The Osteo-Hematon: Modeling the Ontogenic Development of Stem-Cell Niches into Discrete Organotypic Proliferative Units.

Hemogenic stem cells (HSC) emerge at distinct allocation territories during ontogenesis. They irrigate freely the whole body by the blood stream, but the adult HSC expand and differentiate exclusively in the bone marrow (BM). This implies that stem cells are not autonomous units of development; rather, tissue specific niches control their destiny. We investigated the spatio-temporal emergence of stem cells and nest-forming cells, and basic mechanisms by which they shape together the functional-proliferative units in the liver and BM during ontogenesis. Video microscopy in gut/liver primordium cultures provided direct evidence for intrinsic emergence of a primitive vasculo-hemogenic meshwork guided by pulsative, cardiomyogenic cells around nascent hepatic cords in E8.75-E10 mouse embryos. Gain-of-competence and development of hepatic units required firm aggregation of cholangio-hepatic (CK19+/CK18+), hemogenic (CAFC) and vasculogenic stem/progenitor cells and nest-forming cells into compact 3-D structures, similarly to BM hematons. Colonization and expansion of HSC in the mouse liver at E12, then in the BM at postnatal d2, numerically correlated with the increase in isolatable niches/hematons. Time-lapse video microscopy, quantitative colony assays, FACS and immunocytochemistry analyses of the adult human BM showed that the hematon is enriched in mesenchymal stem cells, preadipocytes/adipocytes, contractile myogenic cells, a vascular meshwork, guardian macrophages and hemopoietic stem/progenitor cells (Sca-1/c-kit/CD34/Thy-1/CD31⁺). A hematon module plays the role of an organizing center that emits growth signalling factors and accumulates morphogen/differenciator factors, retinoids and 1,25-di(OH)-vitaminD3. The physical attachment of many hematons to spongy bone suggests that they form the basic structural-functional units in the ultimate, fully competent BM. We suggest designating these units osteo-hematons. A comparative analysis of BM aspirates from healthy donors (n=75) and patients (n=210) revealed the disorganization of osteo-hematon modules in AML prior to therapy and in CML during all stages of the disease.