Srinivas S., Watanabe T.
© Cambridge University Press 2013. Overview and general concepts: The process of embryogenesis is the basis for establishing the final form of the adult. Embryonic development does not consist simply of the growth in size of a ‘preformed’ miniature fetus, but is a tremendously dynamic process, characterized by a great deal of cell movement and tissue rearrangement. An embryo starts out as a single cell, the fertilized egg, which divides to form a mass of cells. Over the course of embryogenesis, a pattern emerges from this initially inchoate collection of cells. This pattern is manifest as changes in cells, resulting in the generation of different types of tissues, and in the stereotypic arrangement of these cells and tissues to generate recognizable fetal form. Coordinated with the morphogenetic events that shape the embryo is the progressive differentiation of cells, to achieve functional specialization. Differentiation is generally accompanied by a reduction in the potential of those cells (in terms of cell types they can subsequently differentiate into). Embryogenesis starts with the zygote, said to be totipotent because it can give rise to all fetal and placental tissues. The zygote undergoes repeated rounds of division to yield cells that differentiate into pluripotent cells, which can give rise to many cell types (including all those found in the fetus), but not certain cell types that make up the placenta. In the course of embryogenesis, these pluripotent cells differentiate further, giving multi, oligo and unipotent cell types. Some of these cell types can be described as stem cells, capable of indefinite self-renewal (given the appropriate niche), while also retaining the potential to differentiate into specific cell types.