Montano Crespo, R. E. | bioRxiv Science

bioRxiv2026

Fine ultrastructural organization of glomerular neuropil in the developing zebrafish olfactory bulb

Across animal phyla, odor information is represented in the first olfactory processing center by combinatorial activation of discrete glomeruli. To examine how this organization of olfactory processing channels emerges during development we combined 2-photon calcium imaging, volume electron microscopy, and large-scale automated neuron reconstruction in zebrafish larvae. We focused on a developmental stage (130 hours post fertilization) when the olfactory bulb (OB) is structured into ~16 large neuropil regions referred to as protoglomeruli but most glomeruli are not yet differentiated. Our ultrastructural reconstructions revealed that protoglomeruli are organized into smaller units referred to as microglomeruli, each of which is defined by a distinct set of mitral cells with overlapping, spatially confined dendritic arbors. Neighboring microglomeruli were innervated by distinct subsets of sensory axons and activity was more highly correlated among mitral cells associated with the same microglomerulus than across microglomeruli of the same or different protoglomeruli. These results reveal a precise subcellular organization of the neuropil in the developing OB into distinct processing channels, indicating that the layout of the adult glomerular array is established already early in development.

neuroscience↗

bioRxiv2026

Deep anatomical and ultrastructural classification of neurons in the zebrafish olfactory bulb

Neuronal circuits in the olfactory bulb (OB) perform computations fundamental to pattern classification including a decorrelation and normalization of odor-evoked activity. These computations are mediated by diverse interneurons but a comprehensive picture of interneuron types and their microcircuit organization is lacking. We provide a deep anatomical classification of neuron types and their synaptic connectivity in the OB of adult zebrafish, a well-established model to analyze olfactory computations. We reconstructed 459 neurons in an image volume acquired by serial block face scanning electron microscopy and defined 13 neuron classes based on morphological and ultrastructural features. These comprised two classes of projection neurons and 11 interneuron classes, some of which were further separated into subclasses. Ultrastructural information including spine shape, variations in neurite diameter and synaptic arrangements contributed significantly to the distinction of cell types. As in other species, reciprocal synaptic connections were abundant. Targeted synapse annotation revealed systematic connectivity between projection neurons and interneurons. These included microcircuit motifs combining reciprocal and unidirectional connectivity that provide possible structural substrates for gain control and lateral inhibition. The results provide detailed insights into the structural organization of the OB and an anatomical foundation for physiological and computational studies of information processing in olfaction.

neuroscience↗