Cells, Embryonic

EMBRYONIC STEM CELLS were first identified in the 1980s in animal models, and the breakthrough of their successful isolation from human embryonic tissue came after about two decades. They are derived from embryos that are in their initial stages of development. After an ovum is fertilized by a sperm, a zygote is formed, which then transforms into a loose clump of cells, called the morula, around the fourth day of embryonic development.

The morula then forms an inner and an outer cell mass, which transform into embryonic and extra embryonic tissue (necessary for the initial development of the embryo), respectively. The inner cell mass gives rise to three germinal lay—ers—ectoderm, [Page 76]endoderm, and mesoderm—which go on to form different organs and tissues as the embryo matures. ...