Adult stem cells are undifferentiated cells that are found in most animal tissues. Adult stem cells can divide by mitosis throughout their lifespan to form identical stem cells (self-renewal) or to form cells that can differentiate into the functioning cells of that tissue. Haematopoietic stem cells (HSCs) are adult stem cells that are located in the bone marrow of bones. HSCs have a role in the formation of blood cells. Fig. 4.1 is an outline summary showing the formation of some of the different types of blood cell that can be formed from HSCs. The first stage is the division of HSCs to produce progenitor cells. These cells are also able to divide by mitosis, but are not stem cells. Key = progenitor cells [Figure 4.1: Flowchart illustrating the differentiation of Haematopoietic Stem Cells (HSCs) into various blood cell types, including progenitor cells (CMP cells, GMP cells, CLP cells, MEP cells), immature red blood cells, megakaryocytes, immature neutrophils, immature monocytes, immature B-lymphocytes, immature T-lymphocytes, mature monocytes, mature B-lymphocytes, mature red blood cells, platelets, mature neutrophils, and mature T-lymphocytes (T-helper and T-killer cells). Cell types X and Y are also indicated.]