Morphometric and ultrastructural changes with ageing in mouse peripheral nerve

Qualitative and quantitative information is reported on the morphological c hanges that occur in nerve fibres and nonneuronal cells of peripheral nerve during the lifetime of the mouse. Tibial nerves of mice aged 6-33 mo were studied. With ageing, collagen accumulates in the perineurium and lipid dro plets in the perineurial cells. Macrophages and mast cells increase in numb er, and onion bulbs and collagen pockets are frequently present. Schwann ce lls associated with myelinated fibres (MF) slightly decrease in number in p arallel with an increase of the internodal length from 6 to 12 mo, but incr ease in older nerves when demyelination and remyelination are common. The u nmyelinated axon to myelinated fibre (UA/MF) ratio was about 2 until 12 mo, decreasing to 1.6 by 27 mo. In older mice, the loss of nerve fibres involv es UA (50% loss of 27-33 mo cf. 6 mo) more markedly than MF (35%). In aged nerves wide incisures and infolded or outfolded myelin loops are frequent, resulting in an increased irregularity in the morphology of fibres along th e internodes. In the mouse there is an adult time period, 12-20 mo, during which several features of degeneration progressively appear, and an ageing period from 20 mo upwards when the nerve suffers a general disorganisation and marked fibre loss.