An electron‐dense extracellular material in the nervous system of Aplysia

An electron dense material is found in the extracellular space of the nervous system of Aplysia, a marine mollusc whose ganglia are widely studied by neurobiologists. This material appears to consist of irregular electron‐dense granules, with diameters of approximately 600 Å. This material is found between the glial cells that surround the neuronal perikarya. It is not found in other regions of the nervous system. Because it is found in the ganglion cell layer of the nervous system and because the neurons contain what may be the precursor of this substance, the electron‐dense material is regarded as most probably being a neuronal product. The importance of this material is that it is one of the few examples of visible structure in the extracellular space of the nervous system.