Qualitative and quantitative information is reported on the morphological changes 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 droplets in the
perineurial cells. Macrophages and mast cells increase in number, and onion bulbs and collagen pockets are
frequently present. Schwann cells associated with myelinated fibres (MF) slightly decrease in number in
parallel with an increase of the internodal length from 6 to 12 mo, but increase in older nerves when
demyelination and remyelination are common. The unmyelinated 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 involves 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 the 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.