A standard model of blood cell development is shown in the diagram below.
As you can see, the progenitor blood cells (hemocytoblasts) can differentiate into any type of blood cell. After the first step in differentiation (shown in the second highest line), the pathway is fixed so that each cell ends up being either an erythrocyte (red blood cell), leukocyte (white blood cell) or thrombocyte (platelet cell). At least, that was the thinking until some experiments, led by Ashley Ng of the Walter and Eliza Hall Institute of Medical Research, proved otherwise.
The researchers found that cells (megakaryoblasts) thought to lead solely to the platelet line could in fact give rise to red blood cells. In fact, many of the progenitor cells had far more flexibility than was previously thought. Apparently, the production of different types of blood cells within bone marrow is regulated, not at the first step of differentiation, but much later on. The scientists could even control the blood cell output by applying different chemical signals. Needless to say, this could turn out to be extremely useful. Being able to boost or deplete specific populations of blood cells could be invaluable in treating blood diseases.
Although this data was only collected in mice, it may very well be applicable to humans. If so, it changes the entire picture of how blood cells are made and regulated.