To isolate bone marrow cells from the mouse , isolate the femur bones and place in a tissue culture plate. Trim the muscle from the bones and remove from the knee cap. Clip both ends of the bone, and flush the cells out into a 50 ml centrifuge tube using 10 mls of media in a syringe with 26 gauge needle . Centrifuge the cells. To isolate RNA from the pelleted cells, I would recommend lysing the cells in Trizol and follow the manufacturers protocol to purify the RNA.