Size distribution of rhodamine-labelled microspheres retrogradely transported in cultured neurons

Rhodamine-labelled latex microspheres (RLM), 10-110 nm in diameter, were used as a retrograde tracer in cultured sympathetic neurons. Retrogradely transported RLM were isolated from the cells and measured. These were all smaller than 50 nm in diameter and represented not more than 2% of the original RLM population. The permeability of RLM as a function of size was also estimated and showed a steep decrease between 20 and 30 nm from 100% to less than 20%. Therefore, RLM with a mean diameter of 20 nm would be optimal for retrograde tracing.