Embryonic Stem Cells - Basic Biology to Bioengineering

Abstract

At the pre-implantation blastocyst stage of development, the mammalian embryo is composed of a unique collection of cells of which three major populations predominate. The outermost layer the trophectoderm (TE) gives rise to the placenta, which acts to sustain the developing fetus connecting it to the mother host. The next is a cluster of cells known as the inner cell mass (ICM) these cells are said to be pluripotent (Fig. 1). A third group of cells known as the primitive endoderm, surrounds the ICM cells at the epiblast stage. As development proceeds the ICM cells rapidly divide and eventually begin to differentiate forming the three embryonic germ layers (ectoderm, mesoderm and endoderm). Effectively these pluripotent ICM cells are the precursors of all adult tissues. As these pluripotent cells commit to a specific cellular lineage, they lose their pluripotency. Embryonic stem (ES) cells are euploid pluripotent cell lines isolated directly from cultured preimplantation embryos. The first stable ES cell lines were isolated by immunosurgery from the ICM of implantationdelayed, mouse blastocysts (Martin, 1981; Evans and Kaufman, 1981). Mouse ES cells are very closely related to early ICM cells in terms of their developmental potential (Beddington and Robertson, 1989). This chapter will focus on mouse ES cells (mES) unless otherwise stated. Three features characterize mES cells; 1. They are isolated directly from the embryo (Robertson, 1987). 2. They can colonize the germ line when introduced to the embryo. 3. They possess unrestricted proliferative potential (Suda et al., 1987). These features effectively mean that under appropriate conditions, a karyotype stable selfrenewing, pluripotent population of cells can be propagated indefinitely in vitro. mES cells have other characteristics, which prove useful when comparing embryo derived stem cells to their differentiated progenies. mES cells have a euploid (2n) chromosome complement, a feature that allows their participation in germ cell development and the formation of chimeras (Bradley et al., 1984; Evans, 1994). The functional demonstration of mES cell developmental potential through chimera formation is the definitive proof of the pluripotent nature of the cell population in question. Biomarkers are often used as indicators of the stem cell state due to the time consuming and technically more difficult nature of getting functional proof of stemness. Many of the common markers are transcription factors expressed in the ICM and mES cells and have been shown to have functional roles in selfrenewal and in the maintenance of pluripotency, in both isolated stem cells or the ICM. The