3D spatial transcriptomics reveals the molecular domain structure of the mouse olfactory bulb.

A core organizing principle of the vertebrate brain is its symmetry along multiple axes. However, the structure of these axes, and the precision with which neurons, circuit modules, and brain regions align to them, remain poorly understood. Here, we used 3D spatial transcriptomics to reconstruct the anatomical and molecular organization of the mouse olfactory bulb. We mapped the positions of nearly one thousand molecularly distinct glomeruli, the structural and functional units of odor processing, revealing highly symmetric organization across brain hemispheres. Within each bulb, we defined a curved axis of symmetry that divides pairs of sister glomeruli. Gene expression programs in olfactory sensory neurons predicted glomerular position with near-glomerular resolution. However, glomerular symmetry was disrupted in deeper layer mitral and granule cells, suggesting a reorganization of olfactory bulb output pathways. Our findings provide the first comprehensive map of the molecular domain structure of the olfactory bulb.