Expression of megakaryocytic and myeloid markers in blasts of transient abnormal myelopoiesis in a stillbirth with Down syndrome: report of histopathological findings of an autopsy case.

Transient abnormal myelopoiesis in neonates with Down syndrome is an unusual leukemia that spontaneously regresses within several months of life and is thought to arise in the fetal liver. It is largely unknown how the leukemic blasts proliferate and differentiate in fetal tissues. We report the histopathological findings of an autopsy case of a stillbirth with transient abnormal myelopoiesis. Blood vessels in almost all organs were filled with immature leukemic cells, most of which expressed megakaryocyte antigen CD42b. In contrast, leukemic cells infiltrating the tissues, including the pericardium, expressed myeloperoxidase. These findings indicate that leukemic progenitors in transient abnormal myelopoiesis can differentiate along both megakaryocytic and myeloid lineages, which may be influenced by microenvironmental factors. Numerous dysplastic mature/immature megakaryocytes and blasts were present in the liver, whereas the bone marrow contained predominantly myeloid cells at various stages of differentiation, suggesting that the fetal liver is the major organ for proliferation of blasts in transient abnormal myelopoiesis.