To the editor

The article by Xu et al. in the October issue describes the successful feeder-free propagation of undifferentiated human embryonic stem (ES) cells (Nat. Biotechnol. 19, 971–974, 2001). The authors reported, however, that in conditioned media from mouse embryonic cell lines and from human immortalized lines, the feeder-free human ES cells did not retain all the features of cells grown on feeder layers.

Did Xu et al. have the occasion to test human embryonic fibroblasts, which are available from cell repositories like the American Type Culture Collection (Manassas, VA) and the European Collection of Cell Cultures (Salisbury, UK), as feeders or as a source of conditioned medium? The answer might be of great help to the growing scientific community involved in the study of human ES cells.

Chunhui Xu and Melissa Carpenter reply:

We demonstrated that human embryonic stem (hES) cells can be maintained on appropriate matrices and media conditioned by irradiated primary mouse feeders (MEF-CM) while maintaining all of the essential features of hES cells grown on feeders. When hES cells were grown in conditioned media from a human telomerase immortalized neonatal fibroblast line (BJ5ta) and embryonic murine fibroblast lines (NHG 190 and STO), fewer colonies with appropriate morphology were observed. Evaluation of additional immortalized human cell lines is currently in progress.

Chunhui Xu and Melissa K. Carpenter, Geron Corporation, Menlo Park, CA (mcarpenter@geron.com)