Recent reports indicate that haematopoietic cells can convert into non-haematopoietic cells after transplantation or injury, but how relevant is haematopoietic plasticity during normal development and in the maintenance of adult organs? The answer, say Stadtfeld and Graf, is not greatly (see p. 203). The researchers derived a mouse line in which essentially all the haematopoietic cells, including foetal and adult stem cells, irreversibly express yellow fluorescent protein, and another line in which macrophages and some nonmyeloid haematopoietic cells are similarly labelled. Tracking experiments show that although haematopoietic cells form some hepatocytes in unperturbed mice, this happens at a very low frequency, which only increases two- to threefold after acute liver damage. No haematopoietic cells convert to endothelial cells. These results, which support the concept that germ layer specification sets subsequent developmental boundaries, have important implications for the use of haematopoietic stem cells in tissue replacement applications.

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