The antibody against autism-linked transcription factor T-Box Brain 1 (TBR1), identifies developing neuroblasts of the subplate (SP) and cortical plate (CP), which provide the first pioneer neurons of the developing cerebral cortex47. TBR1 is necessary for neuronal differentiation of NPCs and is a potential master regulator in autism spectrum disorders48. At 5 weeks of control iPSC organoid development, cells expressing nuclear TBR1 were distributed throughout the entire CZ and IZ (Fig. 4, a1). In contrast, in schizophrenia organoids, TBR1+ cells were absent from the upper cortical region, while cells expressing high levels of TBR1 were found concentrated predominantly in the cortical plate and IZ. The loss of TBR1 expression from the upper cortical layers was documented in all the three schizophrenia organoids, as compared to three controls. Quantitative stereological counting showed 32.2 + 2.0% of DAPI-stained nuclei were positive for TBR1 in organoids from control subjects. This number was significantly reduced in the schizophrenia organoids to 17.1 + 1.4 (p < 0.001) (Fig. 4, a2, Supplementary Fig. 5c). Thus, impaired development of the cortical neurons is associated with an overexpression of TBR1 in the cortical plate and the absence of the superficial pioneer TBR1+ neuroblasts.
