Embryology of Fetal Tissue

Abstract

When the first ever clinical embryologist, Bob Edwards, saw the fertilization of a human oocyte in vitro he was witnessing not only a momentous event in clinical medicine but also the creation of totipotent stem cells [249]. At the point of fertilization and up to the point of early compaction, the blastomeres of the human embryo are generally considered to be totipotent stem cells. These totipotent stem cells can differentiate into all cell types and therefore have the ability to create a complete new individual [60]. It is possible that blastomeres are totipotent to enable correction of early developmental errors in the embryo [87]. The data on totipotent stem cells come from experimental embryology using animal embryos as the legal and ethical restrictions on human embryo experimentation restrict such work [84]. From this animal experimentation, it is known that totipotent stem cells can develop into endoderm, mesoderm and ectoderm, germ cells, extraembryonic tissue, and trophoblast. In the mouse embryo, asymmetric divisions at the eight-cell stage result in two populations of cells [111]. The inner cell mass (ICM) of the blastocyst then develops from cells positioned inside the embryo, and those cells on the outside of the embryo develop into the trophectoderm which subsequently develops into the placenta [58, 72, 203]. Two ICM cell types then develop [76] which are: