Haematopoiesis

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Diagram that shows the development of different blood cells from hematopoietic stem cell to mature cells

Haematopoiesis (from Ancient Greek: haima blood; poiesis to make) (or Hematopoiesis in the United States) is the formation of blood cellular components. All of the cellular components of the blood are derived from haematopoietic stem cells. The term pluripotent refers to the ability of a cell to become many different types of cell. Pluripotent haematopoetic cells can become any type of cell in the blood system. The pluripotent cells determine what type of cell to become, or differentiate, in a step-wise fashion.

1 Lineages
2 Maturation
3 Locations
4 External links

[edit] Lineages

All blood cells are divided into three lineages.

Erythroid cells are the oxygen carrying red blood cells.
Lymphoid cells are the cornerstone of the adaptive immune system. The lymphoid lineage is primarily composed of T-cells and B-cells. (white blood cells)
The third lineage of blood cells is the myeloid lineage which includes granulocytes, megakaryocytes, and macrophages. The myeloid lineage is involved in such diverse roles innate immunity, adaptive immunity, and blood clotting.

[edit] Maturation

As a stem cell matures it undergoes changes in gene expression (the levels of genes change) that limit the cell types that it can become and move it closer to a specific cell type. These changes can often be tracked by monitoring the presence of proteins on the surface of the cell. Each succesive change moves the cell closer to its final choice of cell type and further limits its potential cell type until it is fully differentiated. This process is usually presented as a dendrogram or decision tree starting with a stem cell at the single starting point and branches for the major lineages that branch into intermediate semi-differentiated cell types and eventually to fully differentiated cells.

[edit] Locations

In developing embryos, blood formation occurs in aggregates of blood cells in the yolk sac, called blood islands. As development progresses, blood formation occurs in the spleen, liver and lymph nodes. When bone marrow develops, it eventually assumes the task of forming most of the blood cells for the entire organism. However; maturation, activation, and some proliferation of lymphoid cells occurs in secondary lymphoid organs (spleen, thymus, and lymph nodes). While most haematopoiesis in adults occurs in the marrow of the long bones such as the femurs, it also occurs in spongy bone like ribs and sternum). In some cases, the liver, thymus, and spleen may resume their haematopoietic function if necessary (called extramedullary haematopoiesis).

In some vertebrates, haematopoiesis can occur wherever there is a loose stroma of connective tissue and slow blood supply, such as the gut, spleen, kidney or ovaries.