<P> Erythropoietin is required for a myeloid progenitor cell to become an erythrocyte . On the other hand, thrombopoietin makes myeloid progenitor cells differentiate to megakaryocytes (thrombocyte - forming cells). The diagram to the right provides examples of cytokines and the differentiated blood cells they give rise to . </P> <P> Growth factors initiate signal transduction pathways, which lead to activation of transcription factors . Growth factors elicit different outcomes depending on the combination of factors and the cell's stage of differentiation . For example, long - term expression of PU. 1 results in myeloid commitment, and short - term induction of PU. 1 activity leads to the formation of immature eosinophils . Recently, it was reported that transcription factors such as NF - κB can be regulated by microRNAs (e.g., miR - 125b) in haematopoiesis . </P> <P> The first key player of differentiation from HSC to a multipotent progenitor (MPP) is transcription factor CCAAT - enhancer binding protein α (C / EBP α). Mutations in C / EBPα are associated with acute myeloid leukaemia . From this point, cells can either differentiate along the Erythroid - megakaryocyte lineage or lymphoid and myeloid lineage, which have common progenitor, called lymphoid - primed multipotent progenitor . There are two main transcription factors . PU. 1 for Erythroid - megakaryocyte lineage and GATA - 1, which leads to a lymphoid - primed multipotent progenitor . </P> <P> Other transcription factors include Ikaros (B cell development), and Gfi1 (promotes Th2 development and inhibits Th1) or IRF8 (basophils and mast cells). Significantly, certain factors elicit different responses at different stages in the haematopoiesis . For example, CEBPα in neutrophil development or PU. 1 in monocytes and dendritic cell development . It is important to note that processes are not unidirectional: differentiated cells may regain attributes of progenitor cells . </P>

Where does haemopoiesis take place after day 18