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Unformatted text preview: tiated state in the presence of LIF. If LIF is removed from the culture medium, the cells aggregate to form embryoid bodies and then differentiate into a variety of cell types. Embryoid bodies Cell differentiation Blood cells Epithelial cells Adipocytes Smooth muscle cells Nerve cells Heart muscle cells from human embryos, raising the possibility of using embryonic stem cells in clinical transplantation therapies. Mouse embryonic stem cells are grown in the presence of a growth factor called LIF (for leukemia inhibitory factor), which signals through the JAK/STAT pathway (see Figure 15.40) and is required to maintain these cells in their undifferentiated state (Figure 17.24). If LIF is removed from the medium, the cells aggregate into structures that resemble embryos (embryoid bodies) and then differentiate into a wide range of cell types, including neurons, adipocytes, blood cells, epithelial cells, vascular smooth muscle cells, and even beating heart muscle cells. Human embryonic stem cells do not require LIF but are similarly maintained in the undifferentiated state by other growth factors, which are not yet fully characterized. Importantly, embryonic stem cells can be directed to differentiate along specific pathways by the addition of appropriate growth factors to the culture medium. It may thus be possible to derive populations of specific types of cells, such as heart cells or nerve cells, for transplantation therapy. For example, methods have been developed to direct the differentiation of both mouse and human embryonic stem cells into cardiomyocytes, which have been used to repair heart damage resulting from myocardial infarction in mice. Likewise, considerable progress has been made in directing the differentiation of embryonic stem cells to neurons, which have been used for transplantation therapy in rodent models of Parkinson's disease and spinal cord injury, and to insulin-producing pancreatic cells, which have been used for therapy in mouse models of diabetes. A great deal of current research is therefore focused on the development of culture conditions to promote the differentiation of embryonic stem cells along specific pathways, thereby producing populations of differentiated cells that can be used for transplantation therapy of a variety of diseases. This material cannot be copied, reproduced, manufactured, or disseminated in any form without express written permission from the publisher. 2009 Sinauer Associates, Inc. UNCORRECTED PAGE PROOFS
CHAPTER 17 (A) Unfertilized egg Remove egg chromosomes Transfer adult nucleus to enucleated egg Adult somatic cell (B) Culture to early embryo Blastocyst Implant in foster mother FIGURE 17.25 Cloning by somatic cell nuclear transfer (A) The nucleus of an adult somatic cell is transferred to an unfertilized egg from which the normal egg chromosomes have been removed (an enucleated egg). The egg is then cultured to an early embryo and transferred to a foster mother, who then gives birth to a clone of the donor of the adult nucleus. (B) Dolly (the adult sheep, left) was the first cloned mammal. She is shown with her lamb, Bonnie, who was produced by normal reproduction. (Photograph by Roddy Field; courtesy of T. Wakayama and R. Yanagimachi.) Somatic Cell Nuclear Transfer
Clone The isolation of human embryonic stem cells in 1998 followed the first demonstration that the nucleus of an adult mammalian cell could give rise to a viable cloned animal. In 1997 Ian Wilmut and his colleagues initiated a new era of regenerative medicine with the cloning of Dolly the sheep (Figure 17.25). Dolly arose from the nucleus of a mammary epithelial cell that was transplanted into an unfertilized egg in place of the normal egg nucleus--a process called somatic cell nuclear transfer. It is interesting to note that this type of experiment was first carried out in frogs in the 1950s. The fact that it took over 40 years before it was successfully performed in mammals attests to the technical difficulty of the procedure. Since the initial success of Wilmut and his colleagues, transfer of nuclei from adult somatic cells into enucleated eggs has been used to create cloned offspring of a variety of mammalian species, including sheep, mice, pigs, cattle, goats, rabbits, and cats. However, cloning by somatic cell nuclear transfer in mammals remains an extremely inefficient procedure, such that only 13% of embryos generally give rise to live offspring. Animal cloning by somatic cell nuclear transfer, together with the properties of embryonic stem cells, opens the possibility of therapeutic cloning (Figure 17.26). In therapeutic cloning, a nucleus from an adult human cell would be transferred to an enucleated egg, which would then be used to produce an early embryo in culture. Embryonic stem cells could then be cultured from the cloned embryo and used to generate appropriate types of differentiated cells for transplantation therapy. The major advantage provided by therapeutic cloning is that the embryonic stem cells derived by this procedure would be genetica...
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