Haemopoietic stem cell: a new concept.

It is generally accepted that morphologically recognizable bone marrow (BM) cells are derived from progenitor cells committed to a specific line of haematopoietic differentiation [51]. The origin and morphological identity of such progenitor cells is not yet known and the question, whether there is a single pluripotent haematopoietic stem cell (HSC) or a variety of stem cells each with a capacity of self-replication and maintenance is still unresolved [37]. Although progress has been made in segregating [43, 47, 49, 64] various progenitor cells from one another and functional assays [11, 17, 42] have significantly promoted the study of haemopoietic precursor cells, morphological investigations were less rewarding. Even after enrichment, the morphology of haemopoietic stem- and colony-forming cells only remained hypothetical, 'candidate stem cells' [3], because their precursor cell qualities could not be deduced from their morphology, but only retrospectively, and statistically from their function, i.e. in vitro colony-formation or in-vivo haemopoietic reconstitution from an enriched cell suspension [58]. We postulate from our studies on semi-thin sections of undecalcified BM from healthy human fetuses, normal adults and patients with acute myeloid leukaemia (AML) and chronic granulocytic leukaemia (CGL) that the endosteal cells are the precursors of all haematopoietic cells in the bone marrow and are also capable of transformation into either stromal (fibroblast-like cells) or bone-related cells (osteoblasts and osteoclasts).