GDNF supports the long-term survival of MNs in slice culture. A, Slices were cultured for 6 d in increasing concentrations of GDNF. Nissl-stained LMC motor neurons were counted in 12 μm frozen sections of the slices as described in Materials and Methods. MN survival is expressed as a percentage of the mean number present in 12 μm sections of fresh cord.B, In a separate experiment, slices were cultured for 1, 3, or 7 d in the presence (filled squares) or absence (filled circles) of GDNF (200 ng/ml). *p < 0.05; **p < 0.001.