Ontogenic emergence of the hematon, a morphogenetic stromal unit that supports multipotential hematopoietic progenitors in mouse bone marrow.

Development of the full repertoire of hematopoietic-lymphopoietic cells from a single stem cell requires specific contacts with stromal cells. The spatio-temporal organization of these cell associations in the bone marrow in ontogeny is, however, not well understood. In the adult, 10% of marrow cells form a cohort of compact aggregates, the hematon. In the hematon mesenchymal cells (Stro-1(+)), perivascular lipocytes (desmin(+)), endothelial cells (CD34(+), Flk-1(+), Sca-1(+)), and macrophages amalgamate with the hematopoietic progenitors long-term culture-initiating cells (LTC-IC), cobblestone area-forming cell (CAFC), high-proliferative-potential colony-forming unit (HPP-CFU), granulocyte-macrophage (GM)-CFU, and burst-forming unit-erythroid (BFU-E). During endochondral ossification of the femur, GM-CFU and day 7 CAFC numbers increased progressively from day 17 of gestation, but primitive, day 35 LTC-IC appeared from postnatal day 2. Unexpectedly, bone marrow (BM) taken between embryonic day 17 and day 5 was unable to support myeloid cell production in long-term cultures or to support day 35 LTC-IC growth. However, a gain in stromal cell competence occurred between days 7 and 10, which was correlated with the emergence of hematon in the BM. Thus, acquisition of hematopoietic competence by BM lags behind for approximately 10 days after the initial hematopoietic cell influx. In the adult, the hematon fraction was 3.7-fold enriched in day 35 LTC-IC over the buffy coat. It produced more GM-CFU and HPP-CFU in myeloid culture and more B cells in lymphopoietic "switch" cultures. It is reported that stromal hematopoietic units named hematons are specific morphogenetic structures that emerge at a well-defined postnatal stage of development in long bones, delineate discrete territories for hematopoietic stem cell seeding and development, embody the most productive hematogenous compartment in the BM, and probably enclose a morphogenetic organizer. (Blood. 2000;96:3763-3771)