Impaired dendritic arbor maturation is accompanied by reduced dendritic spine density. For the same population of medium spiny neurons (MSNs) that were sampled for dendritic morphology, spine density was quantified on third-order terminal dendritic branches, which were commonly identified for both groups of neurons. (A–C) Example of third-order terminal branches (white arrows) that were present on the Neurolucida tracing (panel A; purple lines) and the corresponding Golgi-impregnated neuron (B and C). In (A), the yellow, white, purple, green, and blue lines represent first, second, third, fourth, and fifth order branches, respectively. (D) Dendritic spines (white arrows) visualized on a Golgi-impregnated MSN. (E) Spine density in control and HI groups at 4 weeks of recovery. Preterm neurons in the HI group (white bars) had a significant reduction in spines of ~12% vs. controls (black bars); N=108, control and N=99, HI group; p<0.001). Scale bars in B, C = 25 μm and in D = 5 μm.