The action of phospholipase A and lysophosphatidyl choline (LPC) on mature, myelinated peripheral nerve fibres has been studied in vivo and electron microscopically, following sub-perineurial injection of these substances. Within 30 min, demyelination was observed in vivo along cylindrico-conical segments, spreading from Schmidt-Lanterman incisures and nodes of Ranvier. By 96 h, all traces of the myelin sheath had disappeared from the area of the lesion, and had been replaced by debris-laden cells lying in chains parallel to one another and the long axis of the fibre. During the next few weeks these cells gradually disappeared, and numerous finely myelinated axons, running between, and in continuity with, the normal fibres proximal and distal to the lesion were observed. If lower concentrations of LPC were used the number of fibres involved decreased, although the demyelinative changes followed the same time-course.

Ultrastructurally, demyelination involved progressive disruption and removal of the lamellar sheath, observed initially as a splitting of the intraperiod line within 30 min. Subsequent breakdown resulted in the formation of strands of 4-6 nm repeat material which was further degraded through quintuple- and triple-layered lamellar units to foam-like systems of disorganized lamellar fragments. The Schwann cell and axons appeared to be undamaged by phospholipase A and LPC, and retained their normal impermeability to exogenous ferritin.

The significance of the demyelinating capacity of LPC in vivo is discussed in terms of its known action on myelin in vitro, the rapidity and apparent specificity of its action demonstrated in this study, and its potential involvement in pathological demyelination.

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