Songbird connectome reveals tunneling of migratory neurons in the adult striatum

Immature neurons in the adult brain migrate and integrate into existing circuits, where they contribute to plasticity, learning, and complex behaviors. However, how these cells navigate synapse-rich regions of the adult brain remains poorly understood. While prior studies have examined the molecular mechanisms and functional consequences of adult neurogenesis, few have investigated the physical interactions between migrating neurons and their surrounding environment. Here, we use electron microscopy-based connectomics to examine how migrating neurons interact with mature circuit elements in the adult zebra finch striatum. Immature neurons exhibiting migratory features were observed contacting diverse structures in their microenvironment, including the axons, dendrites, synapses, and somas of mature neurons. Surprisingly, these interactions were structurally complex, often involving pronounced deformations of mature somas and the surrounding neuropil. These deformations appeared as "tunnels" made by the migratory neurons as they displaced mature structures along their path. Together, these findings suggest that migrating neurons may physically reshape the mature circuit to reach their targets, revealing an unexpected degree of structural and functional plasticity in the adult brain.