Naked axon bundles enclosed by single segments of myelin sheaths in the nerves of non-dystrophic C57BL-ob/+mice.

Bundles of axons enclosed by a single myelin sheath were found in the peripheral nerves of heterozygous obese C57BL-ob/+mice. Serial sections showed them to consist of two to forty naked axons encompassed by a common cuff of myelin formed by a single Schwann cell, with adjoining non-myelinated Schwann cells protruding from either side into the cuff. These structures are similar to those found in dystrophic mice, but there were no signs of dystrophy in the obese mice. It is suggested that the axon bundles result from a disturbance of early myelination, presumably of the phase of Schwann cell proliferation or of the communication between Schwann cells and axons.