The formation from the Schwann cell surface of myelin in the peripheral nerves of chick embryos.

IN 1953 the presence of the characteristic concentric lamellar structure of the myelin sheath was demonstrated in sciatic nerves of chick embryos (2). In the fibers from embryos incubated for 18 days, the alternate dense and less dense bands of the myelin sheath were closely packed, with a unit of spacing of about 97 A. There were about 24 layers (a layer being one dense band plus the less dense region separating it from the next dense band) around each fiber, it was also reported that in preliminary studies of earlier stages (14 and 15 days of incubation) many fibers with as few as 2 to 12 layers were observed. The authors concluded that the increase in myelinatiori of peripheral nerve fibers in the embryo proceeds by the addition, in sequence, of layers of oriented lipid and protein to the surface of the axon within the cytoplasm of the Schwann cell, rather than by accumulation and aggregation of droplets of lipid-protein with sudden "crystallizing out" of the characteristic fine structure. In this paper, evidence is presented which shows that the layers of oriented lipid and protein are derived from the Schwann cell surface by an infolding process initiated at the time the Schwann cell envelops the outgrowing axon.