Salamanders and fish can regenerate lost structures - why can't we?

See research article http://www.biomedcentral.com/1471-213X/12/9 webciteLoss or serious damage to tissues cannot be repaired - at least not in humans. A severed limb does not grow back, an infarcted heart muscle does not heal by itself. Many animal species do, however, have surprising regenerative abilities. Studies of these natural regenerating systems promise to provide a conceptual understanding of the biology of tissue regeneration, and even partial achievements could revolutionize approaches to regeneration in the clinic.Like other organs, vertebrate appendages are composed of complex tissues that originate from multiple germ layers. The limb, for example, consists of epidermis and a peripheral nervous system, both derived from ectoderm, and other internal tissues such as muscle, bone, dermis, and blood vessels, which have a mesodermal origin. In a regeneration-competent vertebrate, damage or complete loss of an appendage initiates a regenerative response that typically involves the early formation of a growth zone of undifferentiated cells, the blastema, at the distal end of the stump. The origin of the newly formed blastemal cells and their fate during the regeneration process have been on-going topics of debate over the past century. Early studies using the regenerating salamander limb and tail indicated that injured multinucleated myofibers can dedifferentiate, give rise to mononucleate progeny, and contribute to the regenerating blastema. Tracing individually labeled myotubes after transplantation documented their capacity to redifferentiate into different lineages within the regenerate, indicating the multipotent nature of derived progenitors cells [1]. Recent advances in generating green fluorescent protein (GFP)-expressing transgenic frogs, salamanders, and fish, combined with molecular marker analyses, have enabled in vivo tracking of cells with high precision. Revisiting the open questions concerning the overall contribution and transdifferentiation o