An ultrastructural study of sensory nerve endings after neurorrhaphy

Abstract

After the mental nerve of mouse was amputated and left in a denervation state for 12 weeks, neurorrhaphy was performed. Then, the ultrastructural changes in the degeneration and regeneration processes of the sensory nerve endings (encapsulated corpuscles) distributed in the lower lip mucosa were evaluated over time up to the 24th week postoperatively, using a transmission electron microscope.
1. In the encapsulated corpuscles distributed in the lower lip mucosa of normal mouse, axon terminals were found at the center, surrounded by the inner core and further by the outer core. The external shape and internal structure were similar to those of Pacini corpuscle.
2. In the degeneration process of encapsulated corpuscles after amputation of the mental nerve, axon terminals disappeared on one day after amputation, and the inner core was observed with its central portion collapsed. Thereafter, lamellar cells of the inner core showed a gradual atrophic tendency. The lamellar gap expanded, and collagen microfibrils increased. In the 12th week after amputation, only a few cytoplasms were alive near the nuclei of lamellar cells. Further, the basement membrane covering the surroundings still maintained a condition almost similar to the original state. The outer core was slower in degeneration than the inner core but the process was the same.
3. When the mental nerve was re-sutured in the 12th week after amputation, regenerated fine axons were seen along the inner surface of the basement membrane, which remained in a lamellar shape in the 4th week after re-suture. In the lamellar cells, cytoplasms extended along the inner surface of the remaining basement membrane. In the 24th week after re-suture, the axons extending toward the center of corpuscle were as thick as normal cases. Both inner and outer core were regenerated with the structure closer to that of healthy cases, whereas most axons showed no entering into the central region but intruded inside the basement membrane of inner core on the peripheral region. The axons were not thick, and the formation of inner core was also imperfect.
4. When left in a denervation state for a long period, encapsulated corpuscles were in a state different from the preoperative status in most cases even when they were regenerated through neurorrhaphy. The results of the present study suggest that morphological changes in the receptor may be one of the major causes of incomplete sensory restoration.