Chapter 13 - Differentiation in Early Development

Abstract

This chapter reviews the development of the mammalian embryo during pre-implantation and the earliest post-implantation stages as a basis for understanding the developmental potency of embryonic stem (ES) cells. During the first cleavage divisions, totipotent blastomeres of the mammalian embryo segregate and eventually become committed to the extraembryonic, somatic, and germline lineages, losing developmental potency. In mice and humans, pluripotent embryonic cells can be isolated from early embryos and maintained in culture as embryonic stem (ES) cells. A number of basic questions on early mammalian development still remain unanswered. This is due to the small size of early mammalian embryos and the effort that is required to obtain them in the large numbers necessary for analysis of gene and protein expression. It is also because implantation (and later placentation), through which the embryo establishes (and maintains) a physical connection with the mother, makes embryos relatively inaccessible; it has not been possible to mimic implantation in vitro or monitor these developmental stages in vivo. Advances in techniques for analysis of gene expression in small sample sizes, in vitro fertilization, clonal analysis of cultured embryos, and the use of genetic markers are providing new clues on key developmental events and allowing important questions to be addressed.