Towards a definition of recovery of function. 
In this review we consider recovery of function after spinal cord injury, and, in particular, recovery improved following intraspinal cellular transplants.  Some recovery occurs spontaneously and this can be especially dramatic in neonates, supporting the notion that developing and adult spinal cord respond differently to injury.  Recovery can be improved in both neonates and adults by appropriate cellular transplants into the injury site.  We describe several functional tests used in animals with spinal lesions and transplants.  We compare the effects of transplants of fetal tissue and genetically modified fibroblasts into neonatal and adult injury sites on recovery of motor and sensorimotor function.  Fetal tissue transplants support greater recovery and elicit more regeneration in neonates than in adults.  Transplants of fibroblasts modified to produce neurotrophic factors however support both recovery and axonal growth even in adults.  The contribution of the transplant to recovery is shown by the loss of function that follows a second lesion just rostral to the original lesion/transplant site.  The effect of the re-lesion indicates that the recovery is mediated by the presence of the transplant but the way in which transplants act to promote recovery may include a number of mechanisms, including regeneration and sprouting, neuroprotection, and modifications of organization of spared CNS structures.