Megakaryocytes, bone marrow

In a group of six children with acute thrombocytopenia and bone marrow megakaryocytic hyperplasia, the blood plasma, prepared as previously described, showed, in each case, an acid phosphatase activity, as determined by Gutman s method, that was higher than the mean value for that age (p = 0.02). The p value for the comparison of the group as a whole with normals was between 0.01 and 0.02. In all six of these patients the plasma acid phosphatase values returned to normal or near normal levels as the thrombocytopenia was corrected. Oski et al. (02) also studied 15 cases of chronic idiopathic thrombocytopenic purpura in whom the bone marrow showed normal to increased numbers of megakaryocytes. Of these, 13 showed plasma acid phosphatase values that were elevated above the normal mean for their age, albeit some of these differences were small. However, these elevations were statistically significant with a p value less than 0.01. 

MSP stimulated maturation of human megakaryocyte cell lines as well as primary bone marrow megakaryocytes. The criterion for maturation was an increase in ploidy, which was quantified as DNA content per cell, determined by flow cytometry. MSP stimulated increased secretion of IL-6 by these cells. Since the effect on ploidy was abolished by antibodies to IL-6, it appears that MSP acts via IL-6 in this system (Banu et al., 1996). MSP also stimulates se-... 

Platelets are the formed elements of the blood which participate in hemostasis. Platelets are enucleated, discoid fragments which arise from mature megakaryocytes in the bone marrow. Under normal circumstances, platelets do not adhere to endothelial surfaces of blood vessels. However, platelets can adhere to damaged areas of blood vessels and become activated in such a way that they can also bind fibrinogen. 

Blood platelets are key players in the blood-clotting mechanism. These tiny fragments of cytoplasm are shed into the circulation from the surface of megakaryocytes located in the bone marrow. When the lining of a blood vessel is injured, activated platelets release clotting factors, adhere to each other and to damaged surfaces, and send out numerous filopodia. The shape changes that occur in activated platelets are the result of actin polymerization. Before activation, there are no microfilaments because profilin binds to G-actin and prevents its polymerization. After activation, profilin dissociates from G-actin, and bundles and networks of F-actin filaments rapidly appear within the platelet. 

Hematopoiesis is defined as the development and maturation of blood cells and their precursors. In utero, hematopoiesis may occur in the liver, spleen, and bone marrow. However, after birth, it occurs exclusively in the bone marrow. All blood cells are generated from a common hematopoietic precursor, or stem cell. These stem cells are self-renewing and pluripotent and thus are able to commit to any one of the different lines of maturation that give rise to platelet-producing megakaryocytes, lymphoid, erythroid, and myeloid cells. The myeloid cell line produces monocytes, basophils, neutrophils, and eosinophils, whereas the lymphoid stem cell differentiates to form circulating B and T lymphocytes. In contrast to the ordered development of normal cells, the development of leukemia seems to represent an arrest in differentiation at an early phase in the continuum of stem cell to mature cell.1... 

PLATELETS. Platelets are nonnucleated discoid or elliptical cells that originate from the fragmentation of giant polyploid megakaryocytes located in the bone marrow. The average diameter of the platelet is 1.5 pm. Each platelet is surrounded by a trilaminar membrane, and its cytoplasm contains a dense body (delta granule), a surface-connected canalicular system,... 

The third of the cellular elements within the blood are the platelets (thrombocytes). Platelets are actually small, round or oval cell fragments. They are about 2 to 4 pm in diameter and have no nuclei. Platelets are formed in the red bone marrow as pinched-off portions of the very large megakaryocytes. Each megakaryocyte, which is confined to the bone marrow, can produce... 

The first site of myelopoiesis75 in the mouse embryo is the fetal liver, where the common myeloid progenitor, the megakaryocyte-erythrocyte-restricted progenitors and granulocyte-monocyte restricted progenitors are present. Myelopoiesis occurs in the fetal liver in the same manner as in adult bone marrow.81 However, the proliferation capacity, colony forming activity and differentiation capacity is different between the fetal liver and adult bone marrow.81... 

B cell bh4 bp BPI BSA C domain C1-C9 cAMP CAP CD cDNA CFU CFU-GEMM bone-marrow-derived lymphocyte tetrahydrobiopterin base pairs bactericidal/permeability-inducing protein bovine serum albumin constant domain complement components cyclic adenosine monophosphate cationic antimicrobial protein cluster of differentiation complementary deoxynucleic acid colony-forming unit granulocyte-erythroid-monocyte-megakaryocyte CFU... 

In addition to fully differentiated cells, many tissues of the body contain stem cells - precursor cells that are not fully differentiated. These can divide to produce more stem cells but can also give rise to progeny which can differentiate. Stem cells in the bone marrow, for example, give rise to all the different types of immune cells, erythrocytes and megakaryocytes, which give rise to platelets. 

Thrombocytes are cell fragments that arise in the bone marrow from large precursor cells, the megakaryocytes. Their task is to promote hemostasis (see p. 290). 

Physiologically, immunohaematopoietic stem cells give rise to progenitors that mature to recognizable megakaryocytes in the marrow. The latter release platelets into the circulation where they have a mean life-span of 10 days after which they are removed by the reticuloendothelial cells of the spleen, liver and bone marrow. There are three broad categories giving reduced counts. 

Production of blood cells in bone marrow of the central axial skeleton is referred to as medullary hematopoiesis. Hematopoietic tissue in adult bone marrow is well perfused and contains fat cells (adipocytes), and various types of blood and blood precursor cells encased within a protein matrix. Fibroblast, stromal and endothelial cells within bone marrow, serve as sources of matrix proteins as well as a factory for growth factors and chemokines that regulate blood cell production and release matured cells into the circulation [2,3]. Chemokines act as signal lamps for trafficking of lymphocytes in and out of lymphoid tissues. Erythroblasts, neutrophils, lymphoblasts, macrophages, megakaryocytes, and pluripotent stem cells are also found within the calcihed lattice crisscrossing the marrow space. 

Platelets the smallest of the human blood cells are released from megakaryocytes in the bone marrow as anucleated fragments into the circulation. 

Within cell lineage differentiation pathways, the inherent potency of the starting material may be restricted based on original source. Bone marrow derived cells have the ability to differentiate into eosinophils, erythrocytes, megakaryocytes, osteoclasts, and lymphoid cells [20]. However, these hematopoietic lineages have limited potential to become a distinct tissue such as liver. Mesenchymal stem cells, on the other hand, can differentiate into a broader array of distinct tissues such osteocytes, chondrocytes, adipocytes, myocytes, and bone marrow stromal cells [20]. This plurality of outcomes requires identification of circumstances under which an aberrant outcome is manifested. 

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