Embryonic stem cells (ES cells) are stem cells derived from the inner cell mass of an early stage embryo known as a blastocyst. Human embryos reach the blastocyst stage 4-5 days after fertilisation, at which time they consist of 50-150 cells.并非来自胎儿的干细胞都是胚胎干细胞。
至于李负责的课题是《胚胎干细胞增殖及定向分化的研究》和《人胚胎生殖嵴干细胞生物学特性的研究》，In embryology, 生殖嵴（the gonadal ridge or genital ridge) is the precursor to the gonads. 因此应该是一般意义的胚胎干细胞和
来自生殖嵴的干细胞。（骨髓或其他组织来源）间充质干细胞，无论来自成年人体或者胎儿，都不算生殖嵴干细胞，更不是普通意义的胚胎干细胞。