Stromal cells as bystanders. BM cells from CByB6F1 mice treated with 5 Gy TBI and 5 × 10⁶ B6 LN cell infusion 14 days earlier were mixed 1:2 with BM cells from a healthy B6 mouse, and the cell mixtures were then transplanted into new lethally irradiated CByB6F1 recipients. After 2 weeks, BM cells extracted from the recipients were cultured in α-MEM at 33°C with 5% CO₂ along with BM cells from 2 fresh CByB6F1 mice to establish stromal feeder layers After 4 weeks in cell culture, fresh BM cells were overlaid on the established feeder layers. Stromal feeder developed from normal CByB6F1 marrow (left) supported marrow cell growth, but stromal cells from cotransplantation recipients could not establish a feeder layer to support marrow growth (right).