Lewy bodies, α-synuclein (α-syn) immunopositive intracellular deposits, are the pathological hallmark of Parkinson’s disease (PD). Interestingly, Lewybody-like structures have been identified in fetal tissue grafts about one decade after transplantation into the striatum of PD patients. One possible explanation for the accelerated deposition of α-syn in the graft is that the aggregation of α-syn from the host tissue to the graft is spread by a prion disease-like mechanism. We discuss here an in vitro model which might recapitulate some aspects of disease propagation in PD. We found here that in vitro-generated α-syn oligomers induce transmembrane seeding of α-syn aggregation in a dose- and time-dependent manner. This effect was observed in primary neuronal cultures as well as in neuronal cell lines. The seeding oligomers were characterized by a distinctive lithium dodecyl sulfate-stable oligomer pattern and could be generated in a dynamic process out of pore-forming oligomers. We propose that α-syn oligomers form as a dynamic mixture of oligomer types with different properties and that α-syn oligomers can be converted into different types depending on the brain milieu conditions. Our data indicate that extracellular α-syn oligomers can induce intracellular α-syn aggregation, therefore we hypothesize that a similar mechanism might lead to α-syn pathology propagation.