The nervous system is made up of two major cell types: neurons, which transmit electrical signals, and glial cells, which support and regulate neuronal function. Understanding how many of each cell type are present, and how they are arranged, is essential for interpreting how neural circuits are organized. Until recently, however, it was not possible to accurately count every cell within a small invertebrate brain at high resolution. 

This project focuses on the sea slug Berghia stephanieae, an emerging model organism for studying simple nervous systems. Using three-dimensional reconstructions generated from volumetric electron microscopy, we conducted a complete cellular census of the rhinophore ganglion, a sensory processing center in the Berghia brain. Nuclei were manually identified and annotated in Neuroglancer, a web-based 3D visualization platform. Each nucleus was classified as neuronal or glial based on observable structural features, including nuclear morphology, cytoplasmic appearance, and its association with segmented cellular reconstructions (cellular meshes) derived from the EM dataset.

Across the entire rhinophore ganglion, we identified over 8,000 neuronal nuclei and over 800 glial nuclei. Distinguishing between these two cell types allowed us to measure how abundant each population is and how they are arranged within this sensory processing center. By completing this cellular census, we provide the first clear, quantitative picture of the ganglion’s cellular composition. This work lays the groundwork for future studies exploring how this circuit develops, functions, and is organized within the Berghia nervous system.