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M. Watanabe, T. Okada, Y. Tokita, K. Saito, J. Yoshida; Injured Axons of Cat Retinal Ganglion Cells Regenerate Into the Crushed Optic Nerve . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1278.
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Purpose: In adult mammals injured axons of retinal ganglion cells (RGCs) fail to regenerate into the optic nerve (OpN). We examined whether an accelerator of cAMP synthesis or activators of macrophage can make injured axons of RGCs to regenerate into the OpN in adult cats. Methods: The left OpN of anesthetized cats was crushed with a 6–0 thread pulled with 0.2 N, 60 sec (microcrush). Forskolin (30 min before OpN crush), zymozan (60 m after the crush), or oxidized galectin–1 (60 m after the crush), was injected into the vitreous. At day 12, WGA–HRP was intravitrally injected to anterogradely label regenerated axons. On day 14, cats were deeply anesthetized, perfused with fixative. The excised OpN was longitudinally cut on a cryostat, labeled axons were visualized with TMB reaction. Results:Evaluation of Microcrush. Dextran–Texas Red was injected into the both geniculate nuclei immediately before the left OpN was crushed. After 14 days the cat was perfused with fixative, and the left and right retinas were flat–mounted, OpNs were sectioned. We found no labeled RGCs or fibers in the left retina or left (crushed) OpN. This implies that the microcrush completely degenerated RGC axons. Number of Regenerated Axons in the OpN. The OpN with no drug–injection (control) contained very few labeled axons which elongated beyond the crush site, whereas OpN with injections of forskolin (0.1 mg), zymozan (50 µg), or galectin–1 (0.5–1000 ng) had many labeled axons elongating beyond the crush site. We counted labeled axons being longer than 0.5 mm or 1 mm from the crush site in every other section. Mean numbers (N=2 to 4) of labeled axons of 0.5 mm or longer were 24 (control), 341 (forskolin 0.1 mg), 275 (galectin 0.5 ng), 2755 (galectin 100 ng), 1355 (galectin 1000 ng), 601 (zymozan 50 µg), respectively. Similar tendency was found in the mean axon numbers at 1.0 mm; galectin 100 ng injection resulted in 469 labeled axons whereas only 2 axons in the control. Longest Regenerated Axons. We measured length of the longest axons, and found no statistically significant difference between control values to experimental ones. Conclusions: In adult cats, crushed axons regenerated into the OpN when a macrophage activator was injected or intracellular cAMP level was elevated. The elongation rate of regenerated axons was estimated as less than 1 mm/ wk, smaller than the rate of regenerated axons in a peripheral nerve graft, 1.1 mm/d (Maki et al., 2003).
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