Human embryonic stem (ES) cells can be maintained in an undifferentiated state if the culture medium is first conditioned on a layer of mouse embryonic fibroblast (MEF) feeder cells. Here we show that human ES cell proliferation is coordinated by MEF-secreted heparan sulfate proteoglycans (HSPG) in conditioned medium (CM). These HSPG and other heparinoids can stabilize basic fibroblast growth factor (FGF2) in unconditioned medium at levels comparable to those observed in CM. They also directly mediate binding of FGF2 to the human ES cell surface, and their removal from CM impairs proliferation. Finally, we have developed a purification scheme for MEF-secreted HSPG in CM. Using column chromatography, immunoblotting, and mass spectrometry-based proteomic analysis, we have identified multiple HSPG species in CM. The results demonstrate that HSPG are key signaling cofactors in CM-based human ES cell culture.

We have previously reported that high concentrations of basic fibroblast growth factor (bFGF) support feeder-independent growth of human embryonic stem (ES) cells, but those conditions included poorly defined serum and matrix components. Here we report feeder-independent human ES cell culture that includes protein components solely derived from recombinant sources or purified from human material. We describe the derivation of two new human ES cell lines in these defined culture conditions.

Human embryonic stem (ES) cells have most commonly been cultured in the presence of basic fibroblast growth factor (FGF2) either on fibroblast feeder layers or in fibroblast-conditioned medium. It has recently been reported that elevated concentrations of FGF2 permit the culture of human ES cells in the absence of fibroblasts or fibroblast-conditioned medium. Herein we compare the ability of unconditioned medium (UM) supplemented with 4, 24, 40, 80, 100, and 250 ng/ml FGF2 to sustain low-density human ES cell cultures through multiple passages. In these stringent culture conditions, 4, 24, and 40 ng/ml FGF2 failed to sustain human ES cells through three passages, but 100 ng/ml sustained human ES cells with an effectiveness comparable to conditioned medium (CM). Two human ES cell lines (H1 and H9) were maintained for up to 164 population doublings (7 and 4 months) in UM supplemented with 100 ng/ml FGF2. After prolonged culture, the cells formed teratomas when injected into severe combined immunodeficient beige mice and expressed markers characteristic of undifferentiated human ES cells. We also demonstrate that FGF2 is degraded more rapidly in UM than in CM, partly explaining the need for higher concentrations of FGF2 in UM. These results further facilitate the large-scale, routine culture of human ES cells and suggest that fibroblasts and fibroblast-conditioned medium sustain human ES cells in part by stabilizing FGF signaling above a critical threshold.