Results Pattern and Dynamics of Reinnervation. Altogether, our results reveal that the zebrafish PLL is a convenient system to study axonal regeneration in vivo compared with mammalian systems, in which nerve wiring is more complex and less traceable. We conclude that the promoting effect of a first lesion is mostly caused by an intrinsic, local change occurring in the injured axons. We show that although Schwann cells act as guidance cues to help the axons regrow along their original path, they are not involved in this regeneration-promoting effect. Whenever a second cut is made after complete regeneration, the latency of reinnervation is reduced at all ages, provided the second cut is immediately distal to the first one. Our data clearly demonstrate the effectiveness and fidelity of regeneration at all ages studied, from 1 to 15 mpf, although the onset of reinnervation is increasingly delayed with age, thereby linking neuronal aging with a progressive decline in neuronal reactivity to axonal damage.
Here we examine whether the PLL nerve is able to regenerate in adult and aging zebrafish. The line migrates ventrally to reach its final position at the juvenile stage, and an axonal branch follows each neuromast during this migration. More neuromasts develop during larval life, such that the line eventually consists of one neuromast on every intersomitic border, or about 30 altogether ( 8). This line lies originally along the horizontal myoseptum of the embryo and comprises only five neuromasts. Here we concentrate on the most extensive of the four lines, the ventral one. Bud-neuromasts remain closely associated and form dorsoventrally arranged linear clusters, or “stitches” ( 7). This pattern is established around 1 mo postfertilization (mpf) and remains essentially unchanged throughout adulthood, except that each juvenile neuromast gives rise to a number of “accessory” neuromasts through a budding process ( 4, 5) that depends on innervation ( 6). The juvenile PLL of zebrafish comprises four lines of neuromasts that extend at different dorsoventral levels, totaling about 50 organs ( 3). We provide evidence that this factor remains present at the site of the first lesion for several days and is intrinsic to the neurons. This latency is reduced after a second nerve cut at all ages, suggesting that a regeneration-promoting factor induced by the first cut facilitates regeneration on a second cut. We quantified the extent of target reinnervation after a nerve cut and found that the latency before the nerve regenerates increases with age.
We observed that irregularities in the original branching pattern are faithfully reproduced after regeneration, suggesting that regenerating axons follow the path laid down by the original nerve branches. We explore the regeneration potential and dynamics of the PLL nerve in adult zebrafish and report that regeneration occurs throughout adulthood. However, the potential for PLL nerve regeneration has not been tested yet beyond the early larval stage. Axonal regeneration after nerve cut has been demonstrated in this system during the first few days of life, leading to complete regeneration within 24 h. Here we show that the posterior lateral line (PLL) of zebrafish is a suitable system to study axonal regeneration in vivo because of both the superficial location and reproducible spatial arrangement of neurons and targets, and the possibility of following reinnervation in live fish on a daily basis. However, studying this process in vivo is difficult or even impossible in most vertebrates.
Axonal regeneration is a major issue in the maintenance of adult nervous systems, both after nerve injuries and in neurodegenerative diseases.
0 kommentar(er)
