[1] Univ Cambridge, Wellcome Trust, MRC, Cambridge Stem Cell Inst, Cambridge CB3 0ES, England
[2] Univ Cambridge, Dept Vet Med, Cambridge CB3 0ES, England
[3] Univ Western Australia, Eileen Bond Spinal Cord Res Ctr, Sch Anat & Human Biol, Perth, WA 6009, Australia