Golgi-impregnated medium-sized spiny neurons (MSNs) in the caudate nucleus (A, A′, B, B′) and putamen (C, C′, D, D′) of control (A, A′, C, C′) and MPTP-treated monkeys (B, B′, D,D′). A′, B′, C′ and D′: High magnification images of dendrites labeled with an arrow in A, B, C and D, respectively. These dendrites correspond to the longest dendritic processes that were analyzed for quantitative studies. Note the dramatic reduction of spines on the dendrites of MSNs from MPTP-treated monkeys. Scale bar in A and C = 25 μm (valid for B and D). Scale bar in B′ = 5 μm (valid for A′, C′ and D′).