Effect of hGM-CSF on various gestational ages of fetal thymic lobes. Fetal thymic lobes from fetal day (FD) 14, 15, and 16 embryos were harvested from hGMR-tg and normal control fetuses and placed into FTOC with or without 50 ng/mL hGM-CSF. After 7 days of culture, the cells were harvested and analyzed for the expression of CD4 and CD8. FTOC established from day-14 transgenic lobes showed marked inhibition in the generation of DP cells, whereas day-15 and -16 FTOC generated increasing numbers of DP cells.