[Hematopoietic stem cells of the human. Function and morphology]

Pluripotential haematopoietic stem cells and their progeny, the so-called committed precursor cells, i.e., progenitor cells which are already lineage-restricted, may be identified by the membrane-bound expression of CD34. In accordance with this peculiar property it became possible to enrich and characterize primitive precursor cells by using different methods of cell separation techniques, which involved fluorescence staining or ferro-magnetic particles bound to CD34 antibodies. Recently conducted studies demonstrate that CD34-positive (CD34+) stem cells of the peripheral blood represent a relatively uniform cell population with almost round nuclei, a finely dispersed chromatin pattern and a small portion of weakly basophilic cytoplasm. From the cytological viewpoint they resemble so-called large stimulated lymphocytes (virocytes). Ultrastructural studies are compatible with a paucity of organelles and a lymphoid character of these progenitors. In comparison, the stem cell population, derived from the bone marrow consists of more heterogeneous elements. These are generally larger and reveal an admixture of fairly immature as well as more differentiated cells, sharing bean-shaped or indented nuclei with prominent nucleoli and a more extended cytoplasm. CD34+ progenitors from the peripheral blood and those from the bone marrow display a co-expression of CD43 (MT1) and CD45 (LCA). Furthermore, different subpopulations exhibit--dependent on their origin (blood/bone marrow) and to a various extent--lineage-restricted markers like CD33, CD38, CD61, CD20, CD11a/c, glycophorin C und CD15 (LeuM1). The recently developed immuno- and ferromagnetic enrichment methods for CD34+ progenitor cells are considered innovative tools for modern oncology. These techniques play an important role in the treatment of haematological malignancies and advanced tumours in the context of autologous and, although so far rarely applied, heterologous stem cell transplantation procedures.