Expression of Ret and GFRα-1 increases in culture. Sections of fresh cord and of slices cultured for 6 d in the presence of GDNF were stained for the signaling component of the GDNF receptor complex Ret (A, B) and for the preferred GDNF coreceptor GFRα-1 (C, D). A, C, Both Ret and GFRα-1 are clearly expressed in fresh cord, but staining is of variable intensity. B, D, By 6 DIV, in the presence of GDNF both receptor components are clearly upregulated in the surviving MNs. Antibody specificity was confirmed by an absence of staining in Ret^−/− and GFRα-1^−/− mice (data not shown).