Genetically Modifying Animals Using Embryonic Stem Cells

The drawback mentioned for cloning animals by nuclear transfer does not exist for a second technique used to produce genetically

Add gene construct from another organism

Grow for a few cell divisions

Figure 13.1 Making Genetically Modified Animals by Nuclear Transfer. First, the haploid nucleus of an unfertilized egg is sucked out of the egg with a very fine glass syringe. The enucleated egg is then fused with a diploid cell from the body of an adult animal. The egg, containing a diploid nucleus, is stimulated to divide in a petri dish. After a few divisions, the growing embryo is implanted into a surrogate mother that will carry it to term. The resulting cloned animal can be genetically modified by inserting a foreign gene into the diploid cell prior to fusion with the enucleated egg.

Grow for a few cell divisions in a petri dish

Figure 13.1 Making Genetically Modified Animals by Nuclear Transfer. First, the haploid nucleus of an unfertilized egg is sucked out of the egg with a very fine glass syringe. The enucleated egg is then fused with a diploid cell from the body of an adult animal. The egg, containing a diploid nucleus, is stimulated to divide in a petri dish. After a few divisions, the growing embryo is implanted into a surrogate mother that will carry it to term. The resulting cloned animal can be genetically modified by inserting a foreign gene into the diploid cell prior to fusion with the enucleated egg.

modified animals. This is a technique that genetically manipulates embryonic stem cells in the test tube and then injects them into a growing embryo. We saw in chapter 4 that stem cells isolated from very young embryos have the ability to form any kind of tissue. When injected into an embryo at the blastocyst stage (an embryo containing dozens of cells that have not yet formed any tissues or organs), embryonic stem cells join the cells of the developing embryo. Later, they follow the developmental and differentiation pattern of the cells surrounding them. In other words, an injected embryonic stem cell that finds itself next to cells destined to become a heart will also develop into a heart cell. If the embryonic stem cell finds itself next to cells destined to become gametes in the adult, it too will become a gamete.

Embryonic stem cells can be cultivated in the lab and manipulated like any other kind of cells. Thus, scientists can genetically modify embryonic stem cells by adding cloned genes from any source (usually by subjecting them to an electric shock in the presence of DNA) and then inject these genetically modified embryonic stem cells into a blastocyst-stage embryo. The blastocyst is subsequently implanted into a surrogate mother, where it continues to develop. Genetically modified embryonic stem cells thus become an integral part of the growing embryo and, eventually, the adult individual. These adults are then formed partly from the cells of the embryo that was the blasto-cyst and partly from the injected stem cells, and thus they are not all genetically alike. Thus, if the embryonic stem cells were genetically modified, only the cells derived from those embryonic stem cells have the foreign gene. The full procedure is illustrated in figure 13.2. It should be emphasized that genetically modified animals are not yet used on an industrial scale and their products are not yet used for medical purposes.

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