THE EVOLUTION OF ‘ONION BULBS’ IN THE HEREDITARY HYPERTROPHIC NEUROPATHY OF THE TREMBLER MOUSE

The evolution of ‘onion bulbs’ was studied ultrastructurally in the dominantly inherited hypertrophic neuropathy of the Trembler mouse. Demyelination was prominent in the peripheral nerves of mice aged 7 days to 4 weeks. Following demyelination, the Schwann cell divided. One Schwann cell captured the axon while the supernumerary Schwann cell produced many sprouts which were delimited by the original basement membrane which later became discontinuous. While the central (adaxonal) Schwann cell had abundant cytoplasm with a normal content of organelles, degenerative changes were evident in the supernumerary more peripheral Schwann cell sprouts. Cytoplasm degenerated from these, and left behind empty membrane configurations which assumed an approximately circumferential orientation. As more ‘onion bulb’ lamellae were added, a morphological gradation became evident; the outer lamellae were more degenerate than the inner ones. Pari passu with the formation of ‘onion bulbs’, endoneurial collagen fibrils increased in number and marked variations in calibre, and frequently islands of microfibrils, were seen in older Tremblers. The axon in older animals underwent changes in contour, with an increase in neurofilaments and decrease in neuro‐tubules.