stem cell is not one of them.
it really depends on luck, whom you work with, what stage of the project you enter the lab, how nice the graduate/post-doc is to let your name be on the paper, etc.
above goes for most pre-meds. a few actually do conduct signficant research and publish. and if you work very hard in the lab (ie a lot of time), you will get there.
and i know how competitive it is to get into medical school, but you really shouldn't worry about publications. your PI's LOR is more important IMO. Anyhow, good luck!