Effect of hGM-CSF on FTOC development. Day-15 fetal thymic lobes were harvested from hGMR-tg and normal control embryos and placed into FTOC. Lobes were cultured in FTOC-CM alone or with the addition of 10 ng/mL or 50 ng/mL hGM-CSF. FTOC were harvested on days 5 and 11 and examined for expression of CD4 and CD8, as described in the Materials and Methods.