Absolute specificity for retrograde fast axonal transport displayed by lipid droplets originating in the axon of an identified Aplysia neuron in vitro.

Lipid droplets were found to form all along the axon of the giant cerebral neuron (GCN) of the sea hare Aplysia californica when the cell was placed in culture. The emission of yellow fluorescence by the droplets after exposure of the neuron to Nile red and their uniformly dark appearance in electron micrographs of axons fixed with glutaraldehyde and osmium tetroxide identified them as lipid droplets. In contrast to lipid droplets in fat cells and certain other cell types, these droplets were bounded by a membrane, indicating that the lipid droplet is a type of organelle that is membranated in some situations but not others. As observed by video-enhanced contrast-differential interference contrast microscopy, the droplets grew manyfold in place in the axon to diameters of 1-3 micron within 2-3 days. Often they formed coherent tandem arrays of 3-15 droplets. Droplets were usually essentially stationary but occasionally moved tens of microns by fast axonal transport, the largest spherical organelles to have been observed to undergo transport. They usually moved as singlets, sometimes as tandem arrays. The direction of transport was always retrograde (towards the cell body). Thus, an organelle need neither originate nor be modified in the axon terminal to be specified for retrograde transport. Whether or not an organelle is formed in the cell body might determine directionality. Alternatively, size might be a determining factor, with large organelles specified for retrograde transport.