Hematopoietic Stem-Cell Transplantation for Acquired Aplastic Anemia

The advent of clonal assays for pluripotent stem cells and hematopoietic progenitors, as well as the characterization of lymphohematopoietic cell surface antigens by flow cytometry, has resulted in a well-described developmental model of hematopoiesis. This schema provides a background in which to discuss marrow failure, both acquired and inherited. The proliferation and differentiation of pluripotent stem cells give rise to progeny that can populate the entire immunologic and hematopoietic systems (1). The immediate offspring of the stem cell are the committed progenitors of both the lymphoid and myeloid lineages. The multipotent myeloid progenitor, the CFU-GEMM, gives rise to the committed progenitors. In turn, each of these progenitors differentiates into the recognizable precursors of the granulocyte, erythrocyte, monocyte/macrophage, megakaryocyte, eosinophil, and basophil lineages. These progenitors appear as immature, undifferentiated mononuclear cells and are present in small numbers in the bone marrow.