Terminal differentiation

Cell differentiation ultimately proceeds as a response to signals from the extracellu- lar microenvironment. If the cells are competent, that is, if they carry the correspond- ing receptors and signal transduction pathways, they can be triggered in the commit- ment state. In many cases the response includes complex gene expression programs whose progress and completion are relatively autonomous and whose outcome is determined by inherent regulatory factors of the cell type affected.' As a short introduction to the problems of differentiation and aging, we survey the main points that characterize the differentiation of hematopoietic cells. We can identify several main aspects.