We have reported a mesencephalic microisland culture method (J Neurosci 1994; 14: 4769-79) and expansion of dopaminergic neurons with microisland culture (SFN Abstr. 1997;23:593). We explored further expansion of mesencephalic neural progenitors (MNP) and their differentiation. MNPs were prepared from 1mm3 tissue blocks isolating from E14 rat ventral mesenscephalon. Dispersed cells were cultured in 75cm2 flasks (5.0 x 105 cells/flask) using an expansion medium (EM; DMEM/F12+TIPS+ bFGF) with/without serum priming (SP; 24hours-2% FCS). Expanded MNPs were collected at the 14 days in cultures and were frozen for later experiments. Striatal neural progenitors (SNPs) were also prepared by the same procedures. Thawed MNPs and SNPs were cultured 7 days in EM with/without SP. Growth rates of MNPs were 33 folds in SP cultures and 123 folds in serum-free cultures. On the contrary, those of SNPs were 111 folds and 13.2 folds, respectively. These results indicated that the effects of serum to neural progenitors are dif...