Ultrastructural observations on myelinated fibres in the tibial nerve of streptozotocin-diabetic rats: effect of insulin treatment.

Four groups of male Sprague-Dawley rats aged 11 weeks and weighing approximately 450 g were studied over 16 weeks: onset and end controls, untreated diabetics and diabetics treated with a daily subcutaneous injection of Ultralente insulin. Good metabolic control was achieved in the insulin-treated group as judged by daily blood glucose estimations, glycosylated haemoglobin levels (HbA1c) and body weight. Cross-sectional myelinated fibre area significantly increased between onset and end controls; growth thus occurred. In the untreated diabetic rats the values were significantly less when compared with age matched controls and not different to onset controls. The values for the insulin-treated diabetic group did not show a significant increase when compared with untreated diabetics and onset controls and were intermediate between end controls and untreated diabetics without any significant difference when compared with either group. Cross-sectional axonal area was significantly less in the untreated diabetic group as compared with age matched controls and this was corrected by insulin therapy, as there was no difference between end controls and insulin-treated diabetic group. Insulin treatment corrects the reduction in axon size but total myelinated fibre size is not normalised. It seems that in addition to the axon, the myelin sheath and Schwann cells are also affected and these may be less influenced by insulin therapy.