Instead of just waiting for you to provide me with a date on which embryonic stem cell research began... I've taken a quick look and found this reference...
AU Thiennu H. Vu, Gail R. Martin, Pauline Lee, David Mark, Alice Wang, and Lewis T. Williams; TI "Developmentally Regulated Use of Alternative Promoters Creates a Novel Platelet- Derived Growth Factor Receptor Transcript in Mouse Teratocarcinoma and Embryonic Stem Cells."; SO Mol. Cell. Biol. 9:4563-4567(1989).I suspect embryonic stem cell research began before 1989, but I hate to waste time doing the research on an issue you brought up.
Please let me know what you know. Thank you.