Whole-cortex in situ sequencing reveals peripheral input-dependent cell type-defined area identity

The cortex is composed of neuronal types with diverse gene expression that are organized into specialized cortical areas. These areas, each with characteristic cytoarchitecture ([Brodmann 1909][1]; [Vogt and Vogt 1919][2]; [Von Bonin 1947][3]), connectivity ([Zingg et al. 2014][4]; [Harris et al. 2019][5]), and neuronal activity ([Schwarz et al. 2008][6]; [Ferrarini et al. 2009][7]; [He et al. 2009][8]; [Meunier et al. 2010][9]; [Bertolero et al. 2015][10]), are wired into modular networks ([Zingg et al. 2014][4]; [Harris et al. 2019][5]; [Huang et al. 2020][11]). However, it remains unclear whether cortical areas and their modular organization can be similarly defined by their transcriptomic signatures and how such signatures are established in development. Here we used BARseq, a high-throughput in situ sequencing technique, to interrogate the expression of 104 cell type marker genes in 10.3 million cells, including 4,194,658 cortical neurons over nine mouse forebrain hemispheres at cellular resolution. De novo clustering of gene expression in single neurons revealed transcriptomic types that were consistent with previous single-cell RNAseq studies([Yao et al. 2021a][12]; [Yao et al. 2021b][13]). Gene expression and the distribution of fine-grained cell types vary along the contours of cortical areas, and the composition of transcriptomic types are highly predictive of cortical area identity. Moreover, areas with similar compositions of transcriptomic types, which we defined as cortical modules, overlap with areas that are highly connected, suggesting that the same modular organization is reflected in both transcriptomic signatures and connectivity. To explore how the transcriptomic profiles of cortical neurons depend on development, we compared the cell type distributions after neonatal binocular enucleation. Strikingly, binocular enucleation caused the cell type compositional profiles of visual areas to shift towards neighboring areas within the same cortical module, suggesting that peripheral inputs sharpen the distinct transcriptomic identities of areas within cortical modules. Enabled by the high-throughput, low-cost, and reproducibility of BARseq, our study provides a proof-of-principle for using large-scale in situ sequencing to reveal brain-wide molecular architecture and to understand its development. ### Competing Interest Statement A.M.Z. is a founder and equity owner of Cajal Neuroscience and a member of its scientific advisory board. The remaining authors declare no competing interests. [1]: #ref-6 [2]: #ref-80 [3]: #ref-81 [4]: #ref-94 [5]: #ref-29 [6]: #ref-64 [7]: #ref-24 [8]: #ref-31 [9]: #ref-49 [10]: #ref-5 [11]: #ref-33 [12]: #ref-85 [13]: #ref-87