An ultrastructural and biochemical analysis of collagen in rat peripheral nerves: the relationship between fibril diameter and mechanical properties

Mechanical features are distributed heterogeneously within nerve tissue, with compliance increased at articulations. This study explored whether differences in stiffness between joint regions (JRs) and non-joint regions (NJRs) of rat median and sciatic nerves were related to localised variation in collagen content or fibril diameter. There was no significant difference in the amount of collagen detected by biochemical assay in JRs and NJRs of either nerve. Ultrastructural analysis showed collagen fibril diameter ranges of 20-80 nm in the endoneurium and perineurium and 30-130 nm in the epineurium. In the median nerve, but not the sciatic nerve, there were significantly smaller fibrils in JRs compared to NJRs. This corresponded to a greater number density of fibrils in JRs compared to NJRs in the epineurium and endoneurium of the median nerve. We report the presence therefore of a population of thinner collagen fibrils in the JR of the median nerve that corresponds to the location of increased compliance in this tissue, suggesting that localised variation in collagen fibril diameter contributes to the longitudinal heterogeneity of tensile properties in this nerve.