卵和精子的结合产生一个单细胞合子,最终它将发育成包括多种不同细胞和组织的多细胞生物体。起初单个细胞产生两个细胞谱系:外层的细胞谱系产生胎盘,内层的细胞谱系发育成身体的所有组成部分以及胚胎干细胞(ESCs)。“谱系特殊性”是怎样产生的对于科学家来说至今还是个谜,但是了解这种转变将帮助人们认识干细胞能怎样转化成不同类型的组织, The combination of egg and sperm creates a single-cell zygote that eventually will develop into a multicellular organism that includes many different cells and tissues. Initially a single cell produces two cell lineages: the outer cell lineage produces the placenta, the inner cell lineage develops into all parts of the body, and embryonic stem cells (ESCs). How genealogy is produced remains a mystery to scientists, but understanding this shift will help people understand how stem cells can be transformed into different types of tissue,