Undulating course of nerve fibres and bands of Fontana in peripheral nerves of the rat

Abstract

The undulating course of nerve fibres and the optical effect of that course, i.e. the bands of Fontana, were studied in the peripheral nerves of the adult rat using light microscopy. The arrangement of collagen fibres in the endoneurium of these nerves was evaluated using transmission and scanning electron microscopy. No nerve fibres undulation was noted on the intracranial sections of the cranial nerves or on the spinal roots. In their endoneurium a few, irregularly arranged collagen fibrils were found. In contrast, the nerve fibres undulation and Fontana's bands were a constant feature in the peripheral course of the nerve trunks. They were discernible in vivo and on excised unfixed as well as fixed nerves. The nerve fibres follow a sine-curve course of variable frequency and amplitude. Exposed in vivo, the nerve fibres retained their wave-like course even after removal of the epineurium and perineurium. The endoneurium of these nerves contained numerous undulating longitudinally oriented bundles of collagen fibrils. These findings suggest that the undulating course of the nerve fibres in peripheral nerves is conditional upon the quantity and arrangement of their endoneurial collagen fibrils. When the nerve was stretched in the course of movement, the undulation became straightened out until it disappeared. Conversely, nerve shortening enhanced the undulation. Thus the wave-like alignment of the nerve fibres represents a physiological reserve length for nerve stretching.

Extracting structural features of rat sciatic nerve using polarization-sensitive spectral domain optical coherence tomography

“…Previous studies reported that when a nerve is stretched using longitudinal tensile stress, the nerve fibers start losing their undulation and the appearance of the bands begins to fade. 33,34 Similar phenomena were also observed in this study. Representative image and plot for stretched nerve in Fig.…”

“…[32][33][34] Owing to the undulating nature of nerve fibers inside the bundle, dark and light stripes become visible on the surface of the fascicule when light is reflected back from the nerve. Their visibility depends both on the degree of nerve stretching and on the angle of illumination.…”

“…Their visibility depends both on the degree of nerve stretching and on the angle of illumination. [32][33][34] Although an oblique angle is more suitable to see the bands, the normally incident optical beam used in OCT still provides sufficient visualization of the bands, which are visible in both intensity and phase-retardation volumetric image reconstructions. Bands at different depths are more visible (shown with arrows) when moved up and down inside the nerve sample, in comparison to a still image as shown in Fig.…”

“…The average distance between adjacent bands in the unstretched nerve is between 300 and 400 μm, and these values are well within previously reported ranges. 33 …”

“…The structural features and optical properties of rat sciatic nerve characterized in this study are (a) trace of nerve bifurcation, (b) epineurium thickness, (c) extinction coefficient and birefringence of both nerve and muscle tissue, (d) bands of Fontana, [32][33][34][35] an optical effect seen due to the undulating nature of nerve fascicles, and finally (e) the effect on the birefringence of a nerve of stretching by longitudinal tensile stress. These results serve as a foundation for future studies on assessments of peripheral nerve injury as well as the nerve recovery process using OCT and its functional extensions.…”