The smooth endoplasmic reticulum: structure and role in the renewal of axonal membrane and synaptic vesicles by fast axonal transport.

The spatial arrangement of the smooth endoplasmic reticulum (SER) was studied in 0.5-2 mum thick sections of rat spinal and chick ciliary ganglia previously impregnated with heavy metal salts. Electron microscopy at low (10-5 V) or high (10-6 V) voltage showed the impregnated SER as a continuous system extending probably from the perikaryon to the axon terminal. Tubules of the SER, which were running in a parallel direction with the axon, were occasionally seen in close apposition with the axonal membrane. Moreover in the preterminal region, anastomosed tubules of the SER formed a subsurface 'primary network' and gave rise to a deeper 'secondary network' made of thinner tubules; synaptic vesicles bulging at the tip of thin tubules of the SER were frequently observed. To specify the role played by the SER in the fast axonal transport, chicken ciliary ganglia were slighty compressed and radioautographed 3 h after the intracerebral injection of [3-H]lysine. Quantitative analysis of the silver grain distribution indicated that labeled proteins, rapidly conveyed down the axon, piled up in regions containing an accumulation of SER profiles. On the basis of these results, it is concluded that: (1) the SER appears as a continuous intraaxonal pathway bridging the perikaryon and the axon terminal; (2) the SER conveys macromolecular components with the fast axonal transport; (3) the conveyed macromolecules, which are delivered to the axonal membrane and to the synaptic vesicles, are probably transferred by means of connections with the SER.