The morphometric composition of myelinated fibres by nerve, level and species related to nerve microenvironment and ischaemia.

An extensive morphometric evaluation of the number, density and diameter distribution of myelinated fibres (MFs) of different nerves, of different proximal-distal levels of nerves, and of different species is reported. The methodology used provides reliable estimates of number and size because of the use of optimal histological techniques, semi-thin sections, careful systematic sampling, nonsubjective evaluation of transverse MF profiles using computerized imaging and evaluation of large numbers of MFs and nerves. The non-MF percentage of endoneurial area was also measured as an index of the amount of nerve microenvironment. We have confirmed that the MF composition is quite variable between nerves, between levels of nerves, and between species. In general the density of MFs is lower in the nerves of man than in the nerves of other animals evaluated, in distal than in proximal levels, in old age and in disease. Although increased endoneurial fluid may play a role in resistance to ischaemic block it appears that the intrinsic metabolic properties of the neural tissue itself are most important.