, and myelofibrosis. As documented here, the GATA-1 low mutation blocks megakaryocytic maturation between stage I and II, resulting in accumulation of defective megakaryocytes (MKs) in the tissues of GATA-1 low mice. The block in maturation includes failure to properly organize ␣ granules because von Willebrand factor is barely detectable in mutant MKs, and P-selectin, although normally expressed, is found frequently associated with the demarcation membrane system (DMS) instead of within granules. Conversely, both von Willebrand factor and P-selectin are barely detectable in GATA-1 low platelets. IntroductionMegakaryocytes (MKs) are specialized cells of the blood responsible for platelet production. 1 They originate from committed progenitor cells, usually localized in the marrow, through a complex maturation process, during which MK precursors progressively increase in size, while undergoing extensive synchronous morphologic changes in the cytoplasm and nucleus. 2 At the ultrastructural level, the major cytoplasmic modifications are represented by massive compartmentalization into discrete regions, delimited by intrusions of the plasma membranes, bound to give rise to the demarcation membrane system (DMS). 2 The DMS will, in turn, internalize platelet-specific ␣ granules, giving rise to proplatelets through a process defined thrombocytopoiesis. 3 In the meantime, chromosomes undergo several cycles of endo-duplications. As a result, the nucleus itself appears multilobed.Despite few differences, the morphologic changes associated with the maturation of MK precursors are similar between mice and humans. 4 On the basis of distinct ultrastructural characteristics, murine and human MK precursors are divided into 4 classes 2 : the promegakaryoblast, a small mononuclear cell expressing already platelet-specific proteins, such as von Willebrand Factor (VWF); the megakaryoblast (or stage I MK), a cell 15 to 50 m in diameter with a large, oval or kidney-shaped nucleus and several nucleoli, whose cytoplasm presents abundant ribosomes and a welldeveloped rough endoplasmic reticulum (RER); the promegakaryocyte (stage II MK), a cell 20 to 80 m in diameter with an irregularly shaped nucleus and a more abundant cytoplasm, containing a rudimental DMS; and, finally, mature megakaryocytes (stage III MKs) that contain a multilobed nucleus surrounded by abundant cytoplasm divided into a perinuclear (hosting the centrioles, few biosynthetically active organelles, and many ␣ granules), the intermediate (containing a well-developed DMS and platelet territories), and the peripheral (devoid of organelles and enriched of cytoskeletal proteins and microtubules) zone. 2 The complex process of MK maturation is controlled by lineagespecific extrinsic and intrinsic factors represented, respectively, by growth factors (such as thrombopoietin 5 [TPO]) and transcriptio...

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