Quantitative changes with age of the myelinated fibre density, nuclear density, the thickness of the perineurium, and the vasa nervorum were studied histologically in the human sural nerve. Materials were obtained from 79 necropsies of acute death without any accompanying peripheral nerve diseases, ranging from 1 week to 88 years of age.
The average small myelinated fibre density decreased rapidly from the age of 1 week (26300/mm2) to the second decade (9560/mm2), and continued to decrease gradually with age, reaching an average of 9730/mm2 for the eighth decade, 74% of that for the second decade.
Large myelinated fibres appeared first in a 3-month-old infant. The average large myelinated fibre density increased rapidly, attaining the level of a young adult at 3 years. The average was maximum at the third decade (6480/mm2) and thereafter decreased with age, reaching an average of 3480/mm2 for the ninth decade, 54% of that of the third decade.
Nuclear density decreased rapidly from 1 week of age (9800/mm2) to the second decade (3750/mm2). Subsequently, it increased gradually with age up to the eighth decade (6090/mm2), at which time it measured 163% of the average of the second decade.
The decrease of large myelinated fibres could not be related to changes of vasa nervorum due to aging before 60 years, while after 60 years there was a greater reduction of large myelinated fibres when the stenosis of vasa nervorum was more pronounced.
A linear relationship was found between the thickness of the perineurium and the diameter of the fascicle. The perincurial index (the thickness of perineurium/diameter of a fascicle×100) showed wide variation among individuals though it showed a tendency to increase with age.