pediagenosis: Haematology
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Showing posts with label Haematology. Show all posts
Showing posts with label Haematology. Show all posts

Thursday, September 13, 2018

Bone Marrow Stroma

Bone Marrow Stroma


Bone Marrow Stroma
The bone marrow forms a suitable environment for stem cell survival, self‐renewal and formation of differentiated progenitor cells. It is composed of stromal cells and a microvascular network (Fig. 1.4). The stromal cells include mesenchymal stem cells, adipocytes, fibroblasts, osteoblasts, endothelial cells and macrophages and they secrete extracellular molecules such as collagen, glycoproteins (fibronectin and thrombospondin) and glycosaminoglycans (hyaluronic acid and chondroitin derivatives) to form an extracellular matrix. In addition, stromal cells secrete several growth factors necessary for stem cell survival.
Haemopoietic Stem And Progenitor Cells

Haemopoietic Stem And Progenitor Cells


Haemopoietic Stem And Progenitor Cells
Haemopoiesis starts with a pluripotential stem cell that can by asymmetric cell division self‐renew but also give rise to the separate cell lineages. These cells are able to repopulate a bone marrow from which all stem cells have been eliminated by lethal irradiation or chemotherapy. This haemopoietic stem cell (HSC) is rare, perhaps 1 in every 20 million nucleated cells in bone marrow. Many of the cells are dormant and in mice it has been estimated that they enter cell cycle approximately every 20 weeks. Although its exact phenotype is unknown, on immunological testing the HSC is CD34+ CD38− and negative for lineage markers (Lin−) and has the appearance of a small or medium‐sized lymphocyte (see Fig. 23.3). The cells reside in specialized osteoblastic or vascular ‘niches’.
Site Of Haemopoiesis

Site Of Haemopoiesis


Site Of Haemopoiesis
In the first few weeks of gestation the yolk sac is a transient site of haemopoiesis. However, definitive haemopoiesis derives from a population of stem cells first observed on the AGM (aorta‐gonads‐mesonephros) region. These common precursors of endothelial and haemopoietic cells (haemangioblasts) are believed to seed the liver, spleen and bone marrow. From 6 weeks until 6–7 months of fetal life, the liver and spleen are the major haemopoietic organs and continue to produce blood cells until about 2 weeks after birth (Table 1.1; see Fig. 7.1b).

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