[1] Univ Groningen, Univ Med Ctr Groningen, Dept Cell Biol, NL-9713 AB Groningen, Netherlands
[2] Vrije Univ Amsterdam, Dept Anat & Neurosci, Med Ctr, NL-1007 MB Amsterdam, Netherlands
[3] Univ Cambridge, Wellcome Trust, MRC, Stem Cell Inst, Cambridge CB3 0ES, England
[4] Univ Cambridge, Dept Vet Med, Cambridge CB3 0ES, England
[5] Ctr Multiple Sclerosis Res, MRC Ctr Regenerat Med, Edinburgh EH16 4UU, Midlothian, Scotland