The clefts of schmidt‐lantermann: A quantitative electron microscopic study of their structure in developing and adult sciatic nerves of the rat

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Abstract
A quantitative electron microscopic study of the clefts of Schmidt‐Lantermann in fibers of the sciatic nerves of young and adult rats showed that the clefts are formed by two conical “male” and “female” portions of the sheath. The circumference of the female cone is approximately 15% larger than the male cone. The surfaces of the cleft transect the sheath at angle of approximately 9°. The present data indicate, also, that both the configuration of the conical portions of the sheath and their position relative to each other may change prior to fixation. Calculations indicate that each cleft permits a change in axoplasmic volume by 22%, and a change in fiber length by 9% of the fiber diameter.Clefts were not observed in nerves of rats before the twelfth day of age nor in sheaths of fibers composed of less than 20 lamellae. Newly formed clefts were wider than adult clefts. Transitional forms suggest that clefts may develop from fibers with redundant loops of myelin sheath. Radioautographic data on the incorporation of H3−leucine in nerve fibers did not indicate transport of significant amounts of this isotope into the clefts.