Atypical persisting fibres in explants of human muscle cocultured with embryonic nerve cells.

When samples of human muscle are cocultured with embryonic mouse spinal cord, the muscle fibres usually regenerate to form a bundle of new myotubes which become innervated and develop cross-striations and contractions. However, we have noticed that in some cultures of 6 biopsies of human muscle, there were fibres which "persisted" in the cultures for long periods of time without being replaced by regenerated myotubes. At both the light and electron microscopic levels, they appeared to be mature muscle with well-organized myofibrils, intact plasma membranes and closely-apposed basal laminae. At least some of the fibres contained adult myosin heavy chains. They were only found in freshly-cultured samples, and appeared to be associated with younger patients, but were not associated with any particular muscle condition. The nature of these persisting fibres is discussed, and we emphasize the need for sequential observation of cultures to ensure that fibres which have regenerated in culture are distinguished from those which have persisted from the original biopsy.