Pluripotential hemopoietic stem cell

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Note that some complexity is omitted from the diagram. Lymphocytes come from "Lymphoid" line, while granulocytes, monocytes, megakaryocytes, and erythrocytes come from "Myeloid" line. Among myeloid cells, granulocytes and monocytes have a common precursor, "CFU-GM".

Hematopoietic stem cells (HSC) are stem cells found in the bone marrow. HSC are the precursor cells which give rise to all the types of both the myeloid and lymphoid lineages. This includes monocytes and macrophages, neutrophils, basophils, eosinophils, T-cells, B-cells, NK-cells, microglia, erythrocytes (red blood cells), megakaryocytes (e.g. platelets), and dendritic cells. As stem cells, they are defined by their ability to form multiple cell types (multipotency) and their ability to self-renew.

Multipotency: Individual HSC have the ability to give rise to any of the end-stage blood cell types. During differentiation, daughter cells derived from HSC undertake a series of commitment decisions, retaining differentiation potential for some lineages while losing others. Intermediate cells become progressively more restricted in their lineage potential, until eventually lineage-committed end stage cells are generated.

Self-Renewal: Some kinds of stem cells are thought to undertake asymmetric cell division, generating one daughter cell that remains a stem cell and one daughter cell that differentiates. For Hematopoietic Stem Cells, however, whether asymmetric cell division occurs during self-renewal is not known with certainty. It is instead possible that hematopoiesis occurs via symmetrical divisions, that sometimes give rise to two daughter HSC, and that at other times give rise to progeny that are committed to differentiate. The balance between self-renewal versus differentiation would therefore be regulated by the control of these two kinds of symmetrical cell division.

It is known that a small number of HSC can expand to generate a very large number of progeny HSC. This phenomenon is used in bone marrow transplant when a small number of HSC reconstitute the hematopoietic system. This indicates that at least during bone marrow transplant, symmetrical cell divisions that give two progeny HSC must occur, as expansion in HSC numbers seen during bone marrow transplant cannot occur in any other way.

Stem cell self-renewal is thought to occur in the stem cell niche in the bone marrow, and it is reasonable to assume that key signals present in this niche will be important in self-renewal. There is much interest in the environmental and molecular requirements for HSC self-renewal, as understanding the ability of HSC to replenish themselves will eventually allow the generation of expanded populations of HSC ex vivo that can be used therapeutically.

[edit] Source

Sketch of bone marrow and its cells

HSC are found in the bone marrow of adults, which includes femurs, hip, ribs, sternum, and other bones. Cells can be obtained directly by removal from the hip using a needle and syringe, or from the blood following pre-treatment with cytokines, such as G-CSF (granulocyte colony stimulating factors), that induce cells to be released from the bone marrow compartment. Other sources for clinical and scientific use include umbilical cord blood and placenta. For scientific purposes, fetal liver of animals is an occasional source.

[edit] Colony-forming units

There are various kinds of colony-forming units:

• Colony-forming unit lymphocyte (CFU-L)
• Colony-forming unit erythrocyte (CFU-E)
• Colony-forming unit granulo-monocyte (CFU-GM)
• Colony-forming unit megakaryocyte (CFU-Me)
• Colony-forming unit Basophil (CFU-B)
• Colony-forming unit Eosinophil (CFU-Eo)

[edit] Markers

Multipotent hematopoeitic stem cells present various cluster of differentiation markers on their surface: CD34+/- , CD38-, CD90+, CD133+, Lin-, Thy1+, CD105+, CD45+

[edit] Nomenclature

Between 1948 and 1950, the Committee for Clarification of the Nomenclature of Cells and Diseases of the Blood and Blood-forming Organs issued reports on the nomenclature of blood cells.^[1][2] An overview of the terminology is shown below, from earliest to final stage of development:

• [root]blast
• pro[root]cyte
• [root]cyte
• meta[root]cyte
• mature cell name

The root for CFU-E is "rubri", for CFU-GM is "granulo" or "myelo" and "mono", for CFU-L is "lympho" and for CFU-Me is "megakaryo". According to this terminology, the stages of red blood cell formation would be: rubriblast, prorubricyte, rubricyte, metarubricyte and finally erythrocyte. However, the following nomenclature seems to be, at present, the most prevalent:

Osteoclasts also arise from haemopoietic cells of the monocyte/neutrophil lineage, specifically CFU-GM.

[edit] Physical characteristics

Hematopoietic stem cells morphologically resemble lymphocytes. They are non-adherent, rounded, rounded nucleus, and low cytoplasm to nucleus ratio. Since PHSC can not be isolated as a pure population, it is not possible to identify them in a microscope. The above description is based on the morphological characteristics of a heterogeneous population of which PHSC are a component.

[edit] References

1. ^ (1948) "First report of the Committee for Clarification of the Nomenclature of Cells and Diseases of the Blood and Blood-forming Organs.". Amer J Clin Pathol 18: 443-450.
2. ^ (1950) "Third, fourth and fifth reports of the committee for clarification of the nomenclature of cells and diseases of the blood and blood-forming organs.". Am J Clin Pathol 20 (6): 562-79. PMID 15432355.

[edit] Additional images

Hematopoiesis

[edit] See also

• Haematopoiesis
• Dictionary at eMedicine hemocytoblast