A role of the environment in Fgf2-induced proliferative response is also consistent with in vivo studies reporting increased proliferation in Fgf2-treated CPe in pathological conditions, such as in a Parkinson's disease model [23], but no obvious size changes in the CPe of normal animals that developed Fgf2-induced hydrocephalus [23]. Here, FGF2 is linked to Parkinson disease.