Contribution of axonal transport to the renewal of myelin phospholipids in peripheral nerves. I. Quantitative radioautographic study.
Kinetics of phospholipid constituents transferred from the axon to the myelin sheath were studied in the oculomotor nerve (OMN) and the ciliary ganglion (CG) of chicken. Axons of the OMN were loaded with transported phospholipids after an intracerebral injection of [2-3H]glycerol or [3H]labeled choline. Quantitative electron microscope radioautography revealed that labeled lipids were transported in the axons mainly associated with the smooth endoplasmic reticulum. Simultaneously, the labeling of the myelin sheath was found in the Schmidt-Lanterman clefts and the inner myelin layers. The outer Schwann cell cytoplasm and the outer myelin layers contained some label with [methyl-3H]choline, but virtually none with [2-3H]glycerol. With time the radioactive lipids were redistributed throughout and along the whole myelin sheath. Since [2-3H]glycerol incorporated into phospholipids is practically not re-utilized, the occurrence of label in myelin results from a translocation of entire phospholipid molecules and from their preferential insertion into Schmidt-Lanterman clefts. In this way, the axon-myelin transfer of phospholipid contributes rapidly to the renewal of a limited pool of phospholipids in the inner myelin layers. When [methyl-3H]choline was used as precursor of phospholipids, the rapid appearance of the label in the inner myelin layers was interpreted also as an axon-myelin transfer of labeled phospholipids. However, the additional labeling of the outer Schwann cell cytoplasm adjacent to Schmidt-Lanterman clefts and of the outer myelin layers reflects a local re-incorporation of the base released from the axon. By these two processes, the axon contributes to purvey the inner myelin layers with new phospholipids and the Schwann cells with new choline molecules.