 Sketch of bone marrow and its cells Pluripotential hemopoietic stem cells (PHSCs) are stem cells found in the bone marrow. They are called pluripotential because these cells have the ability to differentiate into any of several types of blood cells. Hemopoiesis is the formation of blood cellular components.
Background
The existence of these cells were determined by clever experiments on animals. Bone marrow cells are extracted from an animal, which is then subjected to radiation. This destroys any bone marrow cells. Once this is accomplished, the previously collected cells are then injected into the blood stream and eventually make their way into the spleen (which has a similar environment as bone marrow for hemopoiesis). The cells multiply and form colonies, which contain only certain types of blood cell or blood cell precursors. These cells are defined as colony-forming units (CFU).
Colony-forming units
There are four kinds of colony-forming units:
- Colony-forming unit erythrocyte (CFU-E)
- Colony-forming unit granulo-monocyte (CFU-GM)
- Colony-forming unit lymphocyte (CFU-L)
- Colony-forming unit megakaryocyte (CFU-Me)
CFU-E
These cells are precursors for red blood cells, or erythrocytes. The process of red blood cell formation is called erythropoiesis.
The cell differentiates as follows:
- Proerythroblast
- Polychromatophilic erythrocyte
- Normoblast
- Erythrocyte
CFU-GM
These cells are precursors for granulocytes (basophil, eosinophil, neutrophil) or monocytes. The processes of cell formation are called granulocytopoiesis and monocytopoiesis
The granulocyte precursor cell differentiates as follows:
- Myeloblast
- Promyelocyte
- Eosino/neutro/basophilic myelocyte
- Eosino/neutro/basophil
The monocyte precursor cell differentiates as follows:
- Promonocyte
- Early monocyte
- Monocyte
CFU-L
These cells are precursors for lymphocytes. The process of lymphocyte formation is called lymphocytopoiesis.
The cell differentiates as follows:
- Lymphoblast
- Prolymphocyte
- Large lymphocyte
- Small lymphocyte
Prolymphocyte that released into the blood stream and travel to the spleen or lymph nodes differentiate into B-lymphocytes and those that travel into the thymus differentiate into T-lymphocyte.
CFU-Me
These cells are precursors for megakaryocytes, which later form blood platelets or thrombocytes. The process of thrombocyte formation is called thrombocytopoiesis.
The cell differentiates as follows:
- Megakaryoblast
- Promegakaryocyte
- Megakaryocyte
Platelets are then formed from the fragmentation of the megakaryocyte cytoplasm.
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. 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. . The nomenclature used in the article seems to be, at present, the most prevalent.
Physical Characteristics
The physical characteristic of each stage of a cell's development is somewhat subjective but usually specific. However, since there is no consensus on the number of stages in the development of any given cell, it would be difficult to attribute characteristics since it may not apply across all systems being used. For instance, characteristics that define a metamyelocyte could be considered either the mature cell or a myelocyte in the system used in this article.
However, general trends can be observed as the cell matures:
- Size of the cell and nucleus gradually shrinks
- Ratio of nucleus to cytoplasm decreases
- Color of the nucleus becomes darker
- Nucleoli disappears
- Shape of the nucleus changes from round to irregularly shaped
These are only general guidelines which may not all apply to every cell. A notable exception is the mature lymphocyte, which generally keeps a round nucleus.
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