Ultrastructural aspects of rapid-frozen, deep-etched and rotary-shadowed synaptosomes.

Isolated synaptic terminals were rapid-frozen with a liquid-propane jet, deep-etched, and rotary-shadowed replicas produced. The replicas show a three-dimensional view of the cytoplasmic components. Intraterminal mitochondria display a rough inner membrane with numerous stalked globular particles. Microfilaments connect the diverse organelles of the nerve terminal. The main cytoplasmic structure is a tubular membrane system, probably formed by the smooth endoplasmic reticulum (SER). Rapid-frozen synaptosomes reveal sparse profiles of synaptic vesicles in contrast to the current structural features shown by the techniques employing cryoprotectants and/or fixatives. The structural lability of these SER-profiles seems to be very high, to the extent that varying morphological features are observed in the same replica. It is proposed that the synaptic vesicles originate from cisternae of the SER, and that the synaptic vesicles and the SER are in close structural and functional relation. The morphological relationship of both structures to the cell membrane still remains unclear.