Direct contact between human primitive hematopoietic progenitors and bone marrow stroma is not required for long-term in vitro hematopoiesis

Long-term bone marrow cultures support both differentiation and conservation of primitive human hematopoietic progenitors in the absence of exogenous cytokines. It is believed that hematopoiesis in such cultures requires direct contact between hematopoietic progenitors and stroma. In the present study, we demonstrate that primitive progenitors physically separated from the stromal layer by a 0.45-microns microporous membrane continue to generate differentiated progenitors for at least 8 weeks. Moreover, primitive progenitors are conserved to a greater extent under these conditions, as when cultured in direct contact with the stroma. However, excessive production of granulocyte-macrophage progenitors occurs when primitive progenitors are not allowed to interact directly with the stroma. Thus, direct contact between hematopoietic and stromal cells is not required for either differentiation or conservation of primitive hematopoietic progenitors but is essential for the regulated production of mature blood elements. These findings can now be used to define the role of diffusible factors and cell-cell or cell-extracellular matrix adhesion events in the regulation of conservation, proliferation, and differentiation of primitive human hematopoietic progenitors in vitro.