On the other hand, somatic cells are considered to have
little tumorigenic potential even after substantial manipulations
like in vitro expansion, because they consistently pass into senescence
[5]. Malignant transformation of the cells is believed to occur
through multiple processes involving the accumulation of mutations
in key regulatory genes that promote cell survival and proliferation
[6,7]. Although a few individual groups reported the
spontaneous transformation of human mesenchymal stem cells
(hMSCs) during in vitro culture [8e11], two of them retracted their
papers because the results appeared to be attributable to contamination
with tumorigenic cells (fibrosarcoma, osteosarcoma, or
glioma cell lines) [12,13]. The rest of the groups found the immortalization
of the cells, which is closely associated with tumorigenicity,
during in vitro culture, indicating that the good practices to
avoid contamination with tumorigenic cells and the monitoring of
cell growth are critical for the quality control of hCTPs derived from
human somatic cells.