Background

Any injury to mammalian spinal cord leads to the devastating consequence of paralysis and loss of function. In contrast to mammals, injury response in zebrafish cord is quite different, resulting in repair and regeneration of cord followed by functional recovery. A variety of lesioning protocols have been employed to study spinal cord injury and functional recovery in lower vertebrates (Holtzer 1956; Egar and Singer, 1972; Filoni et al., 1984; Becker et al., 1997; Margotta et al., 1991; Hui et al., 2010; Sîrbulescu and Zupanc, 2011) in last five decades. Among them the most popular experimental protocol to study spinal cord regeneration has been tail amputation (Holtzer, 1956; Egar and Singer, 1972; Filoni et al., 1984; Margotta et al., 1991; Sîrbulescu and Zupanc, 2011). Tail amputation involves complete removal of the caudal part of tail, where muscle, skin, bone and cartilages are also removed along with spinal cord. There is complete regeneration of tail along with the spinal cord leading to functional recovery after tail amputation and a substantial progress has been made to understand the cellular mechanism of spinal cord regeneration. But there are drawbacks of using this model. The major criticism against this model is that, it does not appropriately mimic SCI in human, since there is no tail structure in humans and the nature of the injury is different.

In teleosts, the most important lesion paradigm that has been widely used to study spinal cord regeneration is complete transection (Becker et al., 1997; Goldshmit et al., 2012). Transection refers to complete severing of cord which can often lead to spinal shock in humans. It occurs rarely in comparison to hemisection which commonly occurs in gunshot wounds. Transection could be the appropriate model to study axonal regeneration since there is no axonal sparing after transection injury and some believe that axonal sparing itself could augment regeneration in mammals (Basso et al., 1996).

On the other hand, compression and crush injuries are most prevalent in mammals under experimental conditions and in human accidental injury conditions (Thuret et al., 2006). In search for an appropriate injury model to study the regeneration in teleost, we successfully established a standardized crush injury model in zebrafish, which is comparable to the mammalian mode of injury (Hui et al., 2010). Among all the experimental paradigms mentioned, standardized crush injury is the most suitable model to understand both the mammalian and the teleostean scenario compared to transection or tail amputation models. The outcome of crush injury varies when compared to transection injury, as in crush injury secondary degenerative response elicits axonal degeneration, whereas in transection injury axonal tracts are disengaged almost immediately after injury.