ATRX expression increases in postmitotic neurons. (A–E) Fixed tissue sections were treated with a polyclonal antibody (H300) detecting the full-length ATRX protein. (A) Coronal section through the cortical region at E13.5 reveals highest expression in the preplate (PP). (B) Sagittal section through the cortex at P0.5 shows high levels of ATRX staining in postmitotic neurons of the marginal zone (MZ), cortical plate (CP), and subplate (SP), but lower levels of staining in the proliferative subventricular (SVZ) and ventricular zones (VZ). (C) Sagittal section at P0.5 through the hippocampus demonstrates high ATRX levels in CA1, CA3, and dentate gyrus (DG) subregions. (D and E) Saggital section at P31 through the brain demonstrates that ATRX expression is maintained in the adult throughout the cortex (CX), the hippocampus (CA1 and CA3), and the dentate gyrus subregions. (F–K) Cortical progenitor cells dissected from E12.5 telencephalon were cultured for 6 days and were costained for ATRX (F) and for MAP2 (G), a marker of neuronal differentiation. The merged image (H) reveals coexpression of ATRX and MAP2 in the differentiated cells. In contrast, progenitors costained for ATRX (I) and proliferating cells as measured by BrdU incorporation (J) show low levels of ATRX in the BrdU-positive cells (K, merged image). Higher magnification (×20, K, bottom-right inset) image shows that BrdU and ATRX staining does not significantly overlap. Magnification, ×20 (A, C–K), ×10 (B).