Hemopoietic Staminal Cell (second part)

Physical characteristics
With regard to morphology, hematopoietic stem cells resemble lymphocytes. They are non-adherent, and rounded, with a rounded nucleus and low cytoplasm-to-nucleus ratio. Since PHSC cannot 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.

Markers
In reference to phenotype, hematopoeitic stem cells are identified by their small size, lack of lineage (lin) markers, low staining (side population) with vital dyes such as rhodamine 123 (rhodamineDULL, also called rholo) or Hoechst 33342, and presence of various antigenic markers on their surface, many of which belong to the cluster of differentiation series, like: CD34, CD38, CD90, CD133, CD105, CD45 and also c-kit- the receptor for stem cell factor. The hematopoietic stem cells are negative for the markers that are used for detection of lineage commitment, and are, thus, called Lin-; and, during their purification by FACS, a bunch of up to 14 different mature blood-lineage marker, e.g., CD13 & CD33 for myeloid, CD71 for erythroid, CD19 for B cells, CD61 for megakaryocytic, etc. for humans; and, B220 (murine CD45) for B cells, Mac-1 (CD11b/CD18) for monocytes, Gr-1 for Granulocytes, Ter119 for erythroid cells, Il7Ra, CD3, CD4, CD5, CD8 for T cells, etc. for mice) antibodies are used as a mixture to deplete the lin+ cells or late multipotent progenitors (MPP)s.
There are many differences between the human and mice hematopoietic cell markers for the commonly-accepted type of hematopoietic stem cells.[1].
* Mouse HSC : CD34lo/-, SCA-1+ , Thy1.1+/lo, CD38+, C-kit+, lin-
* Human HSC : CD34+, CD59+, Thy1/CD90+,CD38lo/-, C-kit/CD117+, lin-
However, not all stem cells are covered by these combinations that nonetheless have become popular. In fact, even in humans, there are hematopoietic stem cells that are CD34-/CD38-. [5][6]. Also some later studies suggested that earliest stem cells may lack c-kit on the cell surface[7]. For human HSCs use of CD133 was one step ahead as both CD34+ and CD34- HSCs were CD133+.
Traditional purification method used to yield a reasonable purity level of mouse hematopoietic stem cells, in general, requires a large(~10-12) battery of markers, most of which were surrogate markers with little functional significance, and thus partial overlap with the stem cell populations and sometimes other closely-related cells that are not stem cells. Also, some of these markers 9eg Thy1) are not conserved across mouse species, and use of markers like CD34- for HSC purification requires mice to be at least 8 weeks old. Alternative methods that could give rise to similar or better harvest of stem cells is a hot area of research and are presently emerging. One such method uses a signature of SLAM family of cell surface molecules. SLAM (Signaling lymphocyte activation molecule) family is a group of >10 molecules whose genes are mostly located tandemly in a single locus on chromosome 1 (mouse), all belonging to a subset of immunoglobulin gene superfamily, and originally thought to be involved in T-cell stimulation. This family includes CD48, CD150, CD244, etc., CD150 being the founding member, and, thus, also called slamF1 ie SLAM family member 1.
The signature SLAM code for the hemapoietic higherchy are:
* Hematopoietic stem cells (HSC) : CD150+CD48-CD244-
* Multipotent progenitor cells (MPPs) : CD150-CD48-CD244+
* Lineage-restricted progenitor cells (LRPs) : CD150-CD48+CD244+
For HSCs, CD150+CD48- was sufficient instead of CD150+CD48-CD244- because CD48 is a ligand for CD244, and both would be positive only in the activated lineage-restricted progenitors. It seems that this code was more efficient than the more tedious earlier set of the large number of markers, and are also conserved across the mouse strains; however, recent work has shown that this method excludes a large number of HSCs and includes an equally large number of non-stem cells. [8] [9]. CD150+CD48- gave stem cell purity comparable to Thy1loSca-1+lin-c-kit+ in mice.[10]
Irving Weissman's group at Stanford University that was the first to isolate mouse hematopoietic stem cells in 1988, was also the first to work out the markers to distinguish the mouse long-term (LT-HSC) and short-term (ST-HSC) hematopoietic stem cells (self-renew-capable), and the Multipotent progenitors (MPP, low or no self-renew capability — the later the developmental stage of MPP, the lesser the self-renewal ability and the more of some of the markers like CD4 and CD135):
* LT-HSC : CD34-, SCA-1+ , Thy1.1+/lo, C-kit+, lin-, CD135-, Slamf1/CD150+
* ST-HSC : CD34+, SCA-1+ , Thy1.1+/lo, C-kit+, lin-, CD135-, Slamf1/CD150+, Mac-1 (CD11b)lo
* Early MPP : CD34+, SCA-1+ , Thy1.1-, C-kit+, lin-, CD135+, Slamf1/CD150-, Mac-1 (CD11b)lo, CD4lo
* Late MPP : CD34+, SCA-1+ , Thy1.1-, C-kit+, lin-, CD135high, Slamf1/CD150-, Mac-1 (CD11b)lo, CD4lo

Nomenclature of hematopoietic colonies and lineages
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.[11][12] 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

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

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)
The above CFUs are based on the lineage. Another CFU, the colony-forming unit–spleen (CFU–S) was the basis of an in vivo clonal colony formation, which depends on the ability of infused bone marrow cells to give rise to clones of maturing hematopoietic cells in the spleens of irradiated mice after 8 to 12 days. It was used extensively in early studies, but is now considered to measure more mature progenitor or Transit Amplifying Cells rather than stem cells.

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