Morphometric and ultrastructural changes in the sciatic nerve of the albino rat after a long period of immobilization of the knee and ankle joints

ABSTRACT

The sciatic nerve was used as a model to study the effect of a long period of immobilization on the peripheral nerves. Ten young adult male albino rats were used in this investigation. The left knee and ankle joints were immobilized for 12 weeks using a plaster cast. The contralateral right side was used as control. After removal of the cast, segments from the sciatic nerves of the immobilized and control sides were excised and processed for ultrastructural and morphometric studies. Morphometric study revealed that immobilization of the rat hind limb resulted in a highly significant reduction [43%] in the cross-sectional area of the sciatic nerve, a highly significant reduction [33%] in the total number of the myelinated fibers, a highly significant reduction [21%] in the mean diameter of the myelinated fibers and a highly significant increase [19%] in the myelinated fiber density/mm[2]. The size distribution of the myelinated fibers showed marked changes after immobilization. The statistical data showed a shift toward, the small size distribution and the small diameter myelinated fibers outnumbered the medium-sized and the large diameter fibers. Moreover, myelinated fibers with a diameter >15 micro were observed only in the experimental sciatic nerve. These results indicate that the small myelinated fibers were the least affected and may confirm the assumption that the motor fibers are those who are usually affected by immobilization. Ultrastructural observations revealed splitting of the myelin lamellae, presence of marked intramyelinic edematous clefts between the split myelin lamellae, infolded myelin loops and myelin degeneration. Changes in the axons included formation of myelin rings and figures in some of the larger axons, degeneration, compression, irregularity and shrinkage of axons. Degradation of myelin lipids was reflected by formation of large vacuoles and layered structures inside Schwann cells. These ultrastructural and morphometric changes may shed some light on a definite structural neural involvement in the mechanism underlying the immobilization-associated polyneuropathy