A method is provided, including in vitro stimulating an initiating cell population (ICP) of at least 5 million cells that have a density of less than 1.072 g/ml, and at least 1% of which are CD34+CD45−/dim, to differentiate into a progenitor/precursor cell population (PCP). A method is provided, including in vitro stimulating an initiating cell population (ICP) of at least ten thousand cells that have a density of less than 1.072 g/ml to differentiate into a progenitor/precursor cell population (PCP). A method is provided, including separating lower density cells from higher density cells, the lower density cells defining an initiating cell population (ICP), and in vitro stimulating the ICP to differentiate into a progenitor/precursor cell population (PCP). Other embodiments are also described.