On the occurrence of striated muscle within the spinal leptomeninges.

A clinically normal adult male rabbit was found to possess fascicles of muscle fibers within the leptomeninges over the spinal cord at L6, L7, and S1. The finding is believed to be purely incidental and unrelated to any experimental procedure. Morphologically, the fascicles had the characteristics of striated muscle; and, while most were of normal dimensions, several were made up of small fibers 4-10 micron in diameter. Motor endplates, capillaries, and endomysial and perimysial connective tissue were also present. Axons myelinated by Schwann cells were associated with the normal bundles but were absent from the bundles of small fibers. In such bundles, large, unmyelinated axons predominated, some of which lacked ensheathment while others were invested by perineurial-type cells. The observation of incidental intraleptomeningeal skeletal muscle probably reflects a rare developmental anomaly. Its occurrence in humans is also discussed.