Fig. 1. GAGs are required for responses to GDNF in primary cultures. (a-d) Embryonic dorsal root ganglia form no neurites in the absence of GDNF (a), but form axons quickly in response to 100 ng/ml GDNF (b; arrowheads); inhibition of GAG sulphation with chlorate blocks this response to GDNF (c), but does not block the (stronger) axongenic response to 10 ng/ml NGF (d). (e-h) E10 embryonic intestine develops an extensive network of neurites in culture in response to endogenous GDNF (e); this is reduced by culture in chlorate (f) and inhibited still further by 0.3 U/ml heparinase III (g). Inclusion of 2 mM sulphate in the medium rescues cultures from the effect of chlorate (h) as expected, because chlorate and sulphate compete in the synthesis of the specific donor molecule involved in GAG sulphation.