Versatile stem cells in bone marrow.

The question of whether there is a single pluripotent haemopoietic stem cell or a variety of stem cells each with a capacity for self-replication is still unresolved. Evidence from radiation-chimaeras and from patients with marrow grafts indicates that haemopoietic stem cells are not a homogeneous population. It is suggested that in marrow there is a variety of stem cells, some controlled by recognised factors and others by as yet unknown factors. It is postulated that some of these cells are pluripotent, whereas others are differentiated for a single line, so both polyphyleticists and monophyleticists may have been partly right.