Abstract: A method for the differentiation of mammalian pluripotent stem (PS) cells into a mortal multi-lineage progenitor cell population is provided which comprises culturing the pluripotent stem cells in the presence of Hyaluronan (HA). The mortal multi-lineage progenitor cell population may be a population of mesenchymal stem cells. The mortal multi-lineage progenitor cell population may form cells of the mesodermal lineage, suitably osteoblasts. Alternatively, the mortal multi-lineage progenitor cell population may form cells of the endodermal lineage or of the ectodermal lineage, which may be neuronal progenitors.