Early cytoskeletal changes following injury of giant spinal axons in the lamprey.

The spinal cord of the larval sea lamprey contains identified giant axons that readily regenerate following spinal transection. In this study, we used serial light and electron microscopy to analyze the early ultrastructural consequences of axotomy in the proximal stumps of these axons near the lesion site. Axotomy results in two types of striking ultrastructural changes: 1) changes associated with the degeneration of axoplasm and subsequent retraction of the cut axon from the lesion and 2) changes associated with the early stages of axonal regeneration. Degenerative changes include the disruption of mitochondria to form large vacuoles, the collapse of neurofilaments into closely packed masses (condensed filamentous cores; CFCs), and the appearance of amorphous electron-dense bodies (dense granular masses; DGMs). Events associated with regeneration include the disappearance of vacuoles, DGMs, and CFCs and the appearance of small, sprout-like projections from the axon stump. Thus, we show that degenerative and regenerative events can be clearly separated from one another in identified axons, unlike the situation in the central nervous systems of amniote vertebrates such as mammals.