removed via a core biopsy (to maintain the architecture or relationship of the cells to each other and to the bone). Subtypes A colony-forming unit is a subtype of HSC. (This sense of the term is different from colony-forming units of microbes, which is a cell counting unit.) There are various kinds of HSC colony-forming units: Colony-forming unit–granulocyte-erythrocyte-monocyte-megakaryocyte (CFU-GEMM) Colony-forming unit–lymphocyte (CFU-L) Colony-forming unit–erythrocyte (CFU-E) Colony-forming unit–granulocyte-macrophage (CFU-GM) Colony-forming unit–megakaryocyte (CFU-Meg) Colony-forming unit–basophil (CFU-B) Colony-forming unit–eosinophil (CFU-Eos) 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. Isolating stem cells Since hematopoietic stem cells cannot be isolated as a pure population, it is not possible to identify them in a microscope. Hematopoietic stem cells can be identified or isolated by the use of flow cytometry where the combination of several different cell surface markers (particularly CD34) are used to separate the rare Hematopoietic stem cells from the surrounding blood cells. Hematopoietic stem cells lack expression of mature blood cell markers and are thus called Lin-. Lack of expression of lineage markers is used in combination with detection of several positive cell-surface markers to isolate hematopoietic stem cells. In addition, hematopoietic stem cells are characterised by their small size and low staining with vital dyes such as rhodamine 123 (rhodamine lo) or Hoechst 33342 (side population). Function Haematopoiesis Hematopoietic stem cells are essential to haematopoiesis, the formation of the cells within blood. Hematopoietic stem cells can replenish all blood cell types (i.e., are multipotent) and self-renew. A small number of Hematopoietic stem cells can expand to generate a very large number of daughter Hematopoietic stem cells. This phenomenon is used in bone marrow transplantation, when a small number of Hematopoietic stem cells reconstitute the hematopoietic system. This process indicates that, subsequent to bone marrow transplantation, symmetrical cell divisions into two daughter Hematopoietic stem cells must occur. 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