Histopathology of normal tissues and tumor tissues in the nude mice (H&E 100×). A-D. Absence of tumor formation in nude mice injected with 1 × 106 hUCMSCs for 60 days. The tissue sections of lung (A), liver (B), stomach (C) and spleen (D) show no presence of any tumor cells; E-H. The tumor tissue histopathological sections were made in mice of the control group (E), the hUCMSCs group (F), the hUCMSCs-LV-Vec group (G), and the hUCMSCs-LV- IL-21 group (H), respectively. Section E shows the active growth of the tumor cells and obvious nucleic divisions. A few of the apoptosis tumor cells were observed in the tumor tissues of the mice treated with hUCMSCs (F) and hUCMSCs-LV-Vec (G), but a little more necrotic, apoptotic tumor cells were found in Section H, which was collected from the tumor tissues from the mice treated with the hUCMSCs-LV-IL-21 group.