AXON DIAMETER AND MYELIN SHEATH THICKNESS IN NERVE-FIBERS OF THE VENTRAL SPINAL ROOT OF THE 7TH LUMBAR NERVE OF THE ADULT AND DEVELOPING CAT

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  • 1 January 1983
    • journal article
    • research article
    • Vol. 136  (MAY) , 483-508
Abstract
The axon diameter (d) and the number of myelin sheath lamellae (nl) were estimated in electron micrographs of cross sectioned ventral spinal roots of the 7th lumbar nerve of adult cats, kittens and cat fetuses. Myelination started between the 40th and 45th day after mating (about 3 wk before birth). From birth onwards the caliber spectrum consisted of a group of small fibers and a group of large fibers. During the first 2 postnatal mo. the point distribution of the number of myelin lamellae plotted against the axon diamter appeared more or less linear. This point distribution pattern became curved from the 10th postnatal wk onwards, and a function of the general expression: nl = c0 + c1d + C2log(d) could be fitted to describe the regression. In adult cats, the mean number of myelin lamellae/axon diameter ratio was about 30 in the group of small and about 15 in the group of large fibers. Observations on immature animals indicated that the difference between the 2 fiber groups could have been established already during the 3rd postnatal wk. Considerable myelin production took place after the age of 1 yr. The cross sectional area of the myelin sheath in the alpha fiber group increased about 10% after the age of 1 yr, while in the gamma fiber group it increased as much as about 85%. The absolute amount of myelin per unit fiber length produced after the age of 1 yr was the same in both alpha and gamma fibers. Differences and similarities between myelin production in the 2 fiber groups are discussed.