Hematopoietic Stem Cells

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Hematopoietic stem cells (HSCs) resides within the bone marrow, which serves as niche for the maintenance of HSCs. HSC niche maintain a balance between proliferation and differentiation not only during steady state of tissue homeostasis but also in stress conditions-like during bleeding, viral or bacterial infection or loss of blood cells caused due to toxins or chemotherapeutic agents. Identification of HSC niche has been a major challenge since years due to high mobility of HSCs. Unlike the other stem cell niche, where stem cells reside within their niche or in their proximity, HSCs keep travelling between different bone marrow compartments. Observing the complexity of HSC niche, Hanoun and Frenette said –
“HSC niche is a maze; an amazing niche” (Hanoun and Frenette, 2013)
Recently, diverse cell types have been identified that serve as niche for regulating HSCs. They include-osteoblasts, mesenchymal stem cells, non-myelinating Schwann cells, CXCL12 abundant reticular (CAR) cells, endothelial cells associated with leptin-receptor (LepR)-expressing mesenchymal stromal cells. Each of these cell types uses different signaling molecule for maintaining HSCs proliferation and differentiation (Lander et al., 2012).
First cells to be recognized as HSC niche were-osteoblasts. Osteoblasts are bone- forming cells that line the bone-surface. HSCSs are present in close association with osteoblasts (Gong, 1978). First evidence indicating osteoblast as HSC niche came from the study of
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