Profuse sheathing of muscle fibres was noticed by chance when studying receptors in samples of extra-ocular muscle taken from orbits of six patients after eye enucleation or in the treatment of squint. The sheaths, previously unreported in any skeletal muscle, were examined by light and electron microscopy and their incidence determined. Muscle spindle capsules were rare by comparison. Most sheaths were found in the motor end plate band enclosing single muscle fibres and consisted of perineural epithelial cell extensions at neuromuscular junctions. Their length varied from a few to 225 micrometers, partly or fully covering neuromuscular junctions, and although limited attachments to muscle fibres were seen, the sheathed zones were probably not sealed off from the neighbouring endomysium. The incidence of sheathed neuromuscular junctions in inferior oblique muscles increased with age from none at 3 years to a maximum of 41% of the total in a specimen from the eighth decade. Mean length and thickness of sheaths also increased with age. Sheaths were far less frequent outside the motor end plate band, where most were related to grape endings of Felderstruktur fibres. Others contained from one to four muscle fibres and received their sheaths from passing nerves, but neuromuscular junctions were absent. A few of these were long, extending up to 470 micrometers. Sheath formation is evidently an expression of ageing in the perineurium, perhaps as a subsidiary feature of neuromuscular junction plasticity. The risk of confusing sheathed muscle fibres with muscle spindles is discussed.
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