They are round, non-adherent, with a rounded nucleus and low cytoplasm-to-nucleus ratio. In shape,<unk> s resemble lymphocytes. Location The very first<unk> s during (mouse and human) embryonic development are found in aorta-gonad-mesonephros region and the vitelline and umbilical arteries. Slightly later,<unk> s are also found in the placenta, yolk sac, embryonic head, and fetal liver.<unk> s are found in the bone marrow of adults, especially in the pelvis, femur, and sternum. They are also found in umbilical cord blood and, in small numbers, in peripheral blood. Stem and progenitor cells can be taken from the pelvis, at the iliac crest, using a needle and syringe. The cells can be removed as liquid (to perform a smear to look at the cell morphology) or they can be 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<unk> . (This sense of the term is different from colony-forming units of microbes, which is a cell counting unit.) There are various kinds of<unk>  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<unk>  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<unk> s cannot be isolated as a pure population, it is not possible to identify them in a microscope.<unk> s 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<unk> s from the surrounding blood cells.<unk> s 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<unk> s. In addition,<unk> s are characterised by their small size and low staining with vital dyes such as rhodamine 123 (rhodamine lo) or Hoechst 33342 (side population).
Hematopoietic stem cell
Lymphoblast
Stem cell
Progenitor cell
Megakaryocyte–erythroid progenitor cell
Thymocyte
Monocyte
Dendritic cell