Mapping the structure and organization of hippocampal neurons in the mouse brain

Single-cell projectome analysis has revealed previously unknown spatial organization principles of the brain-wide structure and connectivity of more than 10,000 individual hippocampal neurons in the mouse brain, according to a new study. Specialized projections called axons allow neurons to transmit signals to other neurons across the brain. The hippocampus (HIP) – one of the most extensively studied brain regions – plays a crucial role in many crucial brain functions, including learning, memory, and emotional behaviors. The core circuits within the HIP are connected with various other brain areas by axon projections. However, the spatial organization of intra- and extra-HIP axon projections remains largely unknown. Mapping the connectivity of neurons in the HIP is essential for understanding the region’s neural circuitry and underlying brain functions. Combining sparse-labeling methods with fluorescence micro-optical sectioning tomography (fMOST), Shou Qiu and colleagues systematically mapped 10,100 single-neuron axon projections and their arborization patterns throughout the entire mouse HIP, creating a comprehensive three-dimensional single-cell atlas of the region and its connections. Qiu et al.’s reconstruction revealed 43 projectome subtypes with distinct projection patterns based on axon or dendrite morphology and axon arborization at target areas. Spatial transcriptomic analysis of hippocampal sections also showed that the number of projection targets and axon-tip distribution for HIP neurons was correlated with the location of each cell’s soma along longitudinal and transverse axes. Qiu et al. created an open online database that enables interactive visualization and analysis of the study’s data.