It was the purpose of this investigation to study, by means of electron microscopic examination of serial sections, the routes of uptake of the protein tracers, horseradish peroxidase (0.5%) and ferritin (9%), into definitively identified growth cones of rat sympathetic nerve cells in culture. Tracer enters the cone primarily by way of uncoated and, to a lesser degree, coated vesicles, small vacuoles, membranous tubules and cup-shaped structures. Tracer accumulates rapidly in cup-shaped and multivesicular bodies; the former acquire label directly during their formation at the cone surface and both organelle types receive tracer by way of fusion of vesicles and tubules that ferry the label from the surface. Thus, some tracer protein enters lysosome-like bodies in the cone but, because labelled vesicles and tubules are found in the nerve fibre as well, some label may reach the perikaryon without having entered this system. It is suggested that the label-laden tubules in retrograde movement in the neurite are derived from plasmalemma rather than agranular reticulum which has been implicated in anterograde transport; this difference in membrane packaging could provide a basis for bidirectional particulate traffic within the axon. Tracer is also present in characteristic vesicle aggregates sequestered in mound-like excrescences of the cone plasmalemma and in an associated subjacent system of branching tubules described here; the strikingly consistent location of these areas at the bases of or along filopodia and the labelling characteristics prompt the suggestion that label enters these structures not by an active uptake mechanism but by inclusion during a brief period of filopodial retraction or shortening. Additional points made are that 1. endocytosis occurs at the leading edge rather than at the base of the cone. 2. the presence of more label in the cone than in the adjoining neurite indicates higher endocytotic activity in the tip, and 3. whereas the labelling observed here resembles closely that observed in cultured neurites studied by others, a few differences between ferritin and peroxidase endocytosis were detected in this study.
ASJC Scopus subject areas
- Cell Biology