A liquid culture system is described whereby proliferation of haemopoietic stem cells (CFU-S), production of granulocyte precursor cells (CFU-C), and extensive granulopoiesis can be maintained in vetro for several months. Such cultures consist of adherent and non-adherent populations of cells. The adherent population contains phagocytic mononuclear cells, "epithelial" cells, and "giant fat" cells. The latter appear to be particularly important for stem cell maintenance and furthermore there is a strong tendency for maturing granulocytes to selectively cluster in and around areas of "giant fat" cell aggregations. By "feeding" the cultures at weekly intervals, between 10 to 15 "population doublings" of functionally normal CFU-S regularly occurs. Increased "population doublings" may be obtained by feeding twice weekly. The cultures show initially extensive granulopoiesis followed, in a majority of cases, by an accumulation of blast cells. Eventually both blast cells and granulocytes decline and the cultures contain predominantly phagocytic mononuclear cells. Culturing at 33 degrees C leads to the development of a more profuse growth of adherent cells and these cultures show better maintenance of stem cells and increased cell density. When tested for colony stimulating activity (CSA) the cultures were uniformly negative. Addition of exogenous CSA caused a rapid decline in stem cells, reduced granulopoiesis and an accumulation of phagocytic mononuclear cells.

It has been proposed that during embryonic development haematopoietic cells arise from a mesodermal progenitor with both endothelial and haematopoietic potential called the haemangioblast1,2. A conflicting theory associates instead the first haematopoietic cells with a phenotypically differentiated endothelial cell with haematopoietic potential, i.e. a haemogenic endothelium3-5. Support for the haemangioblast concept was initially provided by the identification during embryonic stem (ES) cells differentiation of a clonal precursor, the blast colony-forming cell (BL-CFC), which gives rise to blast colonies with both endothelial and haematopoietic components6,7. Although recent studies have now provided evidence for the presence of this bipotential precursor in vivo8,9, the precise mechanism of generation of haematopoietic cells from the haemangioblast still remains completely unknown. Here we demonstrate that the haemangioblast generates haematopoietic cells through the formation of a haemogenic endothelium intermediate, providing the first direct link between these two precursor populations. The cell population containing the haemogenic endothelium is transiently generated during BL-CFC development. This cell population is also present in gastrulating embryos and generates haematopoietic cells upon further culture. At the molecular level, we demonstrate that the transcription factor Scl/Tal110 is indispensable for the establishment of this haemogenic endothelium population whereas the core binding factor Runx1/AML111 is critical for generation of definitive haematopoietic cells from haemogenic endothelium. Together our results merge into a single linear developmental process the two a priori conflicting theories on the origin of haematopoietic development.

Nuclear pore complexes (NPCs) are large multiprotein assemblies that allow traffic between the cytoplasm and the nucleus. During mitosis in higher eukaryotes, the Nuclear Envelope (NE) breaks down and NPCs disassemble. How NPCs reassemble and incorporate into the NE upon mitotic exit is poorly understood. We demonstrate a function for the conserved Nup107-160 complex in this process. Partial in vivo depletion of Nup133 or Nup107 via RNAi in HeLa cells resulted in reduced levels of multiple nucleoporins and decreased NPC density in the NE. Immunodepletion of the entire Nup107-160 complex from in vitro nuclear assembly reactions produced nuclei with a continuous NE but no NPCs. This phenotype was reversible only if Nup107-160 complex was readded before closed NE formation. Depletion also prevented association of FG-repeat nucleoporins with chromatin. We propose a stepwise model in which postmitotic NPC assembly initiates on chromatin via early recruitment of the Nup107-160 complex.

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