EARLY CYTOSKELETAL CHANGES FOLLOWING INJURY OF GIANT SPINAL AXONS IN THE LAMPREY

The spinal cord of the larval sea lamprey contains identified giant ax ons that readily regenerate following spinal transection. In this stud y, we used serial light and electron microscopy to analyze the early u ltrastructural 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 o f axoplasm and subsequent retraction of the cut axon from the lesion a nd 2) changes associated with the early stages of axonal regeneration. Degenerative changes include the disruption of mitochondria to form l arge vacuoles, the collapse of neurofilaments into closely packed mass es (condensed filamentous cores; CFCs), and the appearance of amorphou s electron-dense bodies (dense granular masses; DGMs). Events associat ed with regeneration include the disappearance of vacuoles, DGMs, and CFCs and the appearance of small, sprout-like projections from the axo n stump. Thus, we show that degenerative and regenerative events can b e clearly separated from one another in identified axons, unlike the s ituation in the central nervous systems of amniote vertebrates such as mammals. (C) 1995 Wiley-Liss, Inc.