The Regulation Of Haemopoiesis
Haemopoiesis starts with stem cell division in which one cell replaces the stem cell (self‐renewal) and the other is committed to differentiation. These early committed progenitors express low levels of transcription factors that may commit them to discrete cell lineages. Which cell lineage is selected for differentiation may depend both on chance and on the external signals received by progenitor cells. Several transcription factors (see p. 8) regulate survival of stem cells (e.g. SCL, GATA‐2, NOTCH‐1), whereas others are involved in differentiation along the major cell lineages.For instance, PU.1 and the CEBP family commit cells to the myeloid lineage, whereas GATA‐2 and then GATA‐1 and FOG‐1 have essential roles in erythropoietic and megakaryocytic differentiation. These transcription factors interact so that reinforcement of one transcription programme may suppress that of another lineage. The transcription factors induce synthesis of proteins specific to a cell lineage. For example, the erythroid‐specific genes for globin and haemosynthesis have binding motifs for GATA‐1.