Cortical GABAergic interneurons generated in the ventral developing brain travel long distances to their final destinations. While there are examples of interneuron migration in the neonatal human brain, the extent of postnatal migration across species and how it contributes to cortical interneuron composition remains unknown. Here we demonstrate that neonatal gyrencephalic brains, including humans, nonhuman primates and piglets, harbor an elaborate subventricular zone, termed the Arc, due to its curved morphology and expanded neuroblast populations. The Arc is absent in lissencephalic marmoset and mouse brains. Transcriptomic and histological approaches revealed that Arc neurons are diverse interneurons from the medial and caudal ganglionic eminences that migrate into the frontal, cingulate and temporal cortex. Arc–cortical targets exhibit an increase in VIP⁺ neuronal density compared to other regions. Our findings reveal that the Arc is a developmental structure that supports the expansion of postnatal neuronal migration for cortical interneuron patterning in gyrencephalic brains.

During organogenesis, patterns and gradients of gene expression underlie organization and diversified cell specification to generate complex tissue architecture. While the cerebral cortex is organized into six excitatory neuronal layers, it is unclear whether glial cells are diversified to mimic neuronal laminae or show distinct layering. To determine the molecular architecture of the mammalian cortex, we developed a high content pipeline that can quantify single-cell gene expression in situ . The Large-area Spatial Transcriptomic (LaST) map confirmed expected cortical neuron layer organization and also revealed a novel neuronal identity signature. Screening 46 candidate genes for astrocyte diversity across the cortex, we identified grey matter superficial, mid and deep astrocyte identities in gradient layer patterns that were distinct from neurons. Astrocyte layers formed in early postnatal cortex and mostly persisted in adult mouse and human cortex. Mutations that shifted neuronal post-mitotic identity or organization were sufficient to alter glial layering, indicating an instructive role for neuronal cues. In normal mouse cortex, astrocyte layer patterns showed area diversity between functionally distinct cortical regions. These findings indicate that excitatory neurons and astrocytes cells are organized into distinct lineage-associated laminae, which give rise to higher order neuroglial complexity of cortical architecture.