Brain circuits in the neocortex develop from diverse types of neurons that migrate and form synapses. Here we quantify the circuit patterns of synaptogenesis for inhibitory interneurons in the developing mouse somatosensory cortex. We studied synaptic innervation of cell bodies, apical dendrites, and axon initial segments using three-dimensional electron microscopy focusing on the first 4 weeks postnatally (postnatal days P5 to P28). We found that innervation of apical dendrites occurs early and specifically: Target preference is already almost at adult levels at P5. Axons innervating cell bodies, on the other hand, gradually acquire specificity from P5 to P9, likely via synaptic overabundance followed by antispecific synapse removal. Chandelier axons show first target preference by P14 but develop full target specificity almost completely by P28, which is consistent with a combination of axon outgrowth and off-target synapse removal. This connectomic developmental profile reveals how inhibitory axons in the mouse cortex establish brain circuitry during development.
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