Novel fibroblastic onion bulbs in a demyelinating avian peripheral neuropathy produced by riboflavin deficiency.

The finding of novel fibroblastic onion bulb-like structures in peripheral nerves is reported for the first time in avian riboflavin deficiency. Day old broiler meat chickens were fed a riboflavin deficient diet (1.8 mg/kg) and were killed on postnatal days 6, 11, 16, 21 and 31, whereas control chickens were fed a conventional diet containing 5.0 mg/kg riboflavin. The fibroblastic onion bulb-like structures were found in sciatic and brachial nerves from day 11 onwards and consisted of long cytoplasmic processes of hypertrophied fibroblasts surrounding demyelinated, remyelinated and normally myelinated axons. The fibroblast cytoplasmic processes often enveloped more than one nerve fibre to produce a unique compound-like onion bulb structure. These onion bulb-like structures occurred early in the course of segmental demyelination at the same time as tomacula formation and became increasingly more prominent in the later stages of demyelination and remyelination. The molecular basis of formation of these unique structures requires further study as to the basis of the attraction of the fibroblast processes to nerve fibres associated with myelinating Schwann cells. The model may also be useful in investigating the role of endoneurial fibroblasts in endoneurial fibrosis as the early fibroblastic response in the onion bulbs is distinct from the more usual fibroblastic deposition of collagen in end-stage peripheral nerve disease.