Cell culture was carried out at 378C and 5% CO2 in a humidified atmosphere. P19 cells, subclone 18 w10x, were cultivated in a-MEM medium. containing 10% fetal bovine serum . and passaged every other day using 0.2% trypsin-EDTA . For neuronal differentiation 4 = 10 5 cells were seeded onto 10 cm Petri dishes of cell culture grade. and grown for 24 h in a-MEM with 10% FBS. Medium was replaced by a-MEM with 2% FBS containing all-trans retinoic acid ŽSigma. in a final concentration of 1 mM; 48 h after induction culture medium was replaced by a-MEM with 2% FBS without retinoic acid. Medium was changed every other day. Cultures were harvested after 1, 5, 10 and 14 days after addition of retinoic acid. Undifferentiated control cultures were grown exclusively in a-MEM with 10% FBS and harvested 48 h after seeding onto Petri
dishes. For non-radioactive in situ hybridization, cells were grown on glass slides coated with poly-D-lysine
2.3. RNA preparation
RNA was extracted from cultivated cells by the method of Chomczynski and Sacchi w7x
with slight modifications.
Cells were lysed in the Petri dish immediately after removing the culture medium and washing cells with sterile PBS.
Typically, the yield of RNA from a 10 cm culture dish was 10–100 mg.