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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.

Abstract. Riboflavin (vitamin B2) deficiency in young chickens produces a demyelinating peripheral neuropathy. In this study, day-old broiler meat chickens were fed a riboflavin-deficient diet (1.8 mg/kg) and killed on posthatch days 6, 11, 16, 21, and 31, while control chickens were given a conventional diet containing 5.0 mg/kg riboflavin. Pathologic changes were found in sciatic, cervical, and lumbar spinal nerves of riboflavin-deficient chickens from day 11 onwards, characterized by endoneurial oedema, hypertrophic Schwann cells, tomacula (redundant myelin swellings), demyelination/remyelination, lipid deposition, and fibroblastic onion bulb formation. Similar changes were also found in large and medium intramuscular nerves, although they were less severe in the latter. However, by contrast, ventral and dorsal spinal nerve roots, distal intramuscular nerves, and subcutaneous nerves were normal at all time points examined. These findings demonstrate, for the first time, that riboflavin deficiency in young, rapidly growing chickens produces selective injury to peripheral nerve trunks, with relative sparing of spinal nerve roots and distal nerve branches to muscle and skin. These novel findings suggest that the response of Schwann cells in peripheral nerves with riboflavin deficiency differs because either there are subsets of these cells in, or there is variability in access of nutrients to, different sites within the nerves.Key words: Avian species; demyelination; neuropathy; peripheral nerves; riboflavin deficiency.The normal functioning of the nervous system depends on a constant supply of nutrients. A deficiency of vitamins, particularly groups B (thiamine, cobalamin, pyridoxine, and niacin) and E, generally produces an axonal type of polyneuropathy.22,41 However, riboflavin (vitamin B2) deficiency is characterized by demyelination with hypertrophy of Schwann cells, marked lipid accumulation, paranodal tomacula, and fibroblastic onion bulbs. [6][7][8]19,20 There is, at least initially, sparing of axons, and spinal cord and brain are apparently unaffected. 7 Riboflavin (7,8-dimethyl-10-ribityl-isoalloxazine) is a water-soluble vitamin and a dietary requirement in both chickens and humans. It is a precursor of flavin mononucleotide and flavin adenine dinucleotide, and decreased levels of these coenzymes impair oxidative phosphorylation. 36 Riboflavin deficiency leads to decreased beta-oxidation of fatty acids with relative preservation of the tricarboxylic acid cycle. 37 The lipid deposition in Schwann cells and the formation of paranodal redundant loops of normal myelin (paranodal tomacula) suggest a disturbance of lipid metabolism and control of myelin membrane formation in the myelinating Schwann cell.The novel findings in this study that Schwann cells in spinal nerve roots and subcutaneous and distal intramuscular nerves do not show these pathologic changes suggest that Schwann cells in different parts of the peripheral nerve system respond differently to riboflavin deficiency. Materials and MethodsOur a...

The finding of tomacula, focal areas of sausage-shaped hypermyelination in peripheral nerves, is reported for the first time in avian riboflavin deficiency. Day-old, meat-type chickens were fed a riboflavin-deficient diet (1.8 mg/kg) and were killed on postnatal days 6, 11, 16, and 21, while control chickens were fed a conventional diet containing 5.0 mg/kg riboflavin. Tomacula were found in sciatic and brachial nerves from day 11 onward, became more frequent and prominent with increasing time, and preceded the onset of segmental demyelination.

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