A method of preparing a stromal cell conditioned medium useful in expanding undifferentiated hemopoietic stem cells to increase the number of the hemopoietic stem cells is provided. The method comprising: (a) establishing a stromal cell culture in a stationary phase plug-flow bioreactor under continuous flow on a substrate in the form of a sheet, the substrate including a non-woven fibrous matrix forming a physiologically acceptable three-dimensional network of fibers, thereby expanding undifferentiated hemopoietic stem cells; and (b) when a desired stromal cell density has been achieved, collecting medium from the stationary phase plug-flow bioreactor, thereby obtaining the stromal cell conditioned medium useful in expanding the undifferentiated hemopoietic stem cells.