Distribution of transported proteins in the slow phase of axoplasmic flow. An electron microscopical autoradiographic study

Electron microscopic autoradiography has been used to study the distribution of labeled proteins along the axons of the retinal ganglion cells in the pigeon, during the slow phase of axoplasmic flow. After intraocular injection silver grains were found over a small fraction of the axonal population at any given time and at any specific location. Where the slowly advancing wave of radioactivity was detectable, the grains were predominantly located over the axoplasm, microtubules, neurofilaments and mitochondria. When the peak of the radioactivity had passed, the grains were found mostly overlying the axolemma. A large amount of radioactivity was observed over the endings of the retinal ganglion cell axons in the tectum 14 days after the injection of the labeled aminoacid. The grains were for the most part located over the outer membranes of the synaptic processes and over synaptic vesicles. These data support the hypothesis that the slow phase of axoplasmic transport contributes to the continuing growth of the axon and is formed mainly of materials which are distributed to the axonal membrane and to its synaptic endings. In contrast, the fast phase of axoplasmic flow results in a distribution of silver grains over the axolemma and the agranular endoplasmic reticulum in the axons, and over mitochondria and synaptic vesicles in its terminals.