They come from the activity that leads to coming of one of at least one of the gender. Such activity leads to formation of zygote. The zygote develops into human and human have human cells. read more
There are also cell lines that came from blastocysts and these we of course call embryonic stem cells (ESC). I personally do not believe that blastocysts are human beings, but I still believe that scientists who use human ESC should give thought to where they come from. read more
There are several sources of stem cells. Pluripotent stem cells can be isolated from human embryos that are a few days old. Cells from these embryos can be used to create pluripotent stem cell "lines" —cell cultures that can be grown indefinitely in the laboratory. read more
First and foremost, it should be noted that not all stem cells come from embryos. read more