Pinocytotic uptake and intracellular distribution of colloidal thorium dioxide by cultured sensory neurites

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Abstract
Sensory ganglia from 9-day chick embryos were grown on collagen coated coverslips for 36 h in the presence of nerve growth factor, producing a profuse neuritic outgrowth. The cultures were then incubated for varying periods in a colloidal suspension of thorium dioxide, and the pinocytotic uptake of this marker was followed by electron microscopy. Following brief exposures (3 min), most of the labelled organelles consisted of smooth surfaced vesicles and vacuoles; with longer exposures, the bulk of the marker accumulated first in cup-shaped pre-multivesicular bodies and ultimately in multivesicular bodies. The marker was also taken up into coated vesicles, dense-cored and electron lucent tubules, dense-cored vesicles and dense bodies of the multi-layered myelin body configuration. In addition, evidence suggestive of exocytosis was also obtained; views of apparent fusion of labelled multivesicular bodies with the plasmalemma involving extrusion of vesicles and marker particles into the extracellular space were regularly encountered following long exposures.