Embryonic Stem Cells Transplanted In Utero Survive and Produce Teratocarcinomas in a Murine Model.

Introduction: Embryonic stem (ES) cells have the capacity to differentiate into any cell type. We have previously established an in vivo murine model for the study of the in vivo differentiation of ES cells. The aim of this study was to monitor tumor formation in this model. Materials and

Methods: Murine ESC genetically engineered to express yellow fluorescent protein (YFP-ES cells) were cultured on mitomycin-treated feeder layers for four days prior to in utero transplantation in medium containing leukemia inhibitory factor. A cell dose of 5x10⁴ YFP-ES cells (H-2b) was injected in utero intraperitoneally in the fetuses of Balb/c (H-2d) and C57BL/6 (H-2b) pregnant mice at E12- E14. The presence of YFP-ES cell-derived cells was analyzed in various tissues of the liveborn offsprings of different ages by fluorescence microscopy and quantified by digitalized fluorescence microscopy of frozen section from various tissues and organs and analysis of tissue supernatants for the presence of the YFP signal. Tumor formation was monitored in liveborn offsprings for up to one year and tissue sections were examined by routine and fluorescent microscopy.

Results: The YFP-fluorescent cells derived from YFP-ES cells were detected in liver and thymus without any evidence of migration in other organs. Extensive peritoneal teratocarcinomas with supradiaphragmatic involvement were generated in two out of five allogeneic in utero transplantation of YFP-ES cells. Tumor formation was not observed in the groups transplanted in utero with fetal liver or adult bone marrow hematopoietic cells, or in the control group. Clinical and histopathological pictures suggestive of graft-versus -host disease were also observed in two out of five mice transplanted in utero.

Conclusion: The promise and the potential risks of the ESC transplantation have to be carefully considered due to the high risk of teratoma/teratocarcinoma occurrence. Protocols using pre-differentiated cells from ES or adult stem cells may constitute a better approach for cellular therapies.