DEVELOPMENTAL-CHANGES AT THE NODE AND PARANODE IN HUMAN SURAL NERVES - MORPHOMETRIC AND FINE-STRUCTURAL EVALUATION

Developmental alterations of paranodal fiber segments have not been in vestigated systematically in human nerve fibers at the light- and elec tron-microscopic level. We have therefore analyzed developmental chang es in the fine structure of the paranode in 43 human sural nerves duri ng the axonal growth period up to 5 years of age, and during the subse quent myelin development up to 20 years and thereafter. The nodal, int ernodal, and paranodal axon diameters reach their adult values at 4-5 years of age. The ratio between internodal and paranodal axon diameter s remains constant at 1.8-2.0. Despite a considerable increase in myel in sheath thickness, the length of the paranodal myelin sheath attachm ent zone at the axon does not increase correspondingly, because of att enuation, separation from the axolemma, and piling up of myelin loops in the paranode. Separation of variable numbers of terminal myelin loo ps from the underlying axolemma results in the formation of bracelets of Nageotte, whereas the transverse bands of these loops disappear. Th e adaptation of the paranodal myelin sheath to axonal expansion during development probably occurs by uneven gliding of the paranodal myelin loops simultaneously with internodal slippage of myelin lamellae. Sin ce mechanically stabilizing structures (tight junctions and desmosomes between adjacent paranodal myelin processes; transverse bands between myelin loops and paranodal axolemma) are unevenly arranged, especiall y during rapid axonal growth, paranodal axonal growth with simultaneou s adaptation of the myelin sheath is probably discontinuous with time.