Immunohistochemistry for NeuN (a neuronal marker) and ID1 was performed using a published procedure (28 (link)). The sections were incubated in mouse anti-NeuN (1:800; MAB377; EMD Millipore, Billerica, MA, USA) or rabbit anti-ID1 (1:200; sc-488; Santa Cruz Biotechnology, Inc.) as primary antibodies, and in biotinylated horse anti-mouse (1:200; BA-2001; Vector Laboratories, Inc., Burlingame, CA, USA or rabbit anti-goat immunoglobulin G (1:200; BA-1100; Vector Laboratories, Inc.) and streptavidin peroxidase complex (VECTASTAIN® Elite ABC kit 1:200; Vector Laboratories, Inc.) as secondary antibodies. The antibodies were finally visualized with 3,3′-diaminobenzidine tetrahydrochloride. A negative control test was performed to establish the specificity of the immunostaining using pre-blocking serum (S-1000; Vector Laboratories, Inc.) instead of primary antibody. The negative control test showed no immunoreactivity in structures observed.