Qualitative and quantitative morphology of human sural nerve at different ages.

A qualitative and quantitative study was made of 27 sural nerves obtained within 24 hours of death from human subjects without history of disease or ingestion of drugs known to affect peripheral nerve. Parameters studied included fascicular area, internodal length and diameter of teased fibres, density of myelinated and unmyelinated axons, and axonal and fibre diameter. The slope of internodal length-fibre diameter regression lines increased progressively during the first decade but thereafter remained relatively constant until about the age of 60 years, after which degeneration and regeneration, and demyelination and remyelination caused increasing variation in internodal length. Densities of myelinated and unmyelinated axons decreased markedly from birth to the end of the eighth decade, due to increasing size and separation of fibres during the first decade, and axonal degeneration and an increase in endoneurial collagen in the older age groups. Except during the first year of life, the diameter distribution of myelinated fibres was bimodal, the lower mode occurring at 3-5 microns in all age groups and the upper shifting progressively from 5-6 microns at 1 year to 9-12 microns after the first decade. g ratios were generally higher in small than in large fibres and showed increasing scatter with age. The distribution of unmyelinated axons was unimodal until the fifth decade; thereafter the number of fine calibre axons increased. Ageing was associated with reduplication of vascular, and thickening of perineurial, basement membranes.