Uptake of horseradish peroxidase by sensory nerve fibres in vitro.

After 47 3/4 hours in vitro, organotypic cultures of dorsal root ganglia of rat were exposed to horseradish peroxidase at a concentration of 4 mg/ml of nutrient medium for 15 minutes. Observations on the living nerve fibres revealed pinocytotic vesicles in the growth cones which were transported toward the perikarya for several hundreds of micrometer at a rate of 0-5-10 micrometer/min. Electron microscopic sections displayed uptake of peroxidase into the growth cones by means of endocytosis. The enzyme marker was contained in small-sized vesicles, as well as in large vacuoles which measured up to 2 micrometer in diameter. The more proximal parts of the axon contained tiny vesicles which were well below the level of resolution of the light microscope. Pinocytotic phenomena were abundant at the fibre tips, but could also be observed occasionally in the intermediate and more proximal portions of contiguous axons. It is proposed tentatively that the large vacuoles in the distal and intermediate parts of the axon had formed from the small vesicles. Furthermore, it is suggested that the profuse uptake and backward transport of vesicles in these regenerating neurons might be meaningful for the neuron in terms of intercellular and environmental recognition.